Autonomic Modulation with Repeated Exposure to +Gz Acceleration

Introduction

Pilots of high performance aircrafts are routinely exposed to +Gz acceleration (G). Exposure to +GZ forces leads to activation of cardiovascular compensatory mechanisms [1]. Recent observations suggest that frequent G exposures are associated with physiological adaptation. This adaptation to repeated G exposures have been demonstrated in several ways. Newman et. al. [2,3] has suggested that 4-Gz-adapted fighter pilots exhibit an improved orthostatic response in terms of increase in magnitude of heart rate and blood pressure response to orthostatic stress. Convertino et al [4,5,6] demonstrated that repeated G exposures leads to an improved blood pressure regulation during cardiovascular stress and an improved G tolerance. A reduction in G tolerance following layoff from flying even for a week has also been described in high performance aircraft pilots. This is termed G-lay off [1,7]. It has been suggested that the physiological adaptation which would have taken place during routine G exposures is lost during lay-offperiod [7].

The physiological basis of this adaptation is still unclear. A standard text book of Aviation Medicine [1] states that “some” physiological adaptation occurs, which is inconclusive. Few authors have hypothesized that this adaptation is due to improved autonomic function upon frequent G exposures [2,4,5,6]. Recently, an increase in blood volume following frequent G exposures has also been suggested [8]. However, little experimental data exist to corroborate these observations. Therefore, this study was conducted with an aim to test the hypothesis whether repeated G exposure is associated with a change in autonomic function.

The autonomic function is a representation of the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS). Amongst the various methods, heart rate variability (HRV) is a very useful tool for understanding the modulation of ANS [9,10] .It is a widely used method to evaluate sympathetic and vagal influences on the heart.

HRV can be measured by time and frequency domain analysis of heart rate (RR) intervals in the electrocardiogram (ECG).Frequency domain analysis measures separate rhythmic contributions from sympathetic and parasympathetic autonomic activity that modulate heart rate. Sympathetic activity is associated with the low frequency range (0.04-0.15 Hz) while parasympathetic activity is associated with the higher frequency range (0.15-0.4 Hz) of modulation frequencies of the heart rate. This difference in frequency ranges allows HRV analysis to separate sympathetic and parasympathetic contributory evidence [9,10].

Three main spectral components are distinguished in a HRV spectrum calculated from short recordings of 2 to 5 minutes. These are very low frequency (VLF) (upto 0.04 Hz), low frequency (LF) (0.04 - 0.15 Hz) and high frequency (HF) (0.15-0.40 Hz) components [task force]. The HF component reflecting momentary respiratory influences on the heart rate, is decreased by tilting or parasympathetic blocking drugs and is increased by sympathetic blocking drugs or controlled respiration. Therefore, the HF component in humans has been thought to provide a quantitative and specific index of vagal modulation [9,11]. On the other hand, the LF component is increased while standing and the increase is blocked by intravenous propranolol. Moreover, this component is not found in quadriplegic humans who have severe dysfunction of the sympathetic nervous system [11,12]. Therefore, the LF component has been interpreted as an indicator mainly of sympathetic influences (especially when expressed in normalised units) [9]. Interpretation of LF components is, however, controversial. It has been suggested by some to include both sympathetic and vagal influences [9].Consequently. the LF/HF ratio has been reported to be a convenient index of sympatho-vagal interaction [9,11]. The physiological explanation of VLF component is much less defined and the existence of a specific physiological process attributed to these heart rate changes might even be questioned [9]. The non-harmonic component which does not have coherent properties and is affected by algorithms of baseline or trend removal, is commonly accepted as a major constituent of VLF. Thus VLF assessed from short-term recordings is a dubious measure and is recommended to be avoided when HRV of short term ECG is interpreted [9.11].

Measurement of VLF, LF and HF power componentsis usually made in absolute values of power, but LF and HF may also be measured in normalized units (n.u.) [9] which represent the relative value of each power component in proportion to the total power minus the VLF component. The representation of LF and HF in n.u. emphasizes the controlled and balanced behavior of the two branches of the autonomic nervous system. Moreover, normalization tends to minimize the effect on the values of LF and HF components of the changes in total power.

