6 ( 1 , W ITFrCOMPLTMENTS Blood Pressure and Pulse Rate AS INFLUENCED BY DIFFERENT POSITIONS OF THE BODY Read at the Fifty-fifth Annual Session of the American Medical Association , in the Section on Pathology and Physiology , and approved for publication by the Executive Committee: Drs. V. C. Vaughan , Frank B. Wynn and Joseph McFarland O. Z. STEPHENS, A.M., M.D., Department of Physiology, Northwestern University Medical School Chicago Reprinted from The Journal of the American Medical Association , October 1, 190J,. CEIICAGO : PRESS OF AMERICAN MEDICAL. ASSOCIATION ONE HUNDRED AND THREE DEARRORN AVENUE. 1904. REMOTE STORA BLOOD PRESSURE AND PULSE RATE. AS INFLUENCED BY DIFFERENT POSITIONS OF THE BODY. 0. Z. STEPHENS, A.M., M.D. Department of Physiology, Northwestern University Medical School. CHICAGO. HISTORY. The blood pressure in the arteries was first meas¬ ured with some degree of accuracy by Stephen Hales in 1733. He ordered a mare to be tied down on her back, into whose crural artery, about three inches from her belly, he inserted a brass pipe one-sixth inch in diameter. To this pipe, by means of another brass pipe, he fastened a glass tube 9 feet long, and of nearly the same diameter as the pipe in the artery. He then untied the ligature on the crural artery, as a result of which the blood rose 8 feet 3 inches above the level of the ventricle of the heart. 1 While blood pressure can be measured in this way with some degree of accuracy, there are several objec¬ tions to the method, chief among which are the follow¬ ing: 1, Through the uncomfortable position of the ani¬ mal and the consequent effect on the vasomotor nerve fibers, the pressure is likely not to be the same as it would under normal conditions; 2, the inaccuracy due to the speedy coagulation of the blood; 3, the clumsi¬ ness and inconvenience of the apparatus. Poiseuille, many years later, improved this method of Hales by substituting for the glass tube a mercurial manometer. The tube connecting this manometer with the artery was filled with a solution of sodium carbon¬ ate to prevent coagulation of the blood. 2 Later, Ludwig made an improvement on Poiseuille’s mercurial manometer by placing on the mercurial col¬ umn a float carrying a writing style, by means of which 1. “Statistical Essays,” vol. 11, after Hlll-Schafer’s Text-book of Physiol., 1900, vol. II. 2. “Rech. sur la force du coeur aortique.” Th&se, Paris, 1828, after Hill-Schfifer’s Text-book of Physiol., 1900, vol. 11. p15 * o5 2 records of the variation in pressure could be taken on revolving drums. By means of the apparatus a fair degree of accuracy can be obtained, but the oscillations of the mercurial column, due to inertia, render it ob¬ jectionable. The mercurial manometer, however, has been used very extensively and with very valuable re¬ sults. Piorry, a noted French physician, was one of the first to observe the effect of the force of gravity on the circulation, i. e., the effect of the change of position on the blood pressure. His observations, however, were clinical. Having been called to a patient who had lost consciousness, and who was being supported in a sitting position by his friends, he placed him in a hori¬ zontal position, after which the patient at once re¬ gained consciousness. Hill has also found, through a trephine hole in the skull, that the intracranial pressure is negative in the sitting posture and positive when the head was bent down toward the knees. 3 Many other observations of the same nature have been observed clinically on the human subject and experi¬ mentally on the lower animals. Most of the work on the lower animals has been done by inserting a cannula connected with a mercurial manometer into the carotid or femoral artery, and then shifting the animal from one position to another, about a horizontal axis which passes through the point of in¬ sertion of the cannula. In this way, quite accurate re¬ sults have been obtained. Of course the animals were invariably narcotized or anesthetized. So far as we have been able to learn the experimental observations on blood pressure in different positions of the body have been confined to the horizontal, vertical feet-down and vertical feet-up positions. The same is true of the pulse rale. The clinical observations have been confined to the horizontal, head-up and head-down at no particular angle. Since the advent of the sphygmomanometer, experi¬ ments have been performed on the healthy human sub¬ ject in the standing, sitting and horizontal postures. The pulse rate in the human subject in different po¬ sitions of the body, so far as we have been able to learn, * was first studied by Guy, 4 who made observations on 3. Hill: Journal Physiol.. Cambridge and London, 1885, rol. ^ xvlli, p. 15. 3 100 men, averaging 27 years of age, in the standing, sit¬ ting and lying positions. He found the pulse rate to be highest in the standing, lower in the sitting and low¬ est in the lying positions. We see, then, that the subjects of blood pressure and pulse rate in different positions of the body are not new ones; that some of the apparatus used in getting blood pressure was inaccurate; that experimental ob¬ servations were on narcotized or anesthetized animals; that clinical observations on blood pressure in the dif¬ ferent positions of the body are confirmatory of the experimental observations. In our work on blood pressure we have dealt with the subject in a different manner, in some respects, from our predecessors, insomuch as we have chosen more posi¬ tions of the body and have taken the pressure on both sides of the body in each position. METHOD. (a) Subjects .