- Email: [email protected]
Effects of posture on cardiac dynamics during visual search
Interncrtional Journal of Psychophvsiologv, Elsevier
19
5 (1987) 19-23
PSP 00155
Effects of posture on cardiac dynamics Jules P. Harrell, Vernessa Psychology Department,
Key wordr:
R. Clark and Robert
Howard Unioersit.v, Washington, (Accepted
Visual search;
during visual search
17 December
Impedance
Sellers
DC 20059 (U.S.A.)
1986)
cardiography;
Posture
Cardiovascular psychophysiologists are employing emerging non-invasive technologies to chart the changes in a variety of parameters of cardiac activity as psychological tasks are performed. One technique, impedance cardiography, makes it possible to assess cardiac output and contractile force while psychological tasks are being performed. This study was designed to determine if facilitating the venous return of blood from the legs would alter the pattern of cardiac changes that have been noted in earlier impedance cardiographic studies of the effects of psychological challenge. Twenty normotensive males performed visual search in upright, sitting and reclining positions and impedance cardiographic assessments of cardiac output and stroke volume, and an index of contractility were obtained. In addition, heart rate and two systolic time intervals were measured. As expected. decreases in stroke volume occurred as heart rate increased during visual search. Hence cardiac output (heart rate X stroke volume) remained stable. Higher levels of cardiac output, stroke volume and contractility were recorded in the reclining versus the upright position. However, the patterns of cardiac activity did not differ before, during and after performance when body position was altered. Hence, posture altered the absolute levels of several cardiac parameters, but did not affect the configuration of cardiac changes associated with performing a mildly challenging task.
INTRODUCTION A sense of dissatisfaction with the available non-invasive measures of cardiovascular functioning permeated the first edited volume on cardiovascular psychophysiology (Obrist et al., 1974). Later, one promising remedy in the form of impedance cardiography was described in a paper targeted for psychophysiologists by Miller and Horvath (1978). Evidence suggests that the impedance cardiograph gives relative estimates of stroke volume and cardiac output that compare favorably with invasive dye dilution techniques when assessed on an intra-subject basis (Mohapatra, 1981; Miller and Horvath, 1978). Several psychophysiological studies using im-
Correspondence: J.P. Harrell, Psychology Department, University, Washington DC 20059, U.S.A. 0167-X760/87/$03,50
Howard
” 1987 Elsevier Science Publishers
pedance cardiography have been reported. For classically conditioned decreases example, in stroke volume (Gliner et al., 1977) increases in cardiac output in anticipation of psychological stress (Gliner et al., 1979) as well as dissociations between cardiac performance and metabolic demands as stress was anticipated (Gliner et al., 1982) have been demonstrated. In addition. levels of cardiac output, stroke volume and contractility, obtained with the impedance cardiograph, were shown to differ in response to active and passive coping tasks (Lovallo et al., 1985). In their study, active tasks involved exposure to shock and noise and could be avoided with a rapid key press. Exposure to shock and noise in the absence of an avoidance mechanism constituted the passive condition. Contractility, cardiac index (cardiac output/body surface area) and heart rate evidenced greater increases under active tasks. Harrell and Clark (1985) sampled impedance
B.V. (Biomedical
Division)
measures of cardiac activity before, during and after the performance of either visual search or rotary pursuit. The visual search task involved locating a 5-digit number nested in a larger matrix. Several speeds of rotary pursuit were performed in which participants endeavored to keep a stylus in contact with a rotating target. Three interesting aspects of cardiac activity were noted across the periods of measurement. First, a shortening of ventricular ejection time (VET) (a systolic time interval measured from the first to the second heart sound) and increases in Heather’s (1969) index of contractility were most pronounced during the period immediately after the tasks were performed. Secondly, though performing a pursuit rotor task elicited significant increases in cardiac output, different cardiac dynamics mediated these increases during and following performance. Specifically, increases in cardiac output during pursuit rotor performance were mediated by increases in heart rate. However, after the task was performed, cardiac output reached its highest levels and these increases were mediated by increased stroke volume and contractile force, as