Resting HRV has long been used to evaluate autonomic function. However, with so many external factors able to affect resting heart rate, HRV during orthostatic stress using head up tilt test provides a more effective and reliable method [11,13].HRV during head up lilt has been suggested as an effective method to evaluate autonomic function in pilots [14].In this study, the interaction of autonomic function components before and after G exposures was studied by HRV measures during a 70° head up tilt test.

Head up tilt (HUT) test has been considered the “gold standard” as an orthostatic stress test among clinical laboratory diagnostic studies in the evaluation of orthostatic intolerance [15]. A 70° head up tilt table test has been commonly used. Other tests for orthostatic tolerance include lower body negative pressure (LBNP) and passive standing test. It is found that heart rate responses during LBNP are subject to marked day to day variability [16].The cardiovascular effects of Gz stress are mainly due accentuation of hydrostatic pressure gradient from head to toe, where carotid and aortic baroreceptors are differentially stimulated [1,16]. LBNP causes stimulation of both carotid and aortic baroreceptors to an equal degree unlike head up tilt test, where carotid baroreceptors are stimulated predominantly [1,17]. The order of stimulation of baroreceptors during LBNP therefore differs from that of head up tilt and Gz stress [17]. Thus, head up tilt test is a better test to simulate G stress. On the other hand, although a passive standing test is simple and does not require any apparatus, it has been generally objected to. This is because of the likelihood that apposition of the feet to a firm surface would allow activation of the muscle-vein pump in the lower extremities. This can increase venous return and thereby improve orthostatic tolerance [16]. Hence, head up tilt test was chosen as an orthostatic stress test for evaluation of autonomic responses following Gz exposures.

Materials and method

Material and Methods

Results

Hear rate changes during G

Heart rate at different G levels over three Gz exposures were analyzed. Heart rate at 1G, 2G and 3G were compared over three days of Gz exposure (Table 4.5.6). There were no significant differences in the heart rate values at respective G levels.

Table 4: Analysis of heart rate at 1 Gon Day 1,Day 2 and Day 3(n=16)

	One-way analysis of variance of Heart rate at 1 G
p value	0.5825
F	0.5511
R square	0.03787
Post hoc analysis of Heart rate at 1 G
Tukey’s Multiple Comparison Test	Mean Diff.	q	Significant? P<0.05?	95% CI of diff
Day 1 vs Day 2	0.6667	0.3299	No	-6.409 to 7.742
Day 1 vs Day 3	-2.200	1.089	No	-9.276 to 4.876
Day 2 vs Day 3	-2.867	1.419	No	-9.942 to 4.209

Table 5: Analysis of heart rate at 2 Gon Day l, Day 2 and Day 3(n=16)

	One-way analysis of variance of Heart rate at 1 G
p value	0.8242
F	0.1946
R square	0.01371
Post hoc analysis of Heart rate at 1G
Tukey’s Multiple Comparison Test	Mean Diff.	q	Significant? P<:0.05?	95% CI of diff
Day 1 vs Day 2	-2.133	0.8818	No	-10.60 to 6.338
Day 1 vs Day 3	-1.133	0.4684	No	-9.605 to 7.338
Day 2 vs Day 3	1.000	0.4133	No	-7.471 to 9.471

Table 6: Analysis of heart rate at 3 G on Day 1, Day 2 and Day 3 (n=16)

	One-way analysis of variance of Heart rate at 1 G
p value	0.3245
F	1.172
R square	0.07724
Post hoc analysis of Heart rate at 1 G
Tukey’s Multiple Comparison Test	Mean Diff.	q	Significant? p < 0.05?	95% CI of diff
Day 1 vs Day 2	0.9333	0.3244	No	-9.140 to 11.01
Day 1 vs Day 3	-4.867	1.692	No	-14.94 to 5.207
Day 2 vs Day 3	-5.800	2.016	No	-15.87 to 4.273

A difference in heart rate at G tolerance level and 1G was measured (Ä HR). There was no significant difference in Ä HR over the three Gz exposures (Table 7).