—The subjects used in these experiments were twenty-two male medical students, with an aver¬ age age of 24 years and an average height of 170 cm. In one of these subjects, however, the pulse rate was not obtained, hence only twenty-one are used in mak¬ ing computations on pulse rate. In changing from one position to another, they were allowed to remain long enough in the new position for the circulatory ap¬ paratus to become adjusted to the new conditions before observations, either on blood pressure or pulse rate, were taken. Every means was taken to keep them comfortable and to avoid anything which would tend to provoke excitement or muscular effort. The rate of respiration was also kept normal. ( b) Apparatus .—The instrument used in taking the blood pressure was the Riva Rocci sphygmomanometer, as modified by H. W. Cook. The instrument is now so well known among physiologists that a description of it here is not demanded. ( c) Technic .—Each of the twenty-two men were taken through the following positions: 1, Standing; 2, sit¬ ting; 3, supine; 4, head down at an angle of 45 de¬ grees; 5, right lateral; 6, left lateral. The blood pressure was taken in both brachial ar- 4. Hill-Schafer’s Text-book of Physiol., 1900, vol. li. 4 teries in each position by placing the band of the in¬ strument around the upper arm midway between the elbow and shoulder. The pulse rate was also taken in each position, the count being taken through a whole minute of time. In taking the pressure in the standing position, the patient stood erect, the upper arm was abducted to the Fig. 1.—Standing. horizontal plane and the forearm flexed at right angles to the brachium and held perpendicularly, with the hand uppermost. The arm was supported in this po¬ sition by an assistant in order to relieve the subject of any muscular effort in sustaining the arm himself. The opposite arm was allowed to hang laxly by the side. In the sitting position, the subject sat on a stool, with the thighs parallel and flexed at right angles to the axis of the body. The legs were flexed at right angles to the thighs. The arm in which the pressure was taken was held in the same position in relation to the body, as it was in the standing posture (the assist¬ ant supporting it), while the hand of the opposite arm lay laxly on the thigh of the same side. Fig. 2.—Sitting. The subject in the supine posture lay flat on his back with legs parallel, and the arm on the opposite side to the one in which the pressure was taken lay parallel with and along side of the body. The arm in which the pressure was taken was abducted to right angles with the body axis, and the forearm at right angles to the brachium and held perpendicularly. Thus it is 6 seen that the arm did not take the exact relation to the body in this position that it did in the standing and sitting postures. In the latter, the brachium was so rotated that the inner aspect faced forward, while in this position the inner aspect of the brachium faced toward the feet, having rotated through an arc of 90 degrees. The variation in blood pressure, due to this slight difference in the relative position of the arm, if there be any variation at all, would certainly be too insignificant to be taken into account. The subject in the head-down position was placed on a table on his back, around his ankles were placed com¬ fortable straps, which were fastened to one end of the table. The end of the table was then elevated so that the subject hung with his head down, at an angle of 45 degrees. The arms assumed practically the same posi¬ tion, in relation to the body, as they did in the supine position, the forearm of the arm operated on being held perpendicularly, and the opposite arm lying along side of the body. In the right lateral position our subject lay on the right side, the head raised to the horizontal level by a pillow, the legs parallel and straight, and the left arm parallel with and on the body. The right arm was ex¬ tended anteriorly at right angles to the body axis, the forearm flexed at right angles to the upper arm and held perpendicularly when the pressure was taken in this arm. When the pressure was taken in the left arm in this posture, the right arm was allowed to as¬ sume a position most comfortable to the subject, in or¬ der to obviate any nervous influence on the circulation which might arise from the subject being uncomfortable. The left arm assumed the same relation to the body as it did in the supine when the pressure was taken in it. This, of course, put the forearm in a horizontal plane. The left lateral position is a repetition of the right lateral in reversed order, and need not be detailed fur¬ ther. RESULTS. After the pressure in each brachial was taken in a given position, the two results were averaged to obtain a mean pressure in that position. The results of such observations, together with the pulse rate, are shown in the following table: 7 No. Stand¬ ing. Sitting. Supine. Head- down. Right Lateral. Left Lai. 1 Right arm . . . . 