reproducible over time. strikingly similar to those observed earlier in experimental animals.

Transcription

1 J. Physiol. (1977), 269, pp With 8 text-jigure8 Printed in Great Britain BAROREFLEX INHIBITION OF THE HUMAN SINUS NODE: IMPORTANCE OF STIMULUS INTENSITY, DURATION, AND RATE OF PRESSURE CHANGE By DWAIN L. ECKBERG* From the Cardiovascular Center, the Cardiovascular Division, Department of Internal Medicine, and the Veterans Administration and University Hospitals, University of Iowa College of Medicine, Iowa City, Iowa 52242, U.S.A. (Received 13 September 1976) SUMMARY 1. Carotid baroreceptors were stimulated with electronically controlled neck suction in five healthy young men and pulse interval prolongation was measured. Timing of the onset of stimuli in relation to cardiac activity was held constant, and stimulus intensity, duration, and dp/dt were varied independently. 2. In the subjects studied, sinus node responses to neck suction were proportional to dp/dt. However, variations of stimulus dp/dt within or above the normal range for arterial dp/dt did not influence the magnitude of integrated baroreflex responses, or the earliest portion of baroreflex sinus node inhibition. 3. Carotid baroreflex responses were linear over a wide range which extended beyond the normal range for human systolic arterial pressures. 4. Saturation of the carotid baroreceptor-cardiac reflex occurred at distending pressures of about 160 mmhg. 5. The average baroreflex responses of the group studied were highly reproducible over time. 6. Baroreflex gain correlated very strongly with base line pulse interval. 7. The magnitude of baroreflex responses increased linearly with the duration of carotid sinus distension and reached a maximum level with stimuli lasting 0*5 sec or more. 8. The results demonstrate that carotid sinus transfer characteristics can be measured in normal man, and that human response patterns are strikingly similar to those observed earlier in experimental animals. * Clinical Investigator, Veterans Administration. Current address: Department of Medicine, Medical College of Virginia, Richmond, Virginia 23298, U.S.A.

2 562 DWAIN L. ECKBERG INTRODUCTION The quantitative relation between changes of carotid arterial pressure and carotid sinus nerve or effector organ activity (transfer characteristics) has been studied extensively in experimental animals, but very little in man. In anaesthetized animal preparations, experimental conditions can be controlled rigorously: carotid baroreceptor areas can be isolated surgically and stimulated selectively; stimulus parameters can be varied independently and can be controlled precisely; and other reflexogenic areas can be deafferented to permit analysis of the open-loop responses to carotid baroreceptor stimulation. I conducted the present series of experiments to characterize carotid sinus transfer characteristics in normal man. Carotid baroreceptors were stimulated selectively by suction applied to a neck chamber (Eckberg, Cavanaugh, Mark & Abboud, 1975). Individual stimulus parameters were controlled electronically with a pneumatic valve. Measurements were made within one beat after the onset of the stimulus, before closed-loop baroreflex adjustments would have been expected to modify sinus node responses (Eckberg, Abboud & Mark, 1976). A previous report (Eckberg, 1976) stressed the importance of the timing of baroreceptor stimulation within the cardiac cycle as a determinant of the magnitude of sinus node responses. The present study assesses the importance of other baroreflex stimulus parameters, including intensity, duration, and rate of pressure change. The results suggest that carotid sinus transfer characteristics can be measured in man, and that human responses are qualitatively similar to those measured previously (Koch, 1931; Landgren, 1952) in experimental animal preparations. METHODS Carotid baroreceptors of normal young men were stimulated with brief bursts of suction applied to a neck chamber. Rotation of a pneumatic valve controlled stimulus parameters. Stimulus intensity, duration, and rate of pressure reduction comprised input variables. A digital computer measured the output variable, pulse interval, on-line in real time. Subjects. Volunteers comprised five healthy men whose average age was (s.e. of mean) years. All subjects had normal blood pressures and none had a family history of hypertension. Two volunteers were endurance athletes; the others did not engage regularly in strenuous physical exercise. Each subject was studied at least eight times, over a 4-16 (average 11) month period, and all experimental protocols were used with each subject. Prior written, informed consent was obtained from all subjects. Experimental setting. An effort was made to maintain experimental conditions constant for all studies. All experiments were conducted in the morning. Subjects were studied supine, in a quiet, warm (23-24o C), darkened room, following break-

