CHAPTER 7

Blood pressure and limb blood flow

Although the cardiac function during breath-holding may seem explained in principle through Chapters 1 & 6, our knowledge of the circulatory changes occurring during apnoea and the Valsalva maneuvre is still incomplete.

Sharpey-Schafer (195 1, 1961) measured limb blood flow in association with the Valsalva manaeuvre, but the measurements were confined to the periods before and after rather short periods of a Valsalva experiment. Elsner & Scholander ( I 96j) have performed some plethysmographic measurements (strain gauge) during immersion of the face in water, but the character of the breath-hold and the size of the blood flow is not clearly stated. Also Brick (1966) studied the forearm blood flow in relation to face immersion, but neither the intrapulmonic nor the intrathoracic pressure was measured during the breath-holds.

It appears that no systematic investigation has been made of the human limb blood flow during breath-holding with Valsalva experiment or during breath-holding without a marked increase of the intrapulmonic pressure.

Material and procedures

A. PRELIMINARY EXPERIMENTS

The first series of experiments were performed in Lund, Sweden, in collaboration with Dr. I. Dahn. The subjects (three volunteers) lay supine with either both calves, or with one calf and one forearm or hand in the plethysmograph filled with water (34°C). The venous occlusion plethysmo- graphs designed by Dahn were used (Dahn 1964, 1965). Two types of experiments were carried out : PROLONGED BREATH-HOLDING. The procedure was started with a 4 minute period with intermittent registration of the control blood flow. The nose was closed with a foam rubber nose clip. The prolonged breath-holdings were performed after 3 deep inspirations. After the third, during which the subject filled his lungs, he held his breath at increased intrapulmonic pressure (Valsalva manaeuvre). With a little practice the test subjects

8 3

blood flow

ml 1 0 0 m l forearm I rnin / I

0

180 sec

seconds

c 2 40 360

Fig. 2). The blood flow in the right forearm measured before, during and after a breath- hold (Valsalva experiment at ‘TLC’) of three minutes duration. The ordinate is ml blood/Ioo ml forearm x min and the abscissa is the time. Each point indicates the

middle of a plethysmographic occlusion period.

increased their breath-holding time to 215 seconds. The flow of blood through the limb was measured both before, during and after each breath- holding (Fig. z j , 26, and 27).

PROLONGED BREATH-HOLDING DURING SYMPATHETIC BLOCK. The same procedure was used here, but left stellate ganglionic block was performed beforehand, by insertion of a needle between the common carotic artery and the thyroid cartilage, and deposition of 10 ml Lidocaine hydrochloride ( I %) in the retropharyngeal space. A few minutes after the injection the solution reached the lower cervical and the upper thoracic ganglia, and a typical Horner syndrome developed. In the period from 10 to 45 minutes after the injection, plethysmographic recordings were made of the left hand or forearm (and both calves) during Valsalva manoeuvres.

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1

10

5

1 - O L

B. SUPPLEMENTARY EXPERIMENTS

The investigations performed in Lund (in 196j) were followed up by studies carried out in Copenhagen on 27 volunteers, well trained in the procedure. The observations were made in association with the develop- ment of the methods set forth in Chapter 2. Some of the results of these personal studies bearing on the limb blood flow and blood pressure during breath-holding, are given and commented below. All the subjects rested supine during the experiment (Chapter 2).

I. CONTROL EXPERIMENTS. In three subjects the venous pressure was measured via a catheter (Bardic Deseret Intracat model 1614), the tip of which was placed in a vein in the middle of the forearm segment enclosed in a Dohn (air) or a water plethysmograph. The other forearm was placed in a water or air plethysmograph, and its blood flow was measured three times each min as a control. In an attempt to avoid the usual increase in peripheral venous pressure during a Valsalva manaeuvre (Fig. 29), the veins

blood f l o w

rn1/100ml calf x min

x-x r igh t leg

o - - - - - o l e f t l e g

.

\ , x *

\ *.-e kAx/\ \;/- 120 sec

0. /-.