134 142 174 185 172 125 Left arm ., . . 122 132 150 174 124 163 Pulse rate . . . 97 90 72 69 68 72 2 Right arm . . . 146 153 163 170 157 119 Left arm . . . . 146 153 155 171 118 158 Pulse rate . . . 85 81 64 57 59 63 3 Right arm . . . 106 115 128 137 126 81 Left arm .. . . 117 120 134 153 94 133 Pulse rate . . . 70 57 53 50 56 58 4 Right arm . . . 134 146 177 199 193 135 Left arm . . . . 131 145 158 179 128 192 Pulse rate . . . 93 87 83 87 84 78 5 Right arm . . . 110 113 140 148 132 93 Left arm . . . . 112 123 138 147 99 133 Pulse rate . . . 102 88 77 79 80 79 6 Right arm . . . 133 143 157 159 140 108 Left arm .. . . 136 138 159 151 111 170 Pulse rate . . . 118 93 71 71 88 78 7 Right arm . . . 151 154 179 216 186 141 Left arm . . . . 155 171 186 240 147 213 Pulse rate . . . 86 81 71 64 69 63 8 Right arm . . . 124 127 140 143 150 108 Left arm . . . . 102 107 145 193 115 145 Pulse rate . . .. . . . 9 Right arm . 1.27 3.29 i.34 148 144 92 Left arm . . . . 114 118 130 136 96 134 Pulse rate . . . 86 78 64 65 74 70 10 Right arm . . . 115 130 138 159 129 116 Left arm .. . . 109 101 132 153 119 131 Pulse rate . .. 110 98 86 76 90 87 11 Right arm . . . 141 142 149 169 181 112 Left arm .. . . 139 135 149 165 122 151 Pulse rate . .. 70 69 60 56 59 59 12 Right arm . . . 130 139 146 167 154 no Left arm . . . . 137 136 145 171 111 158 Pulse rate . . . 103 97 80 83 78 77 13 Right arm . . . 136 141 154 169 155 89 Left arm .. . . 117 116 141 164 108 137 Pulse rate . . . 75 69 65 74 67 67 14 Right arm . . . 127 124 146 151 134 92 Left arm . . . . 113 115 133 153 90 137 Pulse rate . . . 72 60 53 48 53 50 15 Right arm . . . 152 132 165 169 157 123 Left arm .. . . 134 137 156 168 122 153 Pulse rate . . . 83 81 76 73 77 75 16 Right arm . . . Ill 100 126 132 119 73 Left arm . . . . 115 116 129 133 95 134 Pulse rate . . . 99 92 69 68 67 75 17 Right arm . . . 134 130 154 177 152 105 Left arm .. . . 125 141 161 184 110 157 Pulse rate . . . 71 70 62 58 59 58 18 Right arm . . . 164 152 170 203 195 135 Left arm . . . . 156 161 178 218 134 198 Pulse rate . . . 92 84 74 76 77 75 19 Right arm . .. 154 151 180 198 180 127 Left arm . . . . 151 150 169 183 112 180 Pulse rate . . . 82 83 78 62 71 66 20 Right arm . . . 122 119 142 153 159 111 Left arm . . . . 119 113 127 158 98 155 Pulse rate . . . 76 75 59 58 63 62 21 Right arm . .. 127 124 159 162 143 96 Left arm . . . . 133 127 133 161 111 157 Pulse rate . . . 77 72 62 58 63 66 22 Right arm . . . 132 118 163 185 157 106 Left arm . . . . 141 129 159 189 128 165 Pulse rate . .. 67 65 55 59 61 60 In summarizing the above data, we used the Hall- Quetelet method. 5 This method uses the median value, 5. The Journal A. M. A., Dec. 21, 1901. 8 instead of the average or arithmetical mean. The data collected from each individual are recorded on a card. The cards from the several individuals are grouped as desired. We grouped our cards according to the pulse rate and blood pressure in each arm in the different positions of the body. For illustration, let us take the blood pressure in the right arm in the standing posi¬ tion; and let us take all those cards showing a blood pressure of 105-110 mm. Hg, inclusive, and place them in one group, and all those showing a pressure of 110- 115 mm. Hg in another group, etc., till we have all the Fig. 3.—Supine. twenty-two cards placed in groups, the difference be¬ tween the minimal values of which is 5 mm. Hg pres¬ sure. We shall then have the following table: Table No. 2. Blood pressure 105+ 110+ 115+ 120+ 125+ 130+ 135+ 140+ 145+ 150+ 155+ 160+ No. of ob¬ servations 1 2 1 2 3 6 1 1 1 3 0 1 In adding the number of observations shown in the table, we get a total of 22, which corresponds to the number of subjects in whom the pressure was taken. 9 The next step is to find the median value, which Dr. Hall defines thus: “The median value is that value which is so located in the whole series of observations of a single measurement of a single group that there are as many above it as below it; i. e., that the num¬ ber of values which it exceeds is equal to the number of values which exceed it.” Since the number of obser¬ vations is 22, the median value, therefore, will have on one side of it 11 values, which are less, and on the other side 11 values, which are greater than itself. We must, then, find the eleventh value. In counting from left Fig. 4.