3 SINUS NODE RESPONSES TO CAROTID STRETCH 563 fast. Before each study, volunteers abstained from stimulants, including tobacco and beverages containing caffeine, and did not engage in strenuous physical exercise. Equipment. The neck chamber, pneumatic valve, and digital stimulator were designed and constructed in the laboratory. The neck chamber (Eckberg et al. 1975) comprised an elliptical piece of sheet lead rimmed with sponge rubber. A pressure transducer, and one or two pneumatic valves were mounted directly upon the chamber. A commercial vacuum cleaner was used to provide a continuous vacuum source whose intensity was regulated with a rheostat. Neck suction began when rotation of a pneumatic valve established continuity between the neck chamber and the vacuum source. A rotary solenoid or a stepping motor rotated the pneumatic valve. A digital stimulator controlled the timing of the onset of neck suction and its duration. Neck suction was timed to begin at a preselected interval before the onset of the next anticipated P wave, as follows: Successive pulse intervals were stored in a memory circuit. When the stimulator was activated, pulse intervals for the two previous cardiac cycles were averaged, the stimulator setting was subtracted from the average control interval, and after a delay equal to this result, the pneumatic valve was opened. For example, if the control pulse interval averaged 1 0 sec, and if neck suction were to be initiated 0*8 see before the next anticipated P wave, the valve would be activated 0-2 sec after the P wave. If, on the other hand, the control pulse interval averaged 1.1 sec, the valve would be activated 0 3 sec after the P wave. Experimental results were transcribed by an ink writing recorder at a paper speed of 10 or 50 mm/sec. Six channels were used to record the electrocardiogram, heart rate (measured with a cardiotachometer), respirations (measured with a pneumograph), 1/sec time lines, neck chamber pressure, and its first derivative. Blood pressure was measured with an ultrasonic sensor (Arteriosonde, Cranbury, New Jersey, U.S.A.) and a manually inflated arm cuff. Data reduction. All data were analysed on-line, in real time by a digital computer. The following measurements and calculations were printed after each application of neck suction: control pulse interval; prolongation of the first pulse interval concluded after the onset of neck suction, from control; and the interval between the stimulus and the anticipated appearance of the next P wave. The last calculation is based upon the assumption (validated earlier (Eckberg, 1976)) that the control pulse interval would have recurred (during held expiration) if neck suction had not been applied. There were two inputs to the computer: electronic pulses from threshold detectors triggered by neck chamber pressure, and the electrocardiogram. Neck pressure threshold was set about 4 mmhg below base line pressure. The electrocardiogram threshold was set either on the descending portion of the P wave or the ascending portion of the R wave. The time from the onset of the P wave until the threshold crossing was subtracted from the measured stimulus to P wave interval. In the subjects studied, prolongation of the P-R interval was negligible compared with the prolongation of the P-P interval caused by neck suction. Thus, the measured pulse interval (referred to as the 'P-P interval') accurately reflected the actual P-P interval, although in some experiments it was measured from R wave threshold crossings. Computer accuracy was tested periodically against recordings obtained at fast (250 mm/sec) paper speed. Measurement error was less than 0-5 %. Specific protocols. Neck suction was applied during held expiration in all experiments to minimize spontaneous fluctuations of pulse interval. P-P interval prolongation was calculated by subtracting the control P-P interval from the first P-P interval concluded after the onset of neck suction. Several permutations upon the

