JOURNAL
OF
APPLIED
Vol. 41, No. 1, July
Systolic
PHYSIOLOGY
1976.
Printed
in U.S.A.
time
intervals
during
+ Gz acceleration
THOMAS B. GRABOYS AND EDWARD D. MICHAELSON Biodynamics Branch, Enuirunmental Sciences Diuision, USAF School of Aerospace Medicine, Aerospace Medical Division, AFSC, U.S. Air Force, Brooks Air Force Base, Texas 78235
B., AND EDWARD II. MICHAELSON.SYStime intervaZs during + G, acceleration. 41(1): 52-56. 1976.-Systolic time intervals (STI) were recorded in 8 healthy male volunteers before, during, and after 30-s exposures to +3 G,, + 5 G,, and + 7 G, acceleration. Heart rate (HR) increased at’ all +&, levels, as’did the HR corrected QS, interval, left ventricular ejection time (LVET,.), preejection*‘period (PEP,.) and PEP/LVET. These changes in ST1 were also proportional to the + Gz level. At the higher + G7 levels, PEP,. and PEP/ LVET continued to increase early in ‘the recovery period, but HR and all ST1 returned to control after 60 s of recovery. Although physiological variables other than myocardial contractility, such as preload and afterload, may influence STI during + G,, the effects of + G, on stroke volume (SV) and cardiac output (CO) were estimated using previously described relationships between STI and invasively determined indices of cardiovascular function. In general, CO increased as SV decreased. During recovery, HR and CO fell and CO remained slightly below control levels, primarily because estimated SV remained low. This study demonstrates the feasibility of using STI to estimate noninvasively the transient changes in cardiovascular function during + G, acceleration. GRABOYS,THOMAS
tdic
left ventricular
function;
noninvasive;
hemodynamics
MUCH IS KNOWN about the physiological effects of acceleration (1, 2, 3, 6) and, more recently, the effects of high sustained + G, acceleration (8), to date there has been no quantitative examination of left ventricular function in man via noninvasive techniques during + G,. Previous studies utilizing invasive techniques have been used to measure arterial and central venous blood pressure (8) and cardiac output (CO) via indicator dilution techniques (9). The latter technique, aside from methodological problems at + G,, provides an estimate of average CO over the +G, exposure period and will not detect transients in cardiac function which may occur with +G,. The results of our study of systolic time intervals (STI) during lower body negative pressure (LBNP) (4) and the tilt-table studies of Stafford (12) suggest that ST1 may provide a way to assess continually the cardiac function in man during + G, .acceleration. The aim of this study was to determine the feasibility of using ST1 as a noninvasive mode to study cardiac function during + G, acceleration and to correlate changes in the ST1 with known hemodynamic events. ALTHOUGH
METHODS
Systolic time intervals were recorded at rest and during +G, acceleration on eight healthy male human
volunteers (age range 20-32 yr; mean: 23). All were members of the USAF acceleration panel and were experienced in riding the centrifuge. Each subject had a complete physical examination (Flying Class II), normal 1%lead electrocardiogram (ECG), and chest roentgenogram. All +G, acceleration exposures, using the ZO-ft-radius human centrifuge at the US Air Force School of Aerospace Medicine, consisted of rapid onset (1 G/s) runs to a chosen peak G level, with maintenance of that level for 30 s, unless symptoms of visual impairment, required termination of the run at an earlier time. Visual changes were determined by using a light bar 31.5 in. in front of the subject at eye level. A central red light on the bar was used for fixation and testing of central vision, and two green lights, 14 in. on either side of the central light (visual angle subtended by both green lights of 48”), were used for testing of peripheral vision. The end point for termination of a run was 100% peripheral light loss. G level was recorded from an accelerometer mounted at the subject’s midchest level. Both subject and centrifuge medical monitor have control of the centrifuge brake and may terminate the run at