—Head down. to right, we find that the eleventh value lies in the group 130 mm. Hg, and is the second from the min¬ imal value and fourth from the maximal value of this group. The median value, therefore, lies in this group, which may be called the median group. We know, then, that it must be between 130 mm. Hg, the min¬ imal value of the group, and 135 mm. Hg, the mini¬ mal value of the next higher group. Now, according to the biologic laws, the six values in this median group will be practically evenly distributed throughout the 5 mm. Hg pressure between its minimal value and the minimal of the next higher group. Hence the sec- 10 ond value from the left must be 130 mm. Hg pressure, plus 2/6 of 5 mm. Hg pressure, which equals 131.6 mm. Hg, the pressure for the right arm in the standing po¬ sition. Dr. Hall has also reduced this process to a mathe¬ matical formula, thus: “Let n equal total number of observations; m equal the number of observations in the median group; 1 equal the sum of observa¬ tions to the left of median group; r equal the sum to the right; a equal the minimum value of the median group; d equal the arithmetrical difference between the minimum values of the groups, and M equal the median value to be determined.’’ Then M=a-t-[d(n-f2—1)—m] d (l H "2-l) M=a-f--—=- l M=130+ 5 (f- : 131.6 mm. Hg pressure by substituting the values of the letters in the above cases. In taking the arithmetical mean, however, we get a pressure of 132.2 mm. Hg. In observing tie table, we see that we have one observation of quite high pres¬ sure. This slight increase of the average over the median value is doubtless due to tins one observation. In a large number of observations the dwarf values are likely to balance the giant values, in which case the arithmetical mean is an accurate though time-con¬ suming method of evaluating data. In a small number of observations, however, one extreme is more certain to overbalance the other, and in this case the Hall- Quetelet method is the only accurate one. It is also accurate in handling large numbers and is much sim¬ pler and more easily applied than the old method. Our data pertaining to the pulse rate and blood pres¬ sure in both brachial arteries, in the different positions enumerated above, were handled in this manner, the summary of the results of this evaluation being given in the following table: Table No. 3. Standing. Sitting. Supine. Head down. Rt Lat. Lt. Lat. Right arm. 133.3 152.5 166.2 155 110 Average. . 131.7 150.4 165.6 134.5i 133 Left arm. 130 130 148.3 165 114 156 Pulse rate. 86 82 68.7 65.8 68.1 69.1 11 DISCUSSION OF EFFECTS ON BLOOD PRESSURE. In scanning this table, it is seen that the average blood pressure in the two arms increases in the stand¬ ing, sitting, supine and head-down positions, respect¬ ively, while the pulse rate decreases. It has been con¬ sidered that the blood pressure varies as the heart rate, times the heart strength times the resistance. Expressed in terms of a formula, we have P varies as Hr X Hs X R, where P equals blood pressure, Hr, the heart rate, Hs the heart strength, and R the resistance. This resistance may be due to arterial causes—arte¬ rial resistance; it may be due to the capillaries—capil¬ lary resistance; it may be due to contractions of the arterioles—peripheral resistance; it may be due to venous causes—venous resistance; or it may be due to the effect of gravity on the circulation—hydrostatic re¬ sistance. How, it remains to be seen whether the phenomena observed in Table 3 can be explained by means of this formula. Standing Position .— We see that the average blood pressure in the standing position is lower and the pulse rate is higher than in any other position in the series. Here the current meets with the least arterial and hy¬ drostatic resistance on the arterial side of the circula¬ tion, and with the greatest hydrostatic resistance on the venous side. Both of these factors tend to decrease the pressure in the upper portions of the body by tending to allow the accumulation of blood in the lower portions oT the circulatory system. This is partially compen¬ sated for, however, by the abdominal muscles and con¬ traction of the arterioles in the splanchnic area. 3 Sitting Position .—The average pressure in the sitting position is nearly 1 mm. Hg. greater than in the stand¬ ing position, a difference almost so slight in itself as to be ignored. But when we observe that the pulse rate in this position has decreased four beats to the minute, more importance attaches to this slight rise in pres¬ sure, and we begin to wonder why the pressure did not sink with the lowering of the heart rate. According to the formula, P will vary with Hr where Hs and R remain constant. Therefore, if Hr decreases, P will decrease also. If P does not decrease when Hr decreases, but, on the contrary, remains constant or increases, it is evident that the variation of Hr is coun- 12 terbalanced or more than counterbalanced by the varia¬ tion of either Hs or R, or both, in an opposite direction. Can we account in any way for a sufficient rise in Hs or R, or both, to produce the slight rise of P against the decrease in Hr? It will be recalled that in the sitting position the thighs were flexed at right angles to the body axis, and the legs at right angles to the thighs; that the body was sustained erect on the pelvis, and the hand of the arm not being operated on was lying relaxed on the thigh of the same side. The blood, then, must take a somewhat different course in this position from what it took in the standing position, i. e., it must course around two right angles in each leg—one at the in¬ guinal region and one at the knee. It also deviates slightly from a straight line at the elbow of the arm not being operated on. Thus the blood in two of the largest arteries turns two right angles in each lower extremity, and one large artery turns an angle of approximately 45 degrees at the elbow. The same is true of the veins in the same localities. This introduces an arterial and venous resistance which did not exist in the standing position. It is possible, too, that the capillary resistance may be increased by compression of the gluteal region and upper part of the thigh by the weight of the body on them. It has been stated that through the influence of the vasodilator nerve fibers the flow of blood to a contract¬ ing muscle is increased. 