well as by elevated heart rate. Finally, though visual search failed to lead to changes in cardiac output, performing this task in a challenging context did lead to a more rapid heart rate. It is likely that posture or body position plays an important role in the configuration of cardiac indices obtained in the impedance cardiographic studies. For example, in contrast to Harrell and Clark (1985) the findings of Lovallo et al. (1985) suggested that increases in contractility and stroke volume are obtained with relative ease in response to psychological stimuli. However. Lovallo et al. placed participants in ‘semirecumbent’ positions (p. 285) while Harrell and Clark’s participants assumed upright sitting positions. The latter posture allows a pooling of blood in the legs and a consequent slowing of the venous return of the blood supply to the heart. Hence, the upright sitting posture may have militated against increases in stroke volume and contractile force as the tasks were performed, via the Frank Starling mechanism. As psychophysiologists use new technologies to trace a topography of cardiac adjust-
ments to psychological events, attention will have to be paid to the role of posture in the configuration of responses. The present study examined the pattern of cardiac adjustments as a visual search task was performed in both upright sitting and reclined postures. In the reclined position, the venous return of blood from the legs is facilitated. Therefore, increases in cardiac output and cardiac contractility were thought to be more likely as the task was performed in this position.
METHODS Subjects Twenty male college students, ranging in age from 18 to 22 years, participated in this study. Credit points applied to the grade in an introductory psychology course were provided. All subjects reported being free of a history of cardiovascular disease and were not taking medication known to affect the central and autonomic nervous systems directly. Each individual was given a tour of the laboratory. This included an explanation of the operation of the physiological recording equipment and the opportunity to view pictures of individuals attached to this equipment. Following a summary of the general nature of the study, written consent to participate was solicited. None of the subjects refused to participate. Physiological measures The study was conducted in a sound-attenuated, air-conditioned room, in which physiological recording equipment was separated from the subject by a room partition. Measures of heart rate, stroke volume, cardiac output, Heather (1969) index, pre-ejection period (PEP) and VET were obtained with an Impedance Cardiograph and microcomputer system. These measures were obtained in the manner described by Miller and Horvath (1978). Two mylar electrodes were taped around the neck approximately 3 cm apart. A third band was placed around the trunk at the level of the xiphisternal process. A distance of at least 7 cm between the third and fourth electrodes was maintained in accordance with the sugges-
21
tions of Mohapatra (1981). Hewlett-Packard pre-gelled ECG electrodes were placed in standard positions for ECG recordings, A Hewlett-Packard
subjects were attempting to locate the sequence of numbers. The 15 s following the search period was
heart with an adjustable rubber belt. All signals were fed into a Mode1 304B Impedance Cardio-
designated the recovery period. It was possible to command the microcomputer to sample measures of cardiac activity twice during each of these periods, approximately 5 and 11 s into each period.
graph.
These
heart
sound
microphone
This system
was strapped
removed
ings and minor movements
artifact
over
the
in the read-
of the body. A Mode1
7000 Impedance Cardiograph Microcomputer received the signals from the Mode1 304B and provided a visual display of impedance changes, heart sounds and ECG. PEP was derived by measuring the time interval between the Q wave of the EKG and the first heart sound. The time interval between the occurrence of the first and second heart sounds constituted VET. On command from a Texas Instruments Silent 700 Printer, a digital printout of the value of each cardiac parameter for a given heart cycle was obtained.
samples
Minnesota marketed Medicine,
were
analyzed
Impedance
separately.
Cardiograph
(The
Mode1 304B,
commercially by Instrumentation for Greenwich, CT, removes some of the
artifact in the impedance trace that results from respiration and body movements. However, the influence of respiration on the dZ/dT wave is persistent. Two samples of cardiac activity were obtained during each period to effect sampling at different phases of the respiratory cycle. No significant main effects or interactions involving samples were found. Still, respiration artifact presents a possible confound for impedance cardiography.)