any time. All subjects were seated in the centrifuge at a 13” back angle and were subjected to the following randomized + GZ profile: rapid onset (1 G/s) runs to +3, +5, and + 7 G, for 30 s each separated by a variable recovery period of not less than 3 min. The standard USAF CSU-12/P anti-G suit was used for the +5 and + 7 G7 runs. G-suit inflation begins at +2 G, with pressure in&easing at the rate of 1.5 psi/G above i G. All studies were performed at the same time each day. ECG lead II (0.1-400 Hz); phonocardiogram (40-500 Hz) and carotid pulse tracing (0.1-50 Hz) (obtained by an inflatable cuff secured by Ace bandages) were transmitted via centrifuge slip rings without amplification and were simultaneously recorded on an Electronics-forMedicine DR-8 multichannel recorder. Paper speed was 100 mm/s with time-line intervals of 20 ms. Each STI, as defined by Weissler and Garrard (16), was measured to the nearest 10 ms and an average of 10 cardiac cycles was taken from each observation period. Measurements were: total electromechanical systole (QS,), measured from the onset of the QRS complex to the initial highfrequency component of aortic closure (A,); left ventricular ejection time (LVET), from the beginning of the rapid upstroke to the trough of the incisura of the indirect carotid arterial pulse tracing; and the preejection period (PEP), the difference between LVET and QS,. All ST1 values are given in milliseconds and were corrected for heart rate (HR) in beatslmin using Weissler’s
52
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (137.154.019.149) on January 13, 2019.
ST1
DURING
53
ACCELERATION
regression equations (16, 17) as follows: QS, = QS, + 2.1 HR: LVET,. = LVET + 1.7 HR; PEP,. = PEP + 0.4 HR. Normal corrected intercept index (I) values (corrected to zero heart rate) are: QS, = 546 it 1 (SD); LVET, = 413 -+- 10; PEP, = 131 +- 13. A composite of electromechanical systolic events, the PEP/LVET, was also determined directly from the uncorrected PEP and LVET (18). Stroke volume (SV; ml/stroke) was estimated from the regression equations of Harley et al. (5) where SV = 0.501 LVET + 0.13 HR - 67.1, and cardiac output (CO; mlimin) from SV x HR. Recordings were made prior to the onset of acceleration during the entire centrifuge run and during the first 3 min of recovery. Observation periods were as follows: control 5 and 30 s during +G, and 5, 30, 60, and 120 s after +G, exposure. Figure 1 is a representative tracing obtained at +7 G,. Statistical analyses were carried out using two-factor analysis of variance (time and subject) procedures, each followed by a “multiple range” test (13). Data from 7, 6, and 6 subjects were used for each variable on 3 G, 5 G, and 7 G, respectively, in the analyses. Each analysis also took into account the missing data due to premature termination of a run. Tests are designated as statistically significant when the probability level is less than 0.05 (P < 0.05); however, probability levels of 0.01 and 0.001 have also been noted. RESULTS
Shown in Fig. 2 are the serial ST1 on subjects exposed to +3, +5, and +7 G, acceleration. Corrected data and statistical analysis for each G profile is shown in Tables l-3. QS,. Electromechanical systole prolonged initially in each of the G profiles. During 3 G, the QS, increased from 510 ms to 548 ms at 5 s and subsequently fell to near base-line values in the recovery phase. At 5 G, QS, increased from 512 ms to 580 ms at 5 s, increasing to 597 ms at 30 s and returned to near base-line values in the recovery phase. Similarly, during the 7-G profile, the QS, prolonged from 512 ms to 600 ms at 5 s increasing to 624 ms at 30 s with a return to base-line values by
LEGEND 200r
BASE
FIG.
1. Representative
at +l
G, and
5Gz-----,
3Gz---.
a
I
I
LINE
S k--m-?
I
cc---c
RECOVERY ,;,,,,~?
FIG. 2. Serial ST1 on subjects exposed to +3, +5, and +7 G, acceleration. Each data point represents the mean of all available data at each time interval.
TABLE 1. Time means (ms) and statistical comparisons for each variable at 3 G Variable
-r Mean Base D5S D30s R5S
LVET,
510
OF
APPLIED
Vol. 41, No. 1, July
Systolic
PHYSIOLOGY
1976.