6 If this be true, the flow of blood to the muscles of the lower extremities will be increased while the muscles are contracting to main¬ tain the body in a standing position. This increased flow of blood to the lower portions of the circulatory system will tend to raise the pressure here and lower it in other portions of the arterial system. The relaxation of these muscles in the sitting posi¬ tion, however, with practically the same tension of the abdominal muscles and the muscles of the back in main¬ taining the trunk erect on the pelvis, prevents the in¬ crease of flow of blood to the lower extremities, which re¬ sults in higher pressure in the rest of the arterial sys¬ tem, i. e., the peripheral or arteriolar resistance is in¬ creased in this position. 6. Stewart: Manual of Physiol., 1900, p. 153. 13 The force of gravity also plays an important role. In a man 6 feet high the hydrostatic pressure of a col¬ umn of blood reaching from the vertex to the sole of the foot is equal to 140 mm. Hg, and from the vertex to the middle of the abdomen about 50 mm. Hg. 2 If this statement be true, the hydrostatic pressure in a man 5 feet 8 inches high—the average height of our 22 subjects—will be about 134 mm. Hg. Now, if the av¬ erage distance from the bend of the femoral artery in the inguinal region to the bend of popliteal behind the knee be about 40 cm., the hydrostatic pressure in this Fig. 5.—Right lateral, lower arm. position will not be 134 mm. Hg by 31.5 mm. Hg, the pressure of a column of 40 cm. There will be 40 cm. of the column of 170 c.m (5 feet 8 inches), on which gravity exerts a pressure downward on only the lower wall of the vessel. This column of 40 cm. of blood, meeting resistance to its downward tendency, has Ho effect through its own weight in the column in the leg below it as it had in the standing position, but tends to check the flow of the column above. The latter, by its own weight or hydrostatic resistance, and the elas¬ tic force of the arteries must sweep this 40 cm. of 14 blood through a horizontal distance of 40 cm. In overcoming this extra hydrostatic resistance, the col¬ umn above must necessarily experience a rise in pres¬ sure. Thus the decrease in the hydrostatic resistance due to the decrease in height of the column of blood, is reacted on by the increased pressure due to the hy¬ drostatic resistance of the 40 cm. to be moved in a horizontal plane. This is true on both the arterial and venous sides of the circulatory system. Whether or not these two factors balance we do not know. However, to summarize, we see that we have an increase in the arterial, venous, capillary and peripheral resistances, and also the hydrostatic resistance due to the hori¬ zontal column of blood. We also have a decrease in the hydrostatic resistance due to a decrease in height of the blood column. Now, since we have an increase in P and decrease in Hr, it is evident that the increasing factors of R must more than counterbalance the decreasing factor, since the respiration was kept normal and all nervous stimuli avoided which would tend to increase Hs. This in¬ creased pressure is shared by the coronary arteries, in consequence of which an increase of nutriment is car¬ ried to and increased tension placed on the heart mus¬ cle, both of which tend to increase the heart strength. 7 It is clear, then, that the total increase in P is not due alone to the increased resistance, but is brought about partially by the increase in the heart strength. This increase of the heart strength, however, results from the increased pressure due to the increased re¬ sistance. Just what proportion of the total increase in P is due to the increase in R, and what proportion is due to the increase in Hs, we do not know. We con- ; elude, therefore, that the increased blood pressure in the brachials in the sitting position over that in the standing position is due to an increase in both the re¬ sistance and the heart strength. Supine .—In referring to Table 3, the median pres¬ sure in the supine position is seen to be 150.4 mm. Hg, a distinct rise over that in the two previous positions. We see, also, that the pulse rate has decreased to 68.7 beats per minute, a distinct decrease below that in the two previous positions. Therefore, since P is higher and 7. Roy and Adami: Philosophical Trans, of the Royal Society, 1892. vol. clxxxiii, B., pp. 90, 262. 