.D---o- *.a

*.---- - - -.*-.-- 0. .-- 0.. ~ ”Lx

seconds

. L 0 60 180 300

8 5

of the forearm were drained for blood in 30-60 seconds prior to the venous occlusion period by passive elevation of the forearm (30 cm above the horizontal level); of course, no pressure was applied to the cuffs of this arm during the elevation. Four seconds after the commencement of the Valsalva manaeuvre the arm was lowered into the horizontal position, and on the 7. sec, 60 mm Hg pressure was applied to the distal cuffs of both arms. The proximal cuffs were automatically inflated 2 seconds after the distal ones. Each person performed 9 Valsalva experiments of I minute duration and with 3 minute intervals. During each Valsalva manaeuvre the blood flow was measured only once in the previously elevated arm, either between the 7.-17., the z7.-3j., or the 47.-75. second. The subjects performed the same kind of Valsalva manceuvre each time; after a deep inspiration (‘TLC‘) through a pneumotachograph (which was then closed by an assistant), the subject created an intrapulmonic pressure of 3 0 mm Hg.-In the second phase of the control experiment a narrow cuff (6 cm) was placed around the right arm at the axilla. The cuff was inflated to 3 0 mm Hg for 60 seconds in order to simulate the increase in peripheral venous pressure of the limb, caused by a Valsalva experiment (Fig. 29). Of course, the subject relaxed and breathed normally during this simulated “Valsalva manceuvre”.

2. STANDARD E x P E R r M E N T s with water plethysmographic measurement o f the blood flow through a forearm were carried out in 27 volunteers. Each test subject held his breath at different intrapulmonic pressures, either after a deep inspiration (‘TLC‘), or after a normal expiration (FRC). The breath-holdings lasted 60 (‘TLC’) or 3 5 (FRC) seconds, respectively. During breath-holding the subject kept the intrapulmonic pressure constant at one of the following values: 0, 10, 20, 30, or 40 mm Hg.

3. COMPARISON. The flow of blood was measured before, during and after Valsalva manaeuvres (‘TLC‘ at 3 0 mm Hg). The left calf was placed in a Dohn plethysmograph, and the right forearm in a water plethysmograph (34°C). The left forearm was surrounded by a double loop of mercury- in-rubber, fixed-at one point-with I cm wide Scotch tape. One of Whitney’s strain gauges was used for the loop, but his mounting plate and calibration method was excluded for reasons given in Chapter 2. The experiment was performed several times, with intervals in which control blood flow was measured (Table 12).

4. BLOOD FLOW AND PRESSURE. In 14 of the subjects the flow of blood through a forearm was measured with a water or a Dohn plethysmograph, while the arterial blood pressure, as well as the central and peripheral

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blood flow

ml lOOml forearm x min I ’ 1

x-x leff forearm

O _ _ _ _ _ . + w - a f t e r blockage of left . \ I stellole ganglion I I I I I

seconds

Fig. 21. Forearm blood flow altered by a Valsalva manewre (‘TLC’) performed both before and after sympathetic blockage.

venous pressures, were measured via catheters introduced percutaneously in the vessels of the other arm. Separated by intervals of three minutes of normal breathing, the subject held his breath at different intrapulmonic pressures and lung volumes, as described above.

5 . MUSCLE AND SKIN VESSELS. Hand and finger blood flow has not been the topic of interest, for reasons that will become obvious for anybody who tries to measure this variable. However, the right hand blood flow of one subject was measured. The distal exit of the chamber of the water plethysmograph was closed. The subject was wearing a glove of thin rubber, fixed with latex glue to one of the stiff rubber plates (Fig. 6) which was used to close the proximal hole of the chamber. The plethysmograph chamber was kept at a temperature of 37°C. The flow of blood through the hand was measured for three minutes as a control (9 determinations). The subject then performed a Valsalva experiment of 60 seconds’ duration (‘TLC‘ at an intrapulmonic pressure of 30 mm Hg), during which the blood flow was measured. Then the temperature of the plethysmograph chamber was lowered in steps of 1-3”C, and at each step the above experiment was repeated; the lowest temperature tested was 10°C. The control blood flows decreased from 12.3 at the 37°C to 2.2 FU at 10°C.