Table 7: Ä HR (difference of heart rate at 1 Gand at G tolerance level) on Day 1 and Day 3 of G exposure (n=16)

	Day 1 (bpm)	Day 3 (bpm)	test of significance (p value)
Ä HR	48±19	52±15	0.231 (ns)

Correlation of Ä HR with Gz tolerance on day 1 showed no significant correlation (Fig 1) (r= -0.15, p=0.56). On day 3 the correlation showed minimal change to that on day 1 (r = -0.29. p=0.27) (Fig 2).

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Fig 1:

Correlation of +Gz tolerance with change in heart rate from 1G to G tolerance - Day 1 (n= 16)

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Fig 2:

Correlation of +Gz tolerance with change in heart rate from 1G to G tolerance - Day 3 (n= 16)

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Discussion

The present study was conducted to test the hypothesis that repeated exposure to Gz acceleration would be associated with differences in autonomic functions. The major findings in this study do not support this hypothesis.

Following three sessions of G exposure over 3 consecutive days, the sympathetic and parasympathetic response to orthostatic stress test showed no significant changes. This has been shown by the frequency domain indices of HRV during different stages of the head up tilt test (Table 1). The lack of change in autonomic response following G exposures would suggest either that a physiological adaptation (which has been suggested by several authors) has not occurred or the mechanism of adaptation was not evident through a change in autonomic function.

In addition, Gz tolerance levels did not change significantly over the three G exposures (Table 2,3). Other studies investigating effect of repeated G exposure on G tolerance have demonstrated an increase in +GZ tolerance [4,5,6]. It is reasonable to suggest that the unchanged G tolerance level over 3 exposures indicate lack of physiological adaptation to G.

It is of interest that some underlying mechanism of cardiovascular adaptation can also be deduced from heart rate changes during G exposures. Heart rate increases in response to G. It has been suggested that at low G levels the heart rate is directly correlated with the level and duration of G exposure. In the present study, the heart rate at different G levels was compared over three G exposures. This, however, showed no significant difference (Table 4,5,6). This suggests a lack of change in autonomic response to Gz stress over 3 days of G exposures.

The difference in heart rate (Ä HR) from 1 G to G tolerance level can be considered as a measure of autonomic function. The Ä HR did not change significantly over 3 G exposures (Table 7). Also, the correlation of Ä HR with G tolerance showed no significant difference on day 1 and day 3 of G exposure (Fig 1 ,2). On the other hand, it can be hypothesized that the G tolerance level is independent of increase in heart rate from baseline to G tolerance level (Ä HR).

Together, the data provides indirect evidence that physiological adaptation to three Gz exposures has not occurred. However, the notion that adaptation following repeated G exposures occurs cannot be entirely ruled out. It could be probable that the magnitude and duration of G exposures were not sufficient enough to induce cardiovascular adaptations. Although a previous study has demonstrated cardiovascular adaptation after 3 days of G exposure [5], the magnitude of G exposed was higher. The subject strength for the study could be a limiting factor with less than optimal number of subjects.

Conclusion

This study was an attempt to evaluate autonomic function after repeated +Gz exposures and to test the hypothesis that repeated G exposures would enhance orthostatic stress response. This hypothesis was not supported in the present study. It is evident from the study that a 3 day exposure to gradual onset rate +Gz acceleration neither did bring about a change in autonomic response to orthostatic challenge as measured by HRV, nor a change in +Gz tolerance. The heart rate response to repeated +Gz stress and a change in response also remained unaltered. Although not evident from the present study, physiological adaptation to chronic +Gz acceleration cannot be entirely ruled out. Absence of change in autonomic function in this present study could be due to lack of maximal stimulation of autonomic system, which can be suggested to occur with exposure to high G and a longer duration of exposure to +Gz stress. Less than optimum number of subjects could also be a contributing factor. Further experiments designed to systematically investigate physiological mechanisms of this relationship with longer and greater magnitudes of +Gz exposures are recommended. This could prove important in pilot training and combat effectiveness.