4 564 DWAIN L. ECKBERG technique of neck suction were used to study specific transfer functions and these are described separately. Rate of preeeure change (dp/dt). The pneumatic valve was rotated by a stepping motor whose speed of rotation was a function of the frequency of electronic input pulses. Average P-P interval prolongation was plotted as a function of the intensity of neck suction for each dp/dt. It was assumed on the basis of previous studies (Eckberg, 1976) that the dp/dt measured in the neck chamber slightly exceeded that actually delivered to the carotid sheath. Therefore, two rates of pressure change, and mmhg/sec were chosen to bracket the normal range of human arterial dp/dt (found by Mason, Braunwald, Ross & Morrow (1964) to be about mmhg/sec in the brachial artery), and one level, mmhg/sec, was chosen to fall below the normal range. Intensities of -20 to -60 mmhg were applied for 0-6 sec, sec before the anticipated appearance of the next P wave (see below). In another series of experiments, neck suction of -30 mmhg was applied for 1-0 sec, beginning sec before the anticipated appearance of the P wave. Neck chamber dp/dt was varied between and about mmhg/sec with the stepping motor or solenoid. Prolongation of the cardiac cycle in which neck suction was begun was plotted as a function of stimulus dp/dt. To detect a possible influence of stimulus dp/dt upon the earliest detectable portion of sinus node inhibition, multiple stimuli of -60 mmhg for 0-6 sec, with maximum rates of pressure change of , or mmhg/sec, were timed to begin between 0-2 and 0-6 sec before the anticipated appearance of the next P wave. P-P interval prolongation was averaged at 0 1 see intervals and was plotted as a function of the interval between the onset of neck suction and the anticipated appearance of the next P wave, as described previously (Eckberg, 1976). In all other protocols, the pneumatic valve was rotated by a solenoid. When the solenoid was used, the rate of pressure reduction was proportional to the intensity of neck suction: dp/dt averaged -800 mmhg/sec at -10 mmhg, and fell linearly as neck chamber pressure was reduced, at a rate of -42 mmhg/sec/mmhg neck suction. When the solenoid was used, neck chamber dp/dt ranged between about -800 and mmhg/sec and thus fell within, or above the normal range for human arterial dp/dt. Intenaity-rewponee relation. An earlier study (Eckberg, 1976) showed that although sinus node responses to neck suction vary strikingly as a function of the timing of stimuli in relation to cardiac events, stimuli delivered at the same time within the cardiac cycle evoke comparable P-P interval prolongations. Accordingly, to determine the influence of stimulus intensity, stimulus timing was held constant ( see before the anticipated appearance of the next P wave). Neck chamber pressure was reduced from the ambient level to between -10 and -70 mmhg, for 0-6 sec. Each intensity of neck suction was applied between five and ten times in random sequence, and average P-P interval prolongations were plotted as a function of stimulus intensity. To evaluate further the linearity of carotid baroreceptor responses, the same stimulus intensity, -20 mmhg, for 0-6 sec, was superimposed upon base line levels of continuous neck suction ranging between 10 and 50 mmhg. Two pneumatic valves, and two vacuum sources were used. Base line suction was begun at random times in the cardiac cycle; incremental suction was initiated 4.7 ± 0-4 see later, and was timed to occur between 0 75 and 0-85 sec before the anticipated appearance of the next P wave. Duration-response relation. Neck chamber pressure was reduced from the ambient level to -30 mmhg, see before the anticipated appearance of the next P

5 SINUS NODE RESPONSES TO CAROTID STRETCH 565 wave, and stimulus duration was varied between about 0.1 and 0*7 sec. Neck suction was applied five to ten times at each duration, and average P-P interval prolongation was plotted as a function of stimulus duration. Statit cal analyses. Statistical comparisons were made using analysis of variance with orthogonal contrasts, least squares linear regression, paired t test, and Tukey's test (Huntsberger & Leaverton, 1970). Differences were considered significant when P was less than RESULTS Rate of pressure change (dp/dt). Fig. 1 shows intensity-response relations at three levels of dp/dt. Sinus node responses to and mmhg/sec were comparable (Fig. 1 A), but responses to were significantly (P < 0 05) less than responses to mmhg/sec (Fig. IB). These data support the established view that rate of pressure change is a determinant of integrated baroreceptor reflex responses, but suggest also that this factor is of negligible importance within the normal range of human arterial dp/dt. A 0*5 04, C B 04v1 <1 0*1 _ P <0H05 0 s ' if ' I Neck chamber pressure (mmhg) Fig. 1. Average intensity-response relations of all five subjects at three levels of maximum dp/dt. Vertical bars encompass 1 s.e. of mean. These and subsequent curves were drawn according to the best visual fit. Responses to and mmhglsec stimuli were comparable, but responses to -300 were significantly less than responses to mmhg/sec. Max. dp/dt: 0, -1200; A, -550; *, -300 mmhg/sec. To obtain the data depicted in Fig. 2, the intensity of baroreceptor stimulation was held constant (-30 mmhg), the duration of the stimulus, 1P0 sec, was longer than that used in Fig. 1 (to allow time for very slow onset of neck suction), and dp/dt was varied between and mmhg/sec. With these stimulus parameters, responses began to decline when the dp/dt fell below mmhg/sec. Comparable response patterns were observed in three subjects in whom the onset of neck suction