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6. VENOCONSTRICTION. In three subjects the right forearm was placed in a water plethysmograph, and the venous pressure was measured in a vein in the middle of the segment. The right forearm segment volume and the venous pressure was recorded. A blood pressure cuff (12 cm wide) was placed around the upper right arm and inflated to 290 mm Hg for 3 minutes. After one min of this period had elapsed, the subject held his breath (‘TLC‘ 30, ‘TLC’ 0, FRC 30, FRC 0). Simultaneously as a check the flow of blood through the left forearm was measured intermittently with a Dohn plethys- mograph.

Results and comments

The modified water plethysmograph was found to be more reliable than the air and the strain gauge plethysmographs, probably due to the fixed, local temperature and to the tailor-made sleeves. The Dohn (air displace- ment) plethysmograph was hardly acceptable, since the volume of the limb segment was difficult to obtain with a reasonable accuracy. The strain gauge method, even in the modified form covering practically the whole circumference and with correct calibration, was only applied for measure- ments, expressed in percentage change.

I . Does the plethysmogtaphic method fail to measure essentially ‘true’ blood flows during a Valsalva mancmuvre? Since the peripheral venous pressure increases to a magnitude of 20- 2 j mm Hg, it would be reasonable to assume that the distended veins would create a back-pressure interfering with the arterial inflow (ignoring-for a moment-the distensibility of the capillary bed). In the first control experiment described above, the Valsalva manceuvre-as usual-produced a fall in the flow of blood through both forearms approaching zero. In a forearm which has not been drained of blood prior to the flow measurement the Valsalva maneuvre and the proximal venous occlusion elicit an increase in peripheral venous pressure of approximately 20 mm Hg. However, in the previously elevated (drained) forearm the venous pressure, assumed to be representative of that of the veins of the segment in general, only increases to approximately 10 mm Hg. Consequently, the blood flow reduction is not a function of the increase in venous pressure. On the other hand, no drastic blood flow reduction was observed in the experiments where the peripheral venous pressure was raised with a cuff at the axilla during normal respiration. Thus, the plethysmographic method seems to yield the same results, regardless of the size of the venous back pressure (10-20 mm Hg). It seems reasonable to assume that the flow of blood measured during a Valsalva experiment approximates the ‘true’ value.

8 8

2. The resdts of the preliminary experiments made in Sweden showed that the Valsalva maneuvre was accompanied by a certain reduction of the flow of blood through the forearm (Fig. 2 5 ) and calf (Fig. 26). The results of the standard experiments indicated that the marked drop in the limb blood flow was only seen in the initial phase of a breath-hold at increased intrapulmonic pressure. Breath-holding after a deep inspiration (‘TLC‘) and at an intrapulmonic pressure of zero to 10 mm Hg, reduced the limb blood flow only slightly from the control level, while at 20 mm Hg a considerable drop in blood flow was seen, and at 30 to 40 mm Hg the reduction of the flow of blood through the limb was so drastic that the flow even approached zero. Breath-holding after a normal expiration (FRC) hardly reduced the limb blood flow at intrapulmonic pressures of o to 10 mm Hg, but at 20 to 30 mm the flow of blood was reduced to almost zero in 6 cases. Curiously enough-at that time--21 of the 27 subjects did not show this dramatic blood flow reduction at FRC with an intrapulmonic pressure of 20 to 30 mm Hg.

Simultaneously measured intrapulmonic and central venous pressures showed a linear relationship during Valsalva maneuvres with the glottis open. Owing to the risk that some subjects, contrary to the instructions, closed the glottis and produced the intended intrapulmonic pressure with the upper part of their airways, the central venous pressure was measured during standard experiments. Such ‘false’ Valsalva manaeuvres were not accompanied by a marked fall in limb blood flow, and the central venous pressure did not rise much, although the ‘mouth pressure’ was 3 0 to 40 mm Hg. Consequently, the details of the many standard experiments are without much value, except for the overall picture given above, and conclusions will be based on other experiments (4.).