Printed
in U.S.A.
time
intervals
during
+ Gz acceleration
THOMAS B. GRABOYS AND EDWARD D. MICHAELSON Biodynamics Branch, Enuirunmental Sciences Diuision, USAF School of Aerospace Medicine, Aerospace Medical Division, AFSC, U.S. Air Force, Brooks Air Force Base, Texas 78235
B., AND EDWARD II. MICHAELSON.SYStime intervaZs during + G, acceleration. 41(1): 52-56. 1976.-Systolic time intervals (STI) were recorded in 8 healthy male volunteers before, during, and after 30-s exposures to +3 G,, + 5 G,, and + 7 G, acceleration. Heart rate (HR) increased at’ all +&, levels, as’did the HR corrected QS, interval, left ventricular ejection time (LVET,.), preejection*‘period (PEP,.) and PEP/LVET. These changes in ST1 were also proportional to the + Gz level. At the higher + G7 levels, PEP,. and PEP/ LVET continued to increase early in ‘the recovery period, but HR and all ST1 returned to control after 60 s of recovery. Although physiological variables other than myocardial contractility, such as preload and afterload, may influence STI during + G,, the effects of + G, on stroke volume (SV) and cardiac output (CO) were estimated using previously described relationships between STI and invasively determined indices of cardiovascular function. In general, CO increased as SV decreased. During recovery, HR and CO fell and CO remained slightly below control levels, primarily because estimated SV remained low. This study demonstrates the feasibility of using STI to estimate noninvasively the transient changes in cardiovascular function during + G, acceleration. GRABOYS,THOMAS
tdic
left ventricular
function;
noninvasive;
hemodynamics
MUCH IS KNOWN about the physiological effects of acceleration (1, 2, 3, 6) and, more recently, the effects of high sustained + G, acceleration (8), to date there has been no quantitative examination of left ventricular function in man via noninvasive techniques during + G,. Previous studies utilizing invasive techniques have been used to measure arterial and central venous blood pressure (8) and cardiac output (CO) via indicator dilution techniques (9). The latter technique, aside from methodological problems at + G,, provides an estimate of average CO over the +G, exposure period and will not detect transients in cardiac function which may occur with +G,. The results of our study of systolic time intervals (STI) during lower body negative pressure (LBNP) (4) and the tilt-table studies of Stafford (12) suggest that ST1 may provide a way to assess continually the cardiac function in man during + G, .acceleration. The aim of this study was to determine the feasibility of using ST1 as a noninvasive mode to study cardiac function during + G, acceleration and to correlate changes in the ST1 with known hemodynamic events. ALTHOUGH
METHODS
Systolic time intervals were recorded at rest and during +G, acceleration on eight healthy male human
volunteers (age range 20-32 yr; mean: 23). All were members of the USAF acceleration panel and were experienced in riding the centrifuge. Each subject had a complete physical examination (Flying Class II), normal 1%lead electrocardiogram (ECG), and chest roentgenogram. All +G, acceleration exposures, using the ZO-ft-radius human centrifuge at the US Air Force School of Aerospace Medicine, consisted of rapid onset (1 G/s) runs to a chosen peak G level, with maintenance of that level for 30 s, unless symptoms of visual impairment, required termination of the run at an earlier time. Visual changes were determined by using a light bar 31.5 in. in front of the subject at eye level. A central red light on the bar was used for fixation and testing of central vision, and two green lights, 14 in. on either side of the central light (visual angle subtended by both green lights of 48”), were used for testing of peripheral vision. The end point for termination of a run was 100% peripheral light loss. G level was recorded from an accelerometer mounted at the subject’s midchest level. Both subject and centrifuge medical monitor have control of the centrifuge brake and may terminate the run at any