6 566 DWAIN L. ECKBERG coincided with the P wave (when arterial dp/dt would have been nearly zero (Mason et al. 1964)), and in two subjects in whom neck suction began 0 40 and 0 45 sec after the onset of the P wave. This suggests that the reponse pattern observed was not due merely to superimposition of the experimental stimulus upon the rapid rise of the natural arterial pressure. 0*5 0.4 CL * -30 mmhg 0.1 I 0 sec Neck chamber dp/dt (mmhg/sec) Fig. 2. Average responses of all subjects to neck suction at eight different maximum rates of pressure change (dp/dt). Responses were comparable when maximum dp/dt was mmhg/sec or greater, but were significantly (*P < 0 05) lower when dp/dt was below mmhg/sec. Data depicted in Figs. 1 and 2 suggest that dp/dt may be more important as a determinant of sinus node responses when stimuli are short (0.6 see) than when they are long (.0 sec). It was not possible to apply neck suction for very short durations at very low rates of pressure change, and therefore, the earliest detectable sinus node inhibition (which presumably results from the earliest afferent baroreceptor activity) was examined. Fig. 3 shows that the baroreceptor-cardiac reflex arc duration (the intercept on the x-axis) and the earliest detectable sinus node inhibition were comparable when peak neck chamber dp/dt was or mmhg/sec. This adds further support for the view that variations of dp/dt within the normal range for arterial pressure do not exert a significant influence upon integrated baroreceptor-cardiac reflex responses. Stimulus intensity-sinus node response relation. Fig. 4 shows average responses of all five subjects to neck suction applied on two separate occasions, (range ) days apart. Pulse interval prolongation increased linearly (r = 0-92 and 0.83) with stimulus intensities between -10 and -40 mmhg, and reached an asymptotic plateau after intensities exceeded -50 mmhg. The average intensity-response relation was

7 SINUS NODE RESPONSES TO CAROTID STRETCH 567 highly reproducible over time. Intensity-response relations obtained with the solenoid and with the stepping motor ( mmhg/sec) were comparable. There was a strong correlation between the P-P interval prolongation 0o5 0*4 P n.s. 0.3~ 01 M s <~~~~~~~~ I I I I I I ' * Stimulus to anticipated P wave duration (sec) Fig. 3. Average responsesto multiple applications of neck suction (60mmHg, 0.6 sec) at (@-*) or -550 (A ---A) mmhg/sec, begun between 02 and 0*6 sec before the anticipated appearance of the next P wave (see text). These data show that the earliest detectable sinus node inhibition following baroreceptor stimulation was comparable at both levels of dp/dt. 0*6_ 0*5 U 0.4 /t V 0*3 - V _ Neck chamber pressure (mmhg) Fig. 4. Average intensity-response relations measured on two separate occasions. *, first study; A, second study 60±33 days later. Average responses during the two studies were comparable. Average B.P. 112/70.

8 568 DWAIN L. ECKBERG provoked by 30 mmhg neck suction (in the first study) and the slope of the intensity-response relation between - 10 and -40 mmhg, and the average pulse interval prolongation provoked by -50, - 60, and -70 mmhg. Regression equations were, slope (between -10 and -40 mmhg), in msec/mmhg = -2' (LP-P at -30mmHg), r= 0-96; and average AP-P (at -50, -60 and -70 mmhg), in msec= (AP-P at -30 mmhg), r = Neck chamber Oto -10 to -20 to -30 to -40 to -50 to pressure (mmhg) ~1-4, 1-3 C Fig. 5. Average pulse interval responses to 20 mmhg decrements of neck chamber pressure superimposed upon neck chamber pressures of 0, - 10, - 20, - 30, -40 and -50 mmhg. The average control pulse interval is shown on the left of each upper panel, and the average pulse interval occurring after neck suction is shown on the right. Pulse interval prolongation was comparable when 20 mmhg neck suction was superimposed upon ambient neck chamber pressures of 0, - 10, - 20, - 30, and -40 mmhg, but was significantly (*P < 0-001) less when -20 mmhg was superimposed upon -50 mmhg. Fig. 5 shows average responses of all subjects to stimuli of -20 mmhg superimposed upon neck chamber pressures between 0 and -50 mmhg. Reduction of neck chamber pressure from 0 to -20 mmhg provoked average pulse interval prolongation of sec. Responses to the same decrement of neck chamber pressure, superimposed upon pressures of - 10, - 20, - 30, and -40 mmhg were comparable. However, the average response to reduction of neck chamber pressure from -50 to -70 mmhg. was significantly (P < 0-001) lower than the other responses. The average base line pulse interval increased in proportion to the level