15 Hr lower than in the two other positions, it is clear that Hs or R, or both, have made a greater increase. Hill of London has shown that, when the body of one of the lower animals takes the vertical feet-down position, the blood pressure falls in the carotids and at the same time rises in the femorals. When the body resumes the horizontal position the pressure increases in the carotids and decreases in the femorals, as com¬ pared with what it was at first. When the animal is placed in the vertical feet-up position, the pressure still further rises in the carotids and falls in the fem¬ orals. These phenomena he attributed to the hydro- Fig. 6.—Right lateral, upper arm. static pressure of the blood. Again, if the phrenic nerves be divided when the animal is in the vertical feet-down position, the pressure will still further fall in the caro¬ tids and rise in the femorals. Furthermore, if a cru¬ cial incision be made in the abdominal walls, the pres¬ sure falls still further in the carotids 'and rises in the femorals. These phenomena, he attests, point to a compensatory apparatus in the splanchnic area and the abdominal walls, Since compression of the latter will cause the pressure to rise in the carotids and fall in the femorals. This compensatory apparatus, he thinks, becomes more nearly complete in animals that assume 16 more nearly the vertical position as their natural pos¬ ture; therefore, more nearly complete in man. 3 The clinical experience of Piorry, cited above, and of many others, also shows the effect of gravity on the circulation in the supine position. When the subject is placed on his back the blood which previously tended to gravitate to a plane below the heart, especially into the spacious venous system of the splanchnic area, now tends to become more equally distributed throughout the circulatory system, since this system has taken a horizontal position. This tends to increase the hydro¬ static resistance in the plane of the heart and above it, through an increased flow of blood to these regions and to lower it in the planes below through a correspond¬ ingly decreased flow to those regions. At the same time, the hydrostatic resistance is fur¬ ther increased by the blood in the arterial system hav¬ ing to be moved along a horizontal plane at right angles to the force of gravity. It is decreased, however, in the lower portions of the venous system by a force equivalent to the difference between that required to raise the return circulation to the level of the heart, and that required to move it through a horizontal plane throughout the venous sys¬ tem. It is increased, however, in the upper portions of this system through the general tendency of the blood to become equally distributed throughout the circula¬ tory system. The increased hydrostatic resistance in the circu¬ latory system tends to strengthen the heart beat by increasing the nutriment to and the tension on the heart muscle. This increased force of the heart “may more than counterbalance the increase in the resistance to the contractions of the left ventricle which that rise introduces, so that the ventricle may contract more com¬ pletely than it did before the pressure was raised.” 8 It may be possible that the capillary resistance is slightly increased in this position by the weight of the body on the tissues of the back; this is evidently small, as the body rests largely on bony prominences, such as the sacrum, shoulder blades, etc., leaving the large mus¬ cular areas of the back practically free from pressure. We see, then, that there is an increase in the hydro- 8. Roy and Adami: Philosophical Trans, of the Royal Society, 1892. vol. clxxxiii, B.. p. 269. 17 static resistance in the arterial system and upper por¬ tions of the venous system; that because of this in¬ crease of hydrostatic resistance the heart’s action is strengthened; that there is a possible slight increase in the capillary resistance; that there is a decrease in hy¬ drostatic resistance in the lower portions of the venous system. Now, since P has increased, we are forced to conclude that the increase in Hs and R more than counterbalances the decrease in R in the lower venous system. Further¬ more, that the ultimate factor in bringing about the in¬ crease in P in this position is the hydrostatic resistance. Head-Down Position .—Referring again to Table 3, it is observed that the blood pressure in the head-down position has made a leap of over 15 mm. Hg above what it was in the supine, being now 165.6 mm. Hg. It will also be noticed that the heart rate has been low¬ ered almost three beats per minute, the rate now being 65.8. Now, since the pressure is the greatest and the heart rate is the lowest in this position, it is evident that the heart strength or the resistance, or both, have ex¬ perienced the greatest increase. Since the man is on an inclined plane with the head downward, it is clear that the blood will tend to course toward the head through the influence of gravity. This produces an increased hydrostatic resistance in the up¬ per portions of the circulatory system, which is greater than it was in the supine, since gravity acts on an angle of 45 degrees with the course of the blood, instead of 90 degrees. It is also greater than in the standing and sitting postures, since in the latter gravity acts in a straight line with the blood stream and tends to pull it to the opposite extreme of the circulatory