Heart Rate Analysis during Simulated Push-Pull Effect Manoeuvre

Centrifuge protocol

Each subject was initially given a control +Gz run followed by a push pull run. One minute baseline recordings were taken at +1 Gz before and after each run. +Gz tolerance was determined as 52-56° PLL. Control run had 15 sec epochs of +Gz starting from +3Gz followed by increments of 0.2Gz in subsequent epochs (See Fig-1). The push-pull run had a preceding 10 sec exposure of-1.5Gz for each +Gz epoch. It started from +2Gz with subsequent increments of 0.2Gz (See Fig-2). The onset and offset rate for both the runs were kept at 1 G/sec. A 30 sec interval at baseline was provided between two epochs.

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Fig-1:

+Gz run profile

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Fig-2:

Push-pull run profile

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Analysis of data

The study protocol was part of a larger study comparing +Gz and Push-pull exposures. For the analysis of HR the baseline recording, the recording around the +3Gz epoch of +Gz run and the recording around the Push-pull epoch with comparable +Gz level (closest of +3Gz) of Push- pull run was taken. This was because the lowest +Gz epoch in +Gz run was +3Gz and during the Push-pull run all of the subjects reported PLL before attaining +3Gz level. When the HR was analysed after the study almost all subjects had shown a similar trend. Figure-3 shows the HR response in a subject during +3Gz exposure. It shows that lire HR reaches a peak level from the baseline after a certain time period from the onset of +3Gz and then the HR returns to a lower level once the +3Gz exposure ends. Figure-4 shows that during Push- pull exposure HR reaches a lower level much less than the baseline during the -Gz phase, then returns to higher level during the subsequent +Gz phase and finally returning back to baseline after cessation of +Gz phase.

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Fig-3:

HR response during +3Gz exposure

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Fig-4:

HR response during Push-pull exposure

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For the comparative analysis the lowest resting HR before G exposure were taken as baseline HR (HR_b-min) in the respective profiles. The peak HR achieved during the +3Gz exposure (HR_peak+Gz), duration for attaining the peak HR after reaching the plateau of +3Gz (T_HR-peak+Gz), the lowest HR achieved after offset of +3Gz (HR_min-post+Gz) and the duration for attaining the lowest HR after reaching baseline G (T_{HR-min-post+Gz}) were tabulated from +3Gz run. While from Push-pull profile, the lowest HR during x1.5Gz (HR_min-Gz). duration to reach the lowest HR from onset of xl.5Gz (T_HRmin-Gz). the peak HR achieved during the +Gz exposure (HR_max+GzPP), duration for achieving the peak HR after reaching the plateau of +Gz (T_HRmax+GzPP). the lowest HR achieved after offset of +Gz (HR_min-post-PP) and the duration for attaining the lowest HR after reaching baseline G (T_HRmin-post-PP) were tabulated.

Statistical Analysis

HR and duration to attain the HR were tabulated for +Gz control run and Push-pull run. HR was tabulated as that of baseline, +Gz phase and post +Gz phase of +3Gz run and Push-pull run. They were compared using Student’s two tailed paired ‘f test. The duration to attain peak HR during +Gz phase of control run and Push-pull run were compared using Student’s two tailed paired ‘t’ test.

The duration to attain lowest HR after cessation of +Gz phase were compared between control run and Push-pull run using Student's two tailed paired ‘t’ test. Comparison of HR changes within each profile was done using one-way Anova followed by Tukey’s post hoc analysis. The durations for attaining peak HR during +Gz phase and minimum HR in post +Gz phase within the same run were compared with Student's two tailed paired ‘t’ test. Level of significance (p) was kept less than 5%.

Comparison of peak HR levels

When the min HR recorded before the run. peak HR during +Gz exposure and min HR after +Gz exposures were compared between +Gz and Push-pull exposures using Student ‘t’ test, there were no significant difference (Table-1). This finding is in agreement with those of Goodman et. al., where the Push-pull subjects of different durations of -2Gz had shown similar level of peak HR during subsequent exposure to +2.25Gz which was comparable to peak HR of control subjects exposed to pure +2.25Gz [5]. Hence it may be reasoned that the peak HR achievable during a +Gz exposure is a function of +Gz level, irrespective of the previous “Gz exposure as seen in this study as well as reported by Goodman et. al. [5]. Also in the post +Gz phase the min HR reported in +Gz control run as well as in Push-pull run in this study did not show any significant difference. This also had shown that the HR could come back to similar levels once the +Gz exposure was terminated irrespective of whether pure +Gz or Push-pull exposure. As seen in tables 3 & 4, the minimum pre-run baseline HR and the minimum post +Gz HR did not have any significant differences during both conditions, which again confirmed that the HR attained levels similar to baseline after the G exposure epoch within the 30sec of baseline periods between the epochs of the protocol. Similarly when the HR of different phases were analysed within the profile using One-way Anova and Tukey.s post hoc analysis as given in tables 3 & 4, the -Gz exposure had resulted in significant lowering of HR and the +Gz exposure led to significant increase in HR in accordance with findings reported by earlier studies [6,8,14].

Comparison of time duration

When the time required for attaining peak HR from the start of +Gz plateau was compared between +Gz control and the Push-pull runs, it was significantly higher in the Push-pull run (Table-2). This may be attributed to the delayed autonomic nervous system repose after -Gz exposure reported in several past research works [2,6,12,13]. Similarly when the time to attain minimum HR after +Gz exposure was compared between the profiles, the Push-pull run had shown significant delay compared to pure +Gz run (Table-2). Even though such a finding was not directly reported in the available literature, this can be attributed to the delayed sympathetic vasoconstrictor mechanism as a result of preceding -Gz exposure [2,12,13], which may be still persisting even after subsequent +Gz phase. This therefore can impair the tolerance of subsequent pure +Gz exposure when it occurs during the period of impairment. However measurement of blood pressure and peripheral resistance during the period is important to reach a concrete conclusion.

When the time for reaching