9 SINUS NODE RESPONSES TO CAROTID STRETCH 569 of base line neck suction, from without neck suction, to see during -50 mmhg neck suction, P < The average maximum P-P interval following neck suction was significantly greater when neck chamber pressure was lowered from 0 to -70 than when it was lowered from -50 to -70 mmhg ( vs sec, P < 0.01). This suggests that the P-P interval prolongation which occurred when pressure was reduced from -50 to -70 mmhg was not unusually small because of limitations imposed by the sinus node. 0.6 P < 0* = 0.34 < _ Neck chamber pressure (mmhg) Fig. 6. Average intensity-response relations shown in Fig. 4, replotted according to each subject's average base line pulse interval on the day of each study. Average control P-P interval: *-*, 1P24 ± 0 05; A---A, sec (P n.s.).average responses were significantly greater on the day when the base line pulse interval was longer than when it was shorter. Average systolic blood pressure was insignificantly lower during the study in which base line pulse interval was longer (115±2 vs. 108±2, 0-05 < P < 0-10). The asymptotic pressure level at which saturation of responses occurred (about -50 mmhg) appeared to be comparable, despite differences of reflex gain. The control P-P interval averaged see (range: see) during the first and second intensity-response studies. If peak arterial pressure develops 0*5 sec after the onset of the P wave (D. L. Eckberg, unpublished), neck suction woul d have coincided with peak arterial pressure in eight, and developed slightly (0.08 and 0 05 see) after peak systolic pressure in two studies. Thus, in the majority of these studies, the maximum possible carotid transmural pressure gradient equalled the absolute sum of systolic arterial and neck chamber pressures. Association between base line pulse interval and baroreflex responsiveness. The control P-P interval averaged *05 during the first intensity-

10 570 DWAIN L. ECKBERG response study and *03 see during the second (P n.s.). In three subjects, average control P-P intervals were significantly longer during the first than during the second study; in one subject, the control P-P inter- 18r I E E V_ W #. E0 16 k J.A. 14 0a. la WC _ I I a Average base line P-P interval (msec) Fig. 7. Slopes of the pulse interval prolongation between -10 and -40 mmhg are plotted as a function of the base line pulse interval for each subject, on each of three or four studies on separate days. Minor prolongations of base line pulse interval were associated with striking increases of baroreflex gain. 0*4 r -30 mmhg U-1 V 416 L.1 V 4.1 C 0* a I I I I I I Neck suction duration (sec) Fig. 8. Relation between pulse interval response and the duration of neck suction. Responses increased linearly (r = 0 78) as stimuli were prolonged from 0.1 to 0 5 sec, but did not increase significantly as stimuli were prolonged beyond 0 5 sec.

11 SINUS NODE RESPONSES TO CAROTID STRETCH 571 val was significantly longer during the second study; and in one subject, control P-P intervals during the two studies were comparable. Data from these two studies depicted in Fig. 4 were replotted in Fig. 6 according to the two average base line P-P intervals of each subject. Baroreflex responsiveness was significantly greater during that study during which the average base line pulse interval was longer. The neck chamber pressure at which baroreflex responses reached a plateau did not appear to be influenced by variations of baroreflex responsiveness. Systolic blood pressure was slightly lower during the study with the longer base line pulse interval ( vs mmhg, 0.05 < P < 0.10). Intensity-response relations were measured in each subject on at least three occasions. In Fig. 7, the slope (between -10 and -40 mmhg) of each intensity-response relation was plotted as a function of the average base line pulse interval for each study. The results demonstrate a striking proportionality between base line pulse interval and baroreflex responsiveness, in all subjects. Duration-response relation. Fig. 8 shows that pulse interval prolongation increased linearly (r = 0.78) as stimulus duration was increased from 0*1 to 0*5 sec, but did not increase further as stimulus duration was increased from 0 5 to 0*7 sec. DISCUSSION The most important conclusions from these experiments are that carotid sinus transfer functions can be measured in normal unsedated man and that human response patterns are strikingly similar to those observed earlier in studies of arterial baroreceptors in experimental animals. Rate of pressure change (dp/dt). Bronk & Stella (1935) first suggested that rate of pressure change may be an important determinant of the frequency of afferent carotid sinus nerve activity. Landgren (1952) systematically explored the relation between dp/dt and carotid sinus nerve activity in cats. He found that latency between the onset of pressure rise in the isolated carotid sinus and the first afferent baroreceptor nerve activity, and the threshold for nerve firing were inversely related to dp/dt, and that the initial firing frequency was proportional to rate of pressure change. The present study shows that the integrated baroreceptor-cardiac reflex response to carotid sinus distension in man is proportional to rate of pressure change: pulse interval prolongation was significantly diminished when the dp/dt of neck suction was reduced below about mmhg/ sec. However, when the rate of pressure change was varied within or above (- 550 to about mmhg/sec) the normal range of human arterial dp/dt, there were no differences in the magnitude of the integrated reflex