Procedure After consent was obtained and all sensors and electrodes were attached, the participants were instructed
to sit quietly
for a period
of 5 min.
They were seated in an overstuffed recliner and the lights in the room were dimmed. A similar rest phase was provided at the end of the session. Immediately following the initial rest phase, the experimental task was introduced. The subjects were instructed to perform a visual search task. Slides of an 11 X 11 integer matrix of random numbers were presented using a Kodak 850H carouse1 projector with an automatic timer. target number, 5 digits in length was displayed
A at
the top of the matrix. Participants were instructed to locate the target sequence in the matrix itself within a 15-s period. The time interval between slides varied from 45 to 60 s. Prior to testing, a sample matrix was shown to the participants and they were allowed to find the solution. The task was performed 6 times. Three
trials
were performed in an upright sitting position and 3 in a fully reclined posture. The order of posture was counterbalanced. For each trial, 3 periods were identified. A pretask period was the 15 s before the matrix slide was presented. The performance period was the 15 s during which the slide was exposed and the
Analyses The present split-plot factorial design (Kirk, 1968) was comprised of 4 within-subjects factors and one between-subjects
factor.
Periods (pretask.
performance and recovery), trials, conditions right and reclined) and sampling intervals
(upcon-
stituted the within-subjects factors. Order of assuming the reclined and upright postures was the between-subjects factor. Separate analyses of variance for each cardiac
measure
were computed.
RESULTS Analyses of variance revealed that significant changes in several of the cardiac measures occurred across periods. From pretask to the performance period, heart rate increased and remained faster than pretask levels during the recovery period ( F2,36 = 21.02, P -C 0.01). During performance,
decreases in pretask levels were noted in stroke volume ( F2,34 = 3.39, P < 0.05, the Heather index of contractile force (F& = 4.17, P < 0.05) and VET ( F2,34 = 20.37, P < 0.01. VET evidenced further shortening during the recovery period, while the Heather index and stroke volume tended
22
Ileather rrrdrx
Jlearr rare
IhI’“‘)
StroAecvdlmle
C’CUdlcr~~ *xapu
(CC’)
(Irtetxper
Y
(SD.)
\
/SD.)
.u
(S. D.)
r?ll!l)
\
(S. D.)
Rest
7X.33
(5.X)
300.9
(1X)
6X.5
(X.1)
14.x
(23)
5.94
(1.3)
X0.12
(6.3)
294.3
(1Y)
73.X
14.2
X5.1
(20)
6.16
(1.6)
RCCoWrV
77.96
(6.0)
2X7.9
(16)
12.6
(7.9) (X.7)
(5.6) (5.3)
xx.7
Performance
15.0
(5.X)
X5.Y
(24)
6.07
(1.4)
to return to pretask levels. On the other hand, cardiac output levels did not change significantly across periods, nor did the length of the PEP. Means and standard deviations for these changes are provided in Table 1. Four of the 6 cardiac measures were altered by postural changes. As depicted in Table II, in the reclined position. increases were evident in the Heather index, stroke volume and cardiac output over levels in the upright position. A trend for longer ventricular ejection times was noted also. However, neither heart rate nor PEP period was influenced by the change in posture. No significant periods x conditions interaction effects were found for any of the cardiac measures. Hence, the pattern of cardiac activity occurring before, during and after the search task did not change significantly when posture was altered. Finally, no significant main effects or interactions involving trials, sampling periods or order were obtained.
Curdmc
measure
Upright X
Heart
rate (bpm)
Heather Stroke Cardiac
index (ohms) volume (cc) output (liters per min)
Pre-ejection
period (ms)
The cardiac adjustments that occurred in response to the visual search task were similar to those reported in a previous impedance cardiographic study (Harrell and Clark, 1985). In both studies, cardiac output remained stable because the increase in heart rate that occurred during performance were accompanied by significant decreases in stroke volume. Also, the tendencies for the Heather index to decline as the task was performed and for VET to be shortest in the time period following performance occurred in the previous experiments. It should be noted that though visual search engages participants in an active fashion, it seems to lack the characteristics of active coping found in shock or noise avoidance paradigms (cf. Lovallo et al., 1985; Obrist. 1981). Hence, the failure of visual search to enlist changes in cardiac output or contractility might be due to
Reclrned (SD.)
x
F
P
(S. D.)