system. The hydrostatic resistance in this position is equiva¬ lent to that of a column of blood extending perpendic¬ ularly between the plane of the feet and that of the brachial artery, a distance of about 99 cm., with a pressure of about 78 mm. Hg. According to Hill, 3 the increased hydrostatic pressure in the carotids in the vertical head-down position is par¬ tially compensated for by a decrease in the resistance .in the splanchnic area, brought about through the vaso¬ dilator mechanism; but this compensation is far from complete. The same may be said in regard to the capillary re- 18 sistance here, as was said in case of the supine; if there be an increase it must be slight. The hydrostatic pressure, being greater in the upper portion of the circulatory system than it was in the pre¬ vious postures, will necessarily be shared to a greater extent by the coronary arteries, in consequence of which there will be a greater increase of nutriment to the heart muscle. The tension on the heart will also be greater. We conclude, therefore, that the strength of the heart is greatest in this position. Lateral Positions .—In Table 3 it will be seen that the pressure in the right lateral position is 134.5 mm. Hg, and the pulse rate is 68.1 beats per minute; in the left lateral the pressure is 133 mm. Hg, and the pulse rate 69.1 per minute. Here the same general law holds good—that the pulse rate decreases as the pressure in¬ creases. But we notice that the pulse rate approximates that in the head-down position, while the blood pressure approaches that in the standing and sitting postures, i. e., the pressure is lower compared with the pulse rate than in the other lying positions. How, since the pulse rate is much lower and the blood pressure slightly higher in these positions than in the standing and sitting positions, it is manifest that both the heart strength and resistance, or either one of them, must be increased to a greater extent than in the latter. For convenience we shall take the average pressure and pulse rate in these positions in comparing them with the others, and later take the two separately, in compar¬ ing them with each other. The following table will then be useful: Table No. 4.—Lateral Positions. Right Arm. Left Arm. Average. Lower Arm. Upper Arm. Blood Pressure. Pulse Rate. 134.5 68.1 133 69.1 133.8 68.6 155.5 112 This table shows the average blood pressure in these positions to be 133.8 mm. Hg and the average pulse rate 68.6 beats per minute. By comparing these figures with those in Table 3 we see the pulse rate is nearly the same as in the supine, and but slightly higher than in the head-down positions, while the pressure is much lower. That means that the heart strength, the resistance, or both, have decreased. 19 How can we account for these phenomena? In the first place, how can we account for the slight increase in the pressure over the great fall in the heart rate from what it was in the standing and sitting positions? In the latter a comparatively small amount of blood was moved along a horizontal plane by the heart force, thus overcoming the hydrostatic resistance due to gravity acting at right angles to the blood current, while in the lateral positions the blood moves nearly horizontally through the entire circulatory system. Here, also, the blood tends to become more equally distributed throughout the circulatory system. This re¬ sults in a lowered hydrostatic resistance in the lower portions of the body, with a corresponding increase in the upper portions. In this position it will be recalled that the lower arm was extended anteriorly at right angles to the body axis. Instead of the blood passing from the subclavian artery through the branchial in a straight line to the elbow, it deviates 90° from this course and passes directly for¬ ward. This offers a greater resistance to the blood cur¬ rent above, and at the same time tends to lessen the pressure in the radial below it. We conclude, therefore, that because of this variation the systemic pressure is somewhat higher than is recorded in the above table. This is a greater arterial resistance than we had in the standing and less than in the sitting positions. In the upper arm the forearm was horizontal instead of per¬ pendicular, as it was in the other positions, while the brachium was perpendicular instead of horizontal. Hence, these two differences balance. Because of the increased resistance the heart force is also increased in the same manner as has been given above. We see, then, that the hydrostatic resistance is greater in the planes of the heart and above it, in these posi¬ tions, than in the standing and sitting positions; that the arterial resistance is slightly greater than in the standing, but less than in the sitting position; that the heart strength is increased because of the increased re¬ sistance. Now, since P is slightly higher and Hr much lower, Hs and E, one or both, must be much increased. Now, why is the pressure lower than, and the heart rate nearly the same as, it was in the supine? In the latter the circulatory system lies practically in a hori¬ zontal plane. f n the lateral positions it is “on edge,” as it 20 were, and beside, the upper