12 572 DWAIN L. ECKBERG response, or in the earliest detectable portion of sinus node inhibition. Thus, although these data affirm the existence of this important property of mechanoreceptors in man, they suggest that under physiological conditions, the rate of change of carotid pressure is of negligible importance in determining the magnitude of sinus node responses. These results do not exclude the possibility that rate of pressure change may be an important determinant of sinus node responses at lower arterial pressures (Gero & Gerova, 1967). Non-linear intensity-response relation. Koch (1931) first demonstrated that blood pressure and pulse interval responses to static pressure changes in the isolated canine carotid sinus are non-linear. He showed that there is a threshold for baroreceptor activation, about mmhg, below which variations of intrasinus pressure do not alter pulse interval; a linear range in which changes of intrasinus pressure provoke proportional changes of pulse interval; and a saturation pressure, about mmhg, beyond which further elevations of carotid sinus pressure do not evoke additional pulse interval prolongation. Koch's findings suggested that baroreceptors were maximally responsive to variations of pressure within the normal arterial pressure range. Threshold was not defined in the present subjects, but may be assumed to be about 112 mmhg (their average systolic pressure), or less: proportional pulse interval prolongation always occurred when incremental carotid stretch was produced with very low intensities of neck suction (10 mmhg). In the following study (Eckberg, 1977), continuous neck suction of 10 mmhg provoked sinus node inhibition which persisted for the duration of stimulation (5 sec). This suggests that carotid distending pressure is always above threshold with this intensity of stimulation. The extent of the linear portion of the intensity-response relation also cannot be determined from these data because the threshold is not known. However, the present study suggests that in the five healthy young men studied, the linear range is wide (at least 50 mmhg), and extends beyond the accepted normal range for human systolic arterial pressures. If carotid transmural pressure equals the absolute sum of systolic arterial and neck chamber pressures, the linear range extends from at least mmhg. The extent of the linear range may be overestimated ifthere is significant attentuation of negative pressure in transmission through neck tissues. It is more likely, however, that the extent of the linear range is underestimated by these data because any portion which lies between threshold and 112 mmhg is not included. The threshold for carotid baroreceptor activation is considerably lower than 112 mmhg in experimental animals (Bronk & Stella, 1935; Kalkoff, 1957; Koch, 1931; Landgren, 1952). Since variations of dp/dt within the normal range do not alter the