70.3
(8.3)
71.7
(8.5)
1.77
*.s.
14.1
(5.7)
15.2
(7.6)
9.05
0.008
6.16
0.02
8.63
0.009
4.04
0.060
2.11
n.s
82.9 5.73 288
VET (ms)
DISCUSSION
80.3
(21) (1.5) (19) (6.3)
90.2 6.39 300 77.3
(24) (1.6) (17) (6.X)
23
the lack of the active coping component. Contrary to expectations, the cardiac responses during this task were similar in the upright and
as mildly challenging psychological tasks are encountered, will function to conserve cardiac output. Increases
in cardiac contractility
are not likely.
reclined positions. None of the conditions Xperiods interaction terms in the analyses of variance approached significance, so no argument for an effect of posture on the pattern of changes in cardiac indices can be made. Still, the evidence
ACKNOWLEDGEMENTS
that venous pooling was reduced by the change to a reclining position is compelling. Astrand and Rodahl (1977) noted that a change from a horizontal position on a tilt table to an upright position resulted in a decrease in cardiac output of
Supported by Office of Naval tract 201-47913-24-81 (441) and
approximately one liter. The postural adjustment made in this study was less radical. Nevertheless,
REFERENCES
an average fall in cardiac output of 0.6 liters occurred when the participants assumed and upright sitting as opposed to a reclining position. The fall in stroke volume and in the Heather index after this adjustment was made, provides additional evidence that venous pooling was affected. The failure of postural changes to affect the configuration of cardiac responses to visual search should not be taken to suggest that posture has no influence on the ease with which increases in cardiac output or contractility are elicited in psychophysiological studies. Lengthening the periods during which reclined positions are assumed before encountering the psychological task may increase the likelihood that inotropic increases will be observed. Lovallo et al. (1985) provided 20 min of rest in a reclined position stressors were encountered. passive coping of contractility. In addition,
resulted
before psychological In that study even
in increases
it is possible
that
in the indices if a veritable
horizontal posture were assumed, or if venous return were facilitated by leg movements (Rushmer, 1976), increases in contractility and cardiac output might be more easily demonstrable as a mildly challenging or passive coping psychological task is performed. However, most laboratory studies of psychological stress are likely to request that participants perform tasks in stationary sitting or semi-reclined postures. The present findings indicate that for such postural conditions, the shortterm adjustments in cardiac dynamics that occur
Research ConNIH 2506RR-
801613.