portion is slightly higher than the lower, since the distance from the central axis of the body to the point of the shoulder is somewhat greater than it is from the same axis to the most distant point of the crest of the ilium or great trochanter. This gives a slightly inclined plane, down which the blood tends to gravitate toward the feet, thus raising the pressure below the heart and lessening it above, as compared with what it would be in the supine position. The resistance, there¬ fore, being less, the heart strength will be less because of lessened nutrition to, and tension on, the heart mus¬ cle. The decrease in P, then, must be due to a decrease in Hs and E. The same things are true in the head-down position as in the supine, but in a more marked degree. Here, however, Hs has decreased to a greater degree; hence, we conclude that P has decreased as a result of the decrease in Hr, Hs and E. Now, Table 4 shows that the average pressure is 1.5 mm. Hg greater and the pulse rate a half beat less in the right than in the left lateral position. How may this slight rise of pressure be explained ? The aorta passes from the left ventricle upward and slightly forward and to the right. It then curves back¬ ward, upward and to the left. Therefore, in the right lateral position the heart lifts the greater portion of blood against the force of gravity to the perpendicular distance between the planes of the mouth of the ascend¬ ing and the beginning of the descending aorta, respec¬ tively. It also turns this blood through a semicircle in its course through the arch. In the left lateral this re¬ sistance is absent. The force of gravity alone would be sufficient to take the blood around the arch of the aorta after it reaches the top of the ascending portion. This increased resistance in the right lateral in turn produces a greater heart strength. It is clear, then, that the in¬ crease in P is due to an increase in Hs and E. DISCUSSION' OF EFFECTS ON PULSE RATE. “The mere fact that the centripetal fibers which call the vagus into play by reflex action come chiefly from the heart itself, shows that one part, and a very impor¬ tant part, of the vagus function is to reduce the work done by the heart in the interest of the heart itself. We conclude, then, tliat the vagus acts as a protecting nerve 21 to the heart, reducing the work thrown on that organ when, from fatigue or other causes, such relief is re¬ quired by it. The fact, however, that there exists cen¬ tripetal fibers which call the vagus center into activity, in such nerves as the sciatic and splanchnic, shows that the vagus mechanism may be called on to act in the in¬ terests of other parts of the body whose circulation re¬ quires to be diminished. We conclude, therefore, that the vagus may be used by other parts of the body to di¬ minish the blood pressure and the output of the heart, and thereby reduce the circulation. “Among the organs whose protection against over¬ congestion is of the greatest importance, it need hardly be said that the central nervous system takes the fore¬ most place. It is well known that if the intracranial pressure be raised artificially powerful excitation of the vagus center is produced. Vagus action also results from rise in the blood pressure in the systemic arteries, and the excitation thus produced can be shown to be due to the high pressure within the vessels of the central nerv¬ ous system and not to any direct effect of the rise of pressure on the heart. We must, therefore, look on the vagus mechanism as a means by which the central nerv¬ ous system gains protection against too great congestion. “The dependence on the blood pressure of the degree of vagus action, and the readiness with which the vagus center in the medulla is called into play by a rise of the intracranial pressure, seem to us to indicate that the mechanism in question is especially employed in the interests of the central nervous system as well as the heart itself/ 57 Now, if it be true that the vagus acts as a protecting nerve both to the heart itself and the central nervous sys¬ tem, it is clear that the reduced heart rate in the various positions of the body in the order named is in response to the action of the vagus in endeavoring to protect the heart and central nervous system against the increasing pressure. SUMMARY. 1. The blood pressure increases in the brachials from the standing to the head-down positions, inclusively, in the following order: Standing, sitting, left lateral, right lateral, supine and head-down. 2. The greater the hydrostatic resistance in the upper portions of the circulatory system the greater the in- 22 crease in pressure where the nervous and respiratory sys¬ tems are kept normal. 3. An increase of resistance is accompanied by an in¬ crease in heart strength; the strength of the heart, there¬ fore, will increase in the different positions in the fol¬ lowing order: Standing, sitting, left lateral, right lat¬ eral, supine and head-down. 4. The pulse rate decreases in the same order that the blood pressure increases. 5. The decrease in the pulse rate is a conservative act on the part of nature to protect the heart itself and the central nervous system. I I