13 SINUS NODE RESPONSES TO CAROTID STRETCH 573 magnitude of sinus node responses, the gain (K) of the integrated reflex response for stimuli within the linear range can be described by a simple equation: APP interval = K (A pressure). In the first study, K = 11*6 msec/mmhg (Fig. 4). This constant defines the minimum gain in the subjects studied; a steeper slope would have resulted if there were any attenuation of negative neck pressure in transmission through neck tissues. This expression of reflex gain should not be compared with slopes derived from pulse interval responses to intravenous bolus injections of pressor drugs (found to be 12x8 by Bristow, Honour, Pickering, Sleight & Smyth (1969), and 16-0 msec/mmhg by Eckberg, Drabinsky & Braunwald (1971)). With the pressor injection technique, the rise of systemic arterial pressure probably stimulates aortic as well as carotid arterial baroreceptors. Moreover, the timing of the systolic pulse in relation to sinus node depolarization changes constantly as pulse interval prolongation occurs in response to rising arterial pressure. Variations of baroreceptor stimulus timing would be expected to alter reflex gain independent of stimulus intensity (Eckberg, 1976). Finally, with the pressor injection technique, responses are measured over a long period, from 20 to 30 sec, and may be modified by closed loop baroreflex adjustments. In the present study, average pulse interval prolongations were comparable when stimuli of -20 mmhg were superimposed upon neck chamber pressures of 0, - 10, - 20, - 30, or -40 mmhg (Fig. 5). This provides additional evidence that the gain of carotid baroreceptors is linear over this range of neck chamber pressures, and suggests also that within this range, carotid stretch is proportional to the decrement of neck chamber pressure. This conclusion is supported by the study of Kober & Arndt (1970) who used a neck chamber with human volunteers and found that the diameter of the common carotid artery (measured with a noninvasive echo-ranging device) is a nearly linear function of carotid transmural pressure, between about 50 and 150 mmhg. Baroreftex sensitivity and base line pulse interval. These data also may clarify the relation between base line pulse interval and baroreflex responsiveness. Pickering, Gribbin, Petersen, Cunningham & Sleight (1972) and Eckberg (1976) found that subjects who have the longest average base line pulse intervals also have the highest baroreflex gain. The present study (Figs. 6 and 7) shows that when data obtained on separate occasions from the same subjects are analysed, minor prolongations of average base line pulse intervals are associated with striking augmentations of baroreflex gain. However, when the base line pulse interval was prolonged by continuous neck suction, pulse interval responses to increments of neck suction (Fig.

14 574 DWAIN L. ECKBERG 5) were not proportional to base line pulse interval. I speculate that the prolongations of base line pulse interval measured during different studies (associated with increased baroreflex responsiveness) were due to spontaneous increases of central nervous system baroreflex gain, whereas prolongations of base line pulse interval by continuous neck suction were due to increased baroreceptor afferent nervous activity. Thus, it seems more likely that baroreflex gain determines base line pulse interval, than that base line pulse interval determines sinus node responsiveness to baroreflex inhibition. Pulse interval prolongation increases asymptotically with increasing intensities of neck suction (Fig. 4). The departure from linearity begins at about -50 mmhg. It is assumed that the greatest intensities of neck suction provoke less deformation of stretch receptors per unit pressure change than smaller intensities. If carotid transmural pressure is the absolute sum of systolic arterial and neck chamber pressures, saturation of the human carotid baroreceptor-cardiac reflex occurs at about 160 mmhg. This must be considered a maximum limit, however, because if negative pressure is attenuated in transmission through neck tissues, saturation transmural pressure would be less. It is likely that the pressure level at which saturation occurs reflects intrinsic properties of carotid baroreceptors rather than limits imposed by other elements within the reflex arc. The average pulse interval was significantly longer after neck suction of 0 to - 70, than after pressure changes of -50 to -70 mmhg; this suggests that the plateau observed when neck suction reached -50 mmhg was not due to limits imposed by the sinus node. When sinus node inhibition is caused by electrical stimulation of the carotid sinus nerves (thus bypassing baroreceptors) a ceiling is not found: with intense stimulation, sinus arrest occurs with emergence of a subsidiary pacemaker (Carlsten, Folkow, Grimby, Hamberger & Thulesius, 1958). Finally, although longer base line pulse intervals were associated with greater reflex gain in the present study, the transmural pressure at which saturation occurred appeared to be uninfluenced by base line pulse interval (Fig. 6). Stimulus duration-response relation. An earlier study (Eckberg, 1976) showed that neck suction lasting 0-6 sec provoked greater pulse interval prolongation than suction lasting 0-3 sec. This relation was explored systematically in the present study; the results show that pulse interval prolongation is a linear function of baroreceptor stimulus duration, to about 0-5 sec. It is likely that this relation is complex and may be determined by a multiplicity of factors, including the following: First, the duration of stretch required to activate individual baroreceptor units may be variable. Landgren (1952) and Irisawa & Ninomiya (1967) showed that