Astrand, P. and Rodahl. K. (1977) Textbook of Work Physlology, McGraw Hill, New York. Gliner, J.A., Bedi, J.F. and Horvath, S.M. (1979) Somatic and non-somatic influences on the heart: hemodynamic changes. Psychophysiology, 16: 358-362. Gliner, J.A., Browe, A.C. and Horvath, SM. (1977) Hemodynamic changes as a function of classical aversive condltioning in human subjects. Psychophysiology, 14: 281-286. Gliner, J.A., Bunnell, D.E. and Horvath, SM. (1982) Hemodynamic and metabolic changes prior to speech performance. Physiol. Psychol.. 10: 108-113. Harrell, J.P. and Clark, V.R. (1985) Cardiac responses to psychological tasks. Impedance cardiographic studies. Elol. Psychol., 20: 261-283. Heather. L.W. (1969) A comparison of cardiac output values by impedance cardiography and dye dilution techniques in cardiac patients. In W.G. Kubicek, D.A. Witsoe, R.P. Patterson and A.H.L. Fromm (Eds.), Development and Eoaluatron of an Impedance Ccrrdiographic System to Meusure Cardiac Output and Other Cardtac Parameters. (NASACR-101965), National Aeronautics and Space Administration, Houston. TX. Kirk, R.E. (1968) Experimental Design: Procedures for the Behauroral Scrences. Brooks/Cole, Belmont. CA. Lovallo, W.R., Wilson, M.F., Pincomb, G.A., Edwards, G.L.. Tompkins, P. and Brackett, D.J. (1985) Activation patterns to aversive stimulation in man: passive exposure versus effort to control. Psychophysiology, 22: 283-291. Miller, J.C. and Horvath, S.M. (1978) Impedance cardiograph. Psychophysiology, 15: 80-91. Mohapatra, S.N. (1981) Non-invasive Ccrrdiovrrscular Momtoring. State Mutual Books, New York. Obrist, P.A., (1981) Cardiovascular Psychophysiology: A Perspectiue, Plenum, New York. Obrist, P.A.. Black, A.H., Brener, J. and DiCara, L.V. (1974) Carduwzsculrr Psychophysiology: Current issues in Response Mechanisms, Biofeedback and Methodology, Aldine, Chicago. Rushmer, R.F. (1976) Structure and Function of the Cardiooascular S_vstem. 2nd edn., W.B. Saunders. Philadelphia.
19
5 (1987) 19-23
PSP 00155
Effects of posture on cardiac dynamics Jules P. Harrell, Vernessa Psychology Department,
Key wordr:
R. Clark and Robert
Howard Unioersit.v, Washington, (Accepted
Visual search;
during visual search
17 December
Impedance
Sellers
DC 20059 (U.S.A.)
1986)
cardiography;
Posture
Cardiovascular psychophysiologists are employing emerging non-invasive technologies to chart the changes in a variety of parameters of cardiac activity as psychological tasks are performed. One technique, impedance cardiography, makes it possible to assess cardiac output and contractile force while psychological tasks are being performed. This study was designed to determine if facilitating the venous return of blood from the legs would alter the pattern of cardiac changes that have been noted in earlier impedance cardiographic studies of the effects of psychological challenge. Twenty normotensive males performed visual search in upright, sitting and reclining positions and impedance cardiographic assessments of cardiac output and stroke volume, and an index of contractility were obtained. In addition, heart rate and two systolic time intervals were measured. As expected. decreases in stroke volume occurred as heart rate increased during visual search. Hence cardiac output (heart rate X stroke volume) remained stable. Higher levels of cardiac output, stroke volume and contractility were recorded in the reclining versus the upright position. However, the patterns of cardiac activity did not differ before, during and after performance when body position was altered. Hence, posture altered the absolute levels of several cardiac parameters, but did not affect the configuration of cardiac changes associated with performing a mildly challenging task.
INTRODUCTION A sense of dissatisfaction with the available non-invasive measures of cardiovascular functioning permeated the first edited volume on cardiovascular psychophysiology (Obrist et al., 1974). Later, one promising remedy in the form of impedance cardiography was described in a paper targeted for psychophysiologists by Miller and Horvath (1978). Evidence suggests that the impedance cardiograph gives relative estimates of stroke volume and cardiac output that compare favorably with invasive dye dilution techniques when assessed on an intra-subject basis (Mohapatra, 1981; Miller and Horvath, 1978). Several psychophysiological studies using im-
Correspondence: J.P. Harrell, Psychology Department, University, Washington DC 20059, U.S.A. 0167-X760/87/$03,50
Howard
” 1987 Elsevier Science Publishers
pedance cardiography have been reported. For classically conditioned decreases example, in stroke volume (Gliner et al., 1977) increases in cardiac output in anticipation of psychological stress (Gliner et al., 1979) as well as dissociations between cardiac performance and metabolic demands as stress was anticipated (Gliner et al., 1982) have been demonstrated. In addition. levels of cardiac output, stroke volume and contractility, obtained with the impedance cardiograph, were shown to differ in response to active and passive coping tasks (Lovallo et al., 1985). In their study, active tasks involved exposure to shock and noise and could be avoided with a rapid key press. Exposure to shock and noise in the absence of an avoidance mechanism constituted the passive condition. Contractility, cardiac index (cardiac output/body surface area) and heart rate evidenced greater increases under active tasks. Harrell and Clark (1985) sampled impedance