3 . The resnits of the experiment where the blood flow changes were measured simultaneously with 3 difkrent plethysmographs are given in Table 12. The Valsalva experiment evoked a marked reduction of limb blood flow, and the relative reductions were of a similar magnitude in the forearm and the calf (Table 12).

4. Simdtaneo.w recording of arterial and venous blood pressure and limb blood flow gave the following results. Breath-holding at ‘TLC‘ and intra- pulmonic pressures of 20 to 40 mm Hg produced a marked fall in arterial blood pressure and limb blood flow, along with an increase in the venous pressures (Fig. 29). Breath-holding at ‘TLC’ and intrapulmonic pressures of o to 10 mm Hg only changed the blood flow and pressures to a small extent, and since the main point of interest was the marked blood flow reduction these experiments are only considered as controls. -The short

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Table 12. Percentage reduction of the blood flow by breath-holding after a deep inspiration (‘TLC’) and with an intrapulmonic pressure of 1-30 mm Hg.

FU 74 change

Water pleth., right forearm: 8.1/1.g 77

Strain gauge, left forearm: (5.6/1 . I ) 73

Dohn pleth., left calf: 3.8/1 .o 74

FL denotes flow units (i.~., ml blood/roo ml limb x min). The figure above the oblique line indicate the control blood flow, and below the line is shown the blood flow measured during the initial part of the Valsalva experiment. Thc blood flow is the average of

20 flow observations in one subject.

FRC breath-holds at high intrapulmonic pressure produced a more or less pronounced decrease in the arterial pulse pressure. However, in most cases the systolic pressure increased (Fig. 30), and these subjects showed no marked reduction in the flow of blood through the limbs (Fig. 30). On the other hand, in three cases both the systolic arterial pressure and the pulse pressure dropped during breath-holding at FRC and high intra- pulmonic pressure, and in all of these a marked fall in limb blood flow fol- lowed (Fig. 31) . In Fig. 3 0 the increase in peripheral venous pressure is delayed, as compared to the rise in the cases with marked flow reduction (Fig. 31) .

Catheterisation of all together 64 blood vessels (brachial artery (14), superior vena cam (zz), peripheral vein (28)) was never attended by any complications.

j . In the hand surrounded with water at different temperatures the Valsalva manaeuvre, at each temperature level produced a definite reduction of the blood flow, down to about 0. j ml/roo ml hand x min. Of course, a plethys- mograph used on intact limbs can never reveal with certainty whether skin or muscle blood vessels are involved in the blood flow reduction. However, the hand when cooled down to low temperatures showed a blood flow reduction during Valsalva manaeuvres, to the same absolute value as at higher temperatures. This suggested involvement of the muscle blood vessels, since the cutaneous vessels should be essentially closed beforehand at such low temperatures.

6. Itz order to demonstrate an increase in venous tone it is necessary to show an appreciable pressure rise in veins unable to change their volume. The segment volume of the arm (560 ml), whose circulation was arrested

90

EXP.

FRC 0 w

3 4 1 Y

l o 0 SEC 45

I

Fig. 28. Left forearm blood flow (Dohn plethysm. D) in FU. Variations in the right forearm segment volume measured with a water plethysmograph (W), and in the venous pressure (VP) of the segmental veins, were recorded during arterial occlusion (290 mm Hg) of the right arm. Several seconds after the application of the arterial occlusion (A), the VP reached a certain level whereafter the subject performed a breath-hold (‘TLC‘ 0, ‘TLC’ 30, FRC 0, FRC 30 mm Hg). In the control phase VP was 10 mm Hg, to which must be added a water column equal to 5 mm Hg. The VP level reached before the breath-hold, varied between 20 and 40 mm Hg, dependent upon the content of blood of the veins when the arterial occlusion was applied (A). Pneumotachogram (1’) and

temperatures are recorded as a control.

did not vary essentially with time (Fig. 28) , so input or output of blood through the vessels in the humerus or any other source is excluded. Only breath-holds ('TLC' 30) at high intrapulmonic pressure elicited a marked rise in venous pressure ( I I mm Hg) without essential change of the volume of the segment. Immediately after circulatory arrest the blood volume of the limb will be distributed to the different categories of vessels according to their elastic and active tension. After this equilibration the passive tension of the vein wall will probably be constant, and a marked increase in venous pressure presumably due to an increase in active tension of the venous wall, i.e., venoconstriction.