15 SINUS NODE RESPONSES TO CAROTID STRETCH 575 baroreceptor fibres yielding large afferent spikes may be activated earlier during an abrupt rise of carotid sinus pressure than those yielding spikes of medium or small amplitude. Thus, longer baroreceptor stimuli would be expected to stimulate a larger population of baroreceptor fibres. Secondly, during more prolonged stretch, the aggregate number of baroreceptor spikes is greater (Arndt, Dorrenhaus & Wiecken, 1975). Increased afferent activity would be expected to provoke more pronounced effector organ responses (Kalkoff, 1957). Thirdly, adaptation of baroreceptors (Bronk & Stella, 1935; Eckberg, 1977) may reduce afferent nerve firing frequency during the late phases of prolonged baroreceptor stimuli and introduce non-linearity into the duration-response relation. Finally, the influence of baroreflex stimuli upon the human sinus node is not uniform over time (Eckberg, 1976): very prolonged (more than 05 sec) baroreceptor stimuli may not provoke proportional pulse interval prolongation because some quanta of acetylcholine may be released during a period ofrelative sinus node refractoriness to cholinergic inhibition. Criticism of techniques Experimental approaches in these studies are new and were developed primarily so that human carotid baroreflex responses could be studied quantitatively. There are at least two major shortcomings of these techniques: the actual baroreceptor stimulus, increased carotid transmural pressure gradient, was not measured, and the role of the central nervous system in modulating efferent vagal responses to baroreceptor stimuli is unknown. It may not be of critical importance to quantitate exactly the actual stimulus in these studies, however, because the conclusions derive from qualitative response patterns or quantitative response limits. Limited data published earlier (Eckberg, 1976) suggest that neck suction is transmitted nearly quantitatively andwith negligible delay, to the carotid sheath. The question of central modulation of synaptic transmission is more difficult. Several pieces of inferential evidence regarding central nervous system modulation of baroreflex responses may be gleaned from the present studies. First, although central gain may vary widely from day to day, central modulation does not appear to affect the carotid transmural pressure at which saturation occurs (Fig. 6). Secondly, the patterns of sinus node inhibition observed in the present studies are qualitatively similar to the patterns of afferent carotid sinus nerve activity measured by others in experimental animals. I speculate that when extrinsic factors responsible for baroreceptor-cardiac reflex non-linearities (for example, timing of stimuli in relation to respiratory or cardiac events) are controlled during an experiment, pulse interval prolongation is a linear function of afferent carotid sinus nerve activity.

16 576 DWAIN L. ECKBERG There were several factors that led to selection of the methods used in the present studies. First, the techniques permit precise control of input variables (stimulus intensity, duration, timing, and dp/dt) and extremely accurate measurement of the output variable (P-P interval). Secondly, bradycardia caused by baroreceptor stimulation occurs abruptly. Therefore this effector response can be measured before it is modified by other reflex mechanisms and the technique may be regarded as an open-loop baroreflex forcing. Thirdly, extrinsic factors responsible for non-linearities of sinus node responses, such as the timing of stimulation in relation to respiration and cardiac activity can be controlled completely. An earlier study (Eckberg et al. 1975) suggested that bradycardia occurring following neck suction is due primarily to arterial baroreceptor stimulation rather than to chemoreceptor or tracheal receptor stimulation: The magnitude of responses was not altered by inhalation of 100 % oxygen, and respiratory rate was not altered. Moreover, neck suction does not elicit coughing or swallowing, symptoms which would be expected to accompany activation of tracheal reflexes (Widdicombe, 1964). In conclusion, results from the present study define qualitative sinus node response patterns to a variety of carotid baroreceptor forcings, and establish certain quantitative limits to these responses. The results show a striking similarity between human responses, measured non-invasively, and experimental animal responses, measured by others, invasively. The results pertain only to sinus node responses to carotid baroreceptor stimulation. Directions for future research include measurement of responses of other effector organs to carotid sinus stretch, and measurements of effector organ responses to transient reductions of carotid sinus dimensions. The author acknowledges his appreciation to Professor Francois M. Abboud for his encouragement and critical review of this manuscript, and to Mr Michael S. Cavanaugh and Mr Sukgi Choi for their skilled technical assistance. This research was supported by grants from the Iowa Heart Association, Veterans Administration and the National Institutes of Health (HL and HL 18083). REFERENCES ARNDT, J. O., DORRENHAuS, A. & WIEcKN, H. (1975). The aortic arch baroreceptor response to static and dynamic stretches in an isolated aorta-depressor nerve preparation of cats in vitro. J. Phy8iol. 252, BRISTOW, J. D., HONOUR, A. J., PICKERING, G. W., SLEIGHT, P. & SMYT, H. S. (1969). Diminished baroreflex sensitivity in high blood pressure. Circulation 39, BRONX, D. W. & STELLA, G. (1935). The response to steady pressures of single end organs in the isolated carotid sinus. Am. J. Phy8iol. 110,

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