B.V. (Biomedical
Division)
measures of cardiac activity before, during and after the performance of either visual search or rotary pursuit. The visual search task involved locating a 5-digit number nested in a larger matrix. Several speeds of rotary pursuit were performed in which participants endeavored to keep a stylus in contact with a rotating target. Three interesting aspects of cardiac activity were noted across the periods of measurement. First, a shortening of ventricular ejection time (VET) (a systolic time interval measured from the first to the second heart sound) and increases in Heather’s (1969) index of contractility were most pronounced during the period immediately after the tasks were performed. Secondly, though performing a pursuit rotor task elicited significant increases in cardiac output, different cardiac dynamics mediated these increases during and following performance. Specifically, increases in cardiac output during pursuit rotor performance were mediated by increases in heart rate. However, after the task was performed, cardiac output reached its highest levels and these increases were mediated by increased stroke volume and contractile force, as well as by elevated heart rate. Finally, though visual search failed to lead to changes in cardiac output, performing this task in a challenging context did lead to a more rapid heart rate. It is likely that posture or body position plays an important role in the configuration of cardiac indices obtained in the impedance cardiographic studies. For example, in contrast to Harrell and Clark (1985) the findings of Lovallo et al. (1985) suggested that increases in contractility and stroke volume are obtained with relative ease in response to psychological stimuli. However. Lovallo et al. placed participants in ‘semirecumbent’ positions (p. 285) while Harrell and Clark’s participants assumed upright sitting positions. The latter posture allows a pooling of blood in the legs and a consequent slowing of the venous return of the blood supply to the heart. Hence, the upright sitting posture may have militated against increases in stroke volume and contractile force as the tasks were performed, via the Frank Starling mechanism. As psychophysiologists use new technologies to trace a topography of cardiac adjust-
ments to psychological events, attention will have to be paid to the role of posture in the configuration of responses. The present study examined the pattern of cardiac adjustments as a visual search task was performed in both upright sitting and reclined postures. In the reclined position, the venous return of blood from the legs is facilitated. Therefore, increases in cardiac output and cardiac contractility were thought to be more likely as the task was performed in this position.
METHODS Subjects Twenty male college students, ranging in age from 18 to 22 years, participated in this study. Credit points applied to the grade in an introductory psychology course were provided. All subjects reported being free of a history of cardiovascular disease and were not taking medication known to affect the central and autonomic nervous systems directly. Each individual was given a tour of the laboratory. This included an explanation of the operation of the physiological recording equipment and the opportunity to view pictures of individuals attached to this equipment. Following a summary of the general nature of the study, written consent to participate was solicited. None of the subjects refused to participate. Physiological measures The study was conducted in a sound-attenuated, air-conditioned room, in which physiological recording equipment was separated from the subject by a room partition. Measures of heart rate, stroke volume, cardiac output, Heather (1969) index, pre-ejection period (PEP) and VET were obtained with an Impedance Cardiograph and microcomputer system. These measures were obtained in the manner described by Miller and Horvath (1978). Two mylar electrodes were taped around the neck approximately 3 cm apart. A third band was placed around the trunk at the level of the xiphisternal process. A distance of at least 7 cm between the third and fourth electrodes was maintained in accordance with the sugges-
21
tions of Mohapatra (1981). Hewlett-Packard pre-gelled ECG electrodes were placed in standard positions for ECG recordings, A Hewlett-Packard
subjects were attempting to locate the sequence of numbers. The 15 s following the search period was
heart with an adjustable rubber belt. All signals were fed into a Mode1 304B Impedance Cardio-
designated the recovery period. It was possible to command the microcomputer to sample measures of cardiac activity twice during each of these periods, approximately 5 and 11 s into each period.
graph.