Although it was not proved with simultaneous volume an pressure measurements, previous experiments (Page et al. 19j j -and later other authors) on isolated venous segments in situ, made it likely that the Valsalva manaeuvre provoked venoconstriction in human limbs. The above increase in peripheral venous pressure of 10 mm Hg is difficult to explain on any other basis than venoconstriction.

Cause of the limb blood flow reduction From a theoretical point of view, the fall in the flow of blood through

the limbs during the Valsalva maneuvre must be due to : a fall in the arterio- venous pressure difference (i.e., perfusion pressure), a passive or active increase in the resistance of the small vessels of the limbs, or a combination of these.

Reduced perfusi0n pressure. According to the Poiseuille equation, the flow is identical with perfusion pressure divided by resistance. The mean pressure of the brachial artery drops from about 100 mm Hg in the control period to approximately 75 mm during the initial part of a Valsalva maneuvre (Fig. 29), while the peripheral venous pressure rises from 5 to 20 mm Wg. Thus the arteriovenous pressure difference has fallen less than 50 yo, while the simultaneous percentage drop observed in limb blood flow, was always greater and often approached 100. Therefore, the conclusion must be drawn that only part of the observed fall in blood flow can be accounted for by the reduction in perfusion pressure alone.

Passive increase in resistance. However, the fall in arterial pressure and rise in venous pressure leading to a reduction in perfusion pressure may have produced a change in the dimensions of the resistance vessels of the limb and thus reduced the flow of blood. In rigid straight tubes, as used by Poiseuille, the resistance is constant and-if the coefficient of viscosity and the length of the tube do not vary-inversely proportional to the fourth

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TLC 3 0 Pneumotachogram.

- 2 0 0 m m H g -

- 0 3 0 - ” -0 sec

Fig. 29. Simultaneous recording of blood pressure and blood flow. The breath-hold was performed after a deep inspiration and at an intrapulmonic pressure of 30 mm Hg. On the ordinate o to 10 mm Hg are venous pressures, while 50, 100, I ~ O , and 200 mm Hg are arterial pressures. The blood flow values are given along the plethysmogram. The temperatures are practically constant. Eighty similar experiments confirmed the

covariation between blood flow and arterial pressure shown above.

power of the radius. Since blood vessels are distensible, their radius depends upon their transmural pressure (i.e., the difference between the intravascular pressure and the tissue pressure). The resistance vessels are here defined as those producing the major part of the resistance; it is to be expected that the intravascular pressure of these vessels should, on an average, be close to the mean of the arterial and the venous pressures. It is also likely that the tissue pressure of relaxed limbs is about zero and remains so during a Valsalva manceuvre. Thus-in the control period as well as during the Valsalva maneuvre- the transmural pressure of the resistance vessels should, on an average, be close to half the sum of the arterial and venous pressures. In the control period the transmural pressure will be approximat- ely (IOO+ j)/z mm Hg, while during the Valsalva experiment, at the time where the flow approaches zero, it will be (7j +zo)/z mm Hg (Fig. 29).

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Thus, according to this reasoning, the Valsalva manaeuvre should only lead to a reduction in the transmural pressure of the resistance vessels of a few mm Hg. Even considering the fourth power relation between resist- ance and radius mentioned above, it can hardly be expected that such a moderate reduction in transmural pressure could lead to a decrease in the radius of the resistance vessels responsible for a major increase (toward infinity) in their resistance.