time. All subjects were seated in the centrifuge at a 13” back angle and were subjected to the following randomized + GZ profile: rapid onset (1 G/s) runs to +3, +5, and + 7 G, for 30 s each separated by a variable recovery period of not less than 3 min. The standard USAF CSU-12/P anti-G suit was used for the +5 and + 7 G7 runs. G-suit inflation begins at +2 G, with pressure in&easing at the rate of 1.5 psi/G above i G. All studies were performed at the same time each day. ECG lead II (0.1-400 Hz); phonocardiogram (40-500 Hz) and carotid pulse tracing (0.1-50 Hz) (obtained by an inflatable cuff secured by Ace bandages) were transmitted via centrifuge slip rings without amplification and were simultaneously recorded on an Electronics-forMedicine DR-8 multichannel recorder. Paper speed was 100 mm/s with time-line intervals of 20 ms. Each STI, as defined by Weissler and Garrard (16), was measured to the nearest 10 ms and an average of 10 cardiac cycles was taken from each observation period. Measurements were: total electromechanical systole (QS,), measured from the onset of the QRS complex to the initial highfrequency component of aortic closure (A,); left ventricular ejection time (LVET), from the beginning of the rapid upstroke to the trough of the incisura of the indirect carotid arterial pulse tracing; and the preejection period (PEP), the difference between LVET and QS,. All ST1 values are given in milliseconds and were corrected for heart rate (HR) in beatslmin using Weissler’s
52
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (137.154.019.149) on January 13, 2019.
ST1
DURING
53
ACCELERATION
regression equations (16, 17) as follows: QS, = QS, + 2.1 HR: LVET,. = LVET + 1.7 HR; PEP,. = PEP + 0.4 HR. Normal corrected intercept index (I) values (corrected to zero heart rate) are: QS, = 546 it 1 (SD); LVET, = 413 -+- 10; PEP, = 131 +- 13. A composite of electromechanical systolic events, the PEP/LVET, was also determined directly from the uncorrected PEP and LVET (18). Stroke volume (SV; ml/stroke) was estimated from the regression equations of Harley et al. (5) where SV = 0.501 LVET + 0.13 HR - 67.1, and cardiac output (CO; mlimin) from SV x HR. Recordings were made prior to the onset of acceleration during the entire centrifuge run and during the first 3 min of recovery. Observation periods were as follows: control 5 and 30 s during +G, and 5, 30, 60, and 120 s after +G, exposure. Figure 1 is a representative tracing obtained at +7 G,. Statistical analyses were carried out using two-factor analysis of variance (time and subject) procedures, each followed by a “multiple range” test (13). Data from 7, 6, and 6 subjects were used for each variable on 3 G, 5 G, and 7 G, respectively, in the analyses. Each analysis also took into account the missing data due to premature termination of a run. Tests are designated as statistically significant when the probability level is less than 0.05 (P < 0.05); however, probability levels of 0.01 and 0.001 have also been noted. RESULTS
Shown in Fig. 2 are the serial ST1 on subjects exposed to +3, +5, and +7 G, acceleration. Corrected data and statistical analysis for each G profile is shown in Tables l-3. QS,. Electromechanical systole prolonged initially in each of the G profiles. During 3 G, the QS, increased from 510 ms to 548 ms at 5 s and subsequently fell to near base-line values in the recovery phase. At 5 G, QS, increased from 512 ms to 580 ms at 5 s, increasing to 597 ms at 30 s and returned to near base-line values in the recovery phase. Similarly, during the 7-G profile, the QS, prolonged from 512 ms to 600 ms at 5 s increasing to 624 ms at 30 s with a return to base-line values by
LEGEND 200r
BASE
FIG.
1. Representative
at +l
G, and
5Gz-----,
3Gz---.
a
I
I
LINE
S k--m-?
I
cc---c
RECOVERY ,;,,,,~?
FIG. 2. Serial ST1 on subjects exposed to +3, +5, and +7 G, acceleration. Each data point represents the mean of all available data at each time interval.
TABLE 1. Time means (ms) and statistical comparisons for each variable at 3 G Variable
-r Mean Base D5S D30s R5S
LVET,
510