These
heart
sound
microphone
This system
was strapped
removed
ings and minor movements
artifact
over
the
in the read-
of the body. A Mode1
7000 Impedance Cardiograph Microcomputer received the signals from the Mode1 304B and provided a visual display of impedance changes, heart sounds and ECG. PEP was derived by measuring the time interval between the Q wave of the EKG and the first heart sound. The time interval between the occurrence of the first and second heart sounds constituted VET. On command from a Texas Instruments Silent 700 Printer, a digital printout of the value of each cardiac parameter for a given heart cycle was obtained.
samples
Minnesota marketed Medicine,
were
analyzed
Impedance
separately.
Cardiograph
(The
Mode1 304B,
commercially by Instrumentation for Greenwich, CT, removes some of the
artifact in the impedance trace that results from respiration and body movements. However, the influence of respiration on the dZ/dT wave is persistent. Two samples of cardiac activity were obtained during each period to effect sampling at different phases of the respiratory cycle. No significant main effects or interactions involving samples were found. Still, respiration artifact presents a possible confound for impedance cardiography.)
Procedure After consent was obtained and all sensors and electrodes were attached, the participants were instructed
to sit quietly
for a period
of 5 min.
They were seated in an overstuffed recliner and the lights in the room were dimmed. A similar rest phase was provided at the end of the session. Immediately following the initial rest phase, the experimental task was introduced. The subjects were instructed to perform a visual search task. Slides of an 11 X 11 integer matrix of random numbers were presented using a Kodak 850H carouse1 projector with an automatic timer. target number, 5 digits in length was displayed
A at
the top of the matrix. Participants were instructed to locate the target sequence in the matrix itself within a 15-s period. The time interval between slides varied from 45 to 60 s. Prior to testing, a sample matrix was shown to the participants and they were allowed to find the solution. The task was performed 6 times. Three
trials
were performed in an upright sitting position and 3 in a fully reclined posture. The order of posture was counterbalanced. For each trial, 3 periods were identified. A pretask period was the 15 s before the matrix slide was presented. The performance period was the 15 s during which the slide was exposed and the
Analyses The present split-plot factorial design (Kirk, 1968) was comprised of 4 within-subjects factors and one between-subjects
factor.
Periods (pretask.
performance and recovery), trials, conditions right and reclined) and sampling intervals
(upcon-
stituted the within-subjects factors. Order of assuming the reclined and upright postures was the between-subjects factor. Separate analyses of variance for each cardiac
measure
were computed.
RESULTS Analyses of variance revealed that significant changes in several of the cardiac measures occurred across periods. From pretask to the performance period, heart rate increased and remained faster than pretask levels during the recovery period ( F2,36 = 21.02, P -C 0.01). During performance,
decreases in pretask levels were noted in stroke volume ( F2,34 = 3.39, P < 0.05, the Heather index of contractile force (F& = 4.17, P < 0.05) and VET ( F2,34 = 20.37, P < 0.01. VET evidenced further shortening during the recovery period, while the Heather index and stroke volume tended
22
Ileather rrrdrx
Jlearr rare
IhI’“‘)
StroAecvdlmle
C’CUdlcr~~ *xapu
(CC’)
(Irtetxper
Y
(SD.)
\
/SD.)
.u
(S. D.)
r?ll!l)
\
(S. D.)
Rest
7X.33
(5.X)
300.9
(1X)
6X.5
(X.1)
14.x
(23)
5.94
(1.3)
X0.12
(6.3)
294.3
(1Y)
73.X
14.2
X5.1
(20)
6.16
(1.6)
RCCoWrV
77.96
(6.0)
2X7.9
(16)
12.6
(7.9) (X.7)
(5.6) (5.3)
xx.7
Performance
15.0
(5.X)
X5.Y
(24)
6.07
(1.4)
to return to pretask levels. On the other hand, cardiac output levels did not change significantly across periods, nor did the length of the PEP. Means and standard deviations for these changes are provided in Table 1. Four of the 6 cardiac measures were altered by postural changes. As depicted in Table II, in the reclined position. increases were evident in the Heather index, stroke volume and cardiac output over levels in the upright position. A trend for longer ventricular ejection times was noted also. However, neither heart rate nor PEP period was influenced by the change in posture. No significant periods x conditions interaction effects were found for any of the cardiac measures. Hence, the pattern of cardiac activity occurring before, during and after the search task did not change significantly when posture was altered. Finally, no significant main effects or interactions involving trials, sampling periods or order were obtained.
Curdmc
measure
Upright X
Heart
rate (bpm)
Heather Stroke Cardiac
index (ohms) volume (cc) output (liters per min)
Pre-ejection
period (ms)
The cardiac adjustments that occurred in response to the visual search task were similar to those reported in a previous impedance cardiographic study (Harrell and Clark, 1985). In both studies, cardiac output remained stable because the increase in heart rate that occurred during performance were accompanied by significant decreases in stroke volume. Also, the tendencies for the Heather index to decline as the task was performed and for VET to be shortest in the time period following performance occurred in the previous experiments. It should be noted that though visual search engages participants in an active fashion, it seems to lack the characteristics of active coping found in shock or noise avoidance paradigms (cf. Lovallo et al., 1985; Obrist. 1981). Hence, the failure of visual search to enlist changes in cardiac output or contractility might be due to
Reclrned (SD.)
x
F
P
(S. D.)
70.3
(8.3)
71.7
(8.5)
1.77
*.s.
14.1
(5.7)
15.2
(7.6)
9.05
0.008
6.16
0.02
8.63
0.009
4.04
0.060
2.11
n.s
82.9 5.73 288
VET (ms)
DISCUSSION
80.3
(21) (1.5) (19) (6.3)
90.2 6.39 300 77.3
(24) (1.6) (17) (6.X)
23
the lack of the active coping component. Contrary to expectations, the cardiac responses during this task were similar in the upright and
as mildly challenging psychological tasks are encountered, will function to conserve cardiac output. Increases
in cardiac contractility
are not likely.
reclined positions. None of the conditions Xperiods interaction terms in the analyses of variance approached significance, so no argument for an effect of posture on the pattern of changes in cardiac indices can be made. Still, the evidence
ACKNOWLEDGEMENTS
that venous pooling was reduced by the change to a reclining position is compelling. Astrand and Rodahl (1977) noted that a change from a horizontal position on a tilt table to an upright position resulted in a decrease in cardiac output of
Supported by Office of Naval tract 201-47913-24-81 (441) and
approximately one liter. The postural adjustment made in this study was less radical. Nevertheless,
REFERENCES
an average fall in cardiac output of 0.6 liters occurred when the participants assumed and upright sitting as opposed to a reclining position. The fall in stroke volume and in the Heather index after this adjustment was made, provides additional evidence that venous pooling was affected. The failure of postural changes to affect the configuration of cardiac responses to visual search should not be taken to suggest that posture has no influence on the ease with which increases in cardiac output or contractility are elicited in psychophysiological studies. Lengthening the periods during which reclined positions are assumed before encountering the psychological task may increase the likelihood that inotropic increases will be observed. Lovallo et al. (1985) provided 20 min of rest in a reclined position stressors were encountered. passive coping of contractility. In addition,
resulted
before psychological In that study even
in increases
it is possible
that
in the indices if a veritable
horizontal posture were assumed, or if venous return were facilitated by leg movements (Rushmer, 1976), increases in contractility and cardiac output might be more easily demonstrable as a mildly challenging or passive coping psychological task is performed. However, most laboratory studies of psychological stress are likely to request that participants perform tasks in stationary sitting or semi-reclined postures. The present findings indicate that for such postural conditions, the shortterm adjustments in cardiac dynamics that occur
Research ConNIH 2506RR-
801613.
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