Active iticrease it1 resistance. Since even the combined effect of the 2 factors just mentioned, cannot alone explain the observed fall in flow to nearly zero, one seems compelled to assume that contraction of smooth muscle in the wall of the resistance vessels is also an important contribution to these falls in blood flow. Such a contraction, or vasoconstriction, is probably brought about reflexly, since vasoconstriction was not found during Valsalva maneuvres in limbs with sympathetic block (Fig. 27). Part of the limb blood flow reduction can therefore most likely be explained by a reflex vasoconstriction with a sympathetic efferent path to the resistance vessels.

The initial marked fall in the flow of blood during a Valsalva maneuvre was followed by a slow rise (Fig. 2 5 ) . This may in part be accounted for by a gradual increase in arterial blood pressure (Fig. Z I ) , but some relaxation o f the resistance vessels, possibly evolved by local hypoxia may also be involved, especially during some of the prolonged breath-holds. However, in most cases the blood flow was reduced also in the later part of the manmuvres (Fig. 26, 29), suggesting a marked vasoconstriction, especially since the arterial blood pressure had reached normal levels (Fig. 29).

The afferent path of the reflex arc An essential part of the limb blood flow reduction during a Valsalva

experiment, is due to a reflex vasoconstriction of the resistance vessels with a sympathetic efferent path, so it seems worth while to search for receptors related to the afferent side of the reflex arc.

Since the venoconstriction shown above is seen only during the same types of breath-holds as is the marked constriction of the resistance vessels, the trigger mechanism is assumed to be the same for both phenomena.

First the above results mill be compared with results obtained by others, in an attempt to solve the receptor problem. Sharpey-Schafer (196~) recorded limb blood flow immediately after short breath-holds with unknown lung volume and sometimes unknown intrapulmonic pressure (in certain cases the intra-esophageal pressure was approximately 40 mm

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Hg), and found a blood flow reduction, which-because of the short duration of the breath-hold-may have certain characteristics in common with that observed during a Valsalva experiment here. Sharpey-Schafer (1965) assumed the existence of “a reflex constriction of both arteries and veins” reducing the blood flow, and concentrated his interest upon the “paradoxical role of vasoconstriction” in the period after the Valsalva manaeuvre. The marked degree of peripheral vasoconstriction with a limb blood flow approaching zero during the initial phase of the Valsalva manceuvre, was not recognized. The important effect of a fall in perfusion pressure, and of a fall in transmural pressure of the resistance vessels, was not considered. However, Sharpey-Schafer’s ( I 96 5 ) interpretation is in accordance with the essential part of the explanation given above (i.e., active increase in resistance).

Recently Brick (1966) reported a mean decrease in the flow of blood through the human forearm of Z J yo on immersion of the face in water with the subject prone. The intrapulmonic pressure was not measured.- We repeated Brick’s experiment and found the forearm blood flow to decrease by 68 % (mean) during 60 sec breath-holding with the subject prone (‘TLC’ and an intrapulmonic pressure of 20 mm Hg). The same blood flow decrease was found during similar breath-holds with the face immersed in water at 1 5 and 28°C. Thus, the blood flow reduction, which Brick attributed to face immersion, seems to be elicited simply by breath-holding at increased intrapulmonic pressure, and his suggestion is not a fair solution to the receptor problem.

Other possibilities must be considered.- Receptors might be situated in the thoracic wall, pulmonary tissue (stretch receptors), or in other compart- ments of the respiratory system. Assuming an adequate stimulus of such receptors to be an increase in receptor length per unit of time, it is clear that pulmonary stretch receptors can be excluded as a possible explanation, since the marked blood flow reduction is also seen during breath-holds at FRC (Fig. 3 I), namely where both pulse pressure and the arterial systolic pressure fell. The basis for these exceptions may be a relatively insufficient blood volume (Nordenfelt 1964). The limb blood flow drops with the arterial blood pressure, regardless of the lung volume during the Valsalva experiment (Fig. 30, 3 r).-Receptors might be located in the venous system, in the right half of the heart, or in the pulmonary circulation. Especially the “low pressure baroreceptor reflex to skeletal muscle vessels of man” (Barcroft 1963), is a highly attractive explanation of the vasoconstriction. The evidence for the low pressure receptors is the increase in forearm blood flow by raising the legs of recumbent subjects or by negative pressure

95

01 sec

Fig. 30. Record of a 35 sec breath-hold after a normal expiration (FRC) and at an intra- pulmonic pressure of 3 0 mm Hg. Variables as in Fig. 29. Twelve similar records (FRC, pressure of 20 to 30 mm Hg) and eight control experiments (FRC, pressure of zero to

10 mm Hg), are available.

35

breathing. The effect of these experiments on the arterial blood pressure is not considered of importance in such argumentation. Other reflexes are considered in the monograph by Heymans & Neil (195 8). Unfortunately, most of the evidence of their existence and function is indirect, as shown in the above example.-Receptors might also be located in the left half of the heart and the systemic arteries. Receptors other than sino-aortic have been postulated (Heymans & Neil 1958), but most of the evidence

96

put forward is indirect and the functional role of such receptors is neither clear nor proved. The baroreceptors believed to exist in the myocardium of the left ventricle may be an exception. In contrast, the receptors found in the carotid sinuses and in the wall of the aortic arch have a proved func- tional importance (Heymans & Neil 1 9 1 8 ) ~ but even their function is not yet properly understood. Adequate stimulus of these receptors is not only the mean transmural pressure of the vessels, but also the size of the pulse pressure, and the rate at which it changes. The dramatic fall in arterial blood pressure due to a Valsalva manaeuvre, is a strong “negative” stimulus to the sino-aortic baroreceptors (possibly the receptors cease to fire). The reaction to such a condition is in healthy persons-after a certain delay-a vasoconstriction of both arterioles and veins. The profound vasoconstriction was demonstrated above and the problem seems explained by well-known physiological concepts, without assuming the “existence” of “receptors of unknown origin” or other hardly informative hypotheses as is commonly done in the literature.

In some cases breath-holding at FRC and high intrapulmonic pressure did not lead to a marked fall in limb blood flow. These cases differred from those represented in Fig. 3 I , because the systolic and the mean arterial pressure rises although the pulse pressure fell (Fig. 30), in contrast to the cases represented in Fig. 3 I . In these cases the “negative” stimulus to the baroreceptors may not have been present, indicating sufficient venous return, probably mainly from the abdominal blood deposits (Nordenfelt 1964). Moreover, the high arterial mean pressure and relatively low peripheral venous pressure counteracts the fall in perfusion pressure and in transmural pressure of the resistance vessels.

Although a major part of the reduction of the flow of blood through the limbs seems explainable by the classical high pressure baroreceptor reflex, this does not necessarily exclude influence of other parts of the nervous system. Afferent impulses from many sources, including the central nervous system itself, may influence the activity of the circulatory control centers. Besides the output from these centers to the heart, the arterial, and the venous systems may not always follow the same relative pattern from one situation to the next.

Since the flow of blood through the limbs approaches zero during breath-holding at high intrapulmonic pressure, a greater blood flow reduction could not be expected during the same maneuvre performed in water (regardless of its temperature). During breath-hold dives to depths of some meters, the pressure difference across the lungs and chest wall will most likely be small and not lead to marked changes in limb blood

97 7

10-mm HQ - - - Peripher_al v e m ' .

flow, except where the subject performs a Valsalva manceuvre. During deep dives toward the limit of human performance, the intrapulmonic pressure will probably be negative, and the blood content of the thoracic vessels will increase. This rather special condition is possibly accompanied by a small relative increase in limb blood flow. In general the cardiovascular changes are not likely to potentiate essentially the serious consequences o f gas accumulation during breath-hold diving in otherwise healthy subjects. --The Valsalva experiment and unconsciousness have been analyzed by

Kordenfelt (1964).

1 - I

0 70 see

3SLj 0- - -

In the above argumentation it was mentioned that the function of the high pressure baroreceptors is not completely understood at present and that even more information is needed as far as other receptors are concerned. For persons interested in circulation and neurophysiology these problems should be highly attractive, especially when sufficient knowledge from animal experiments is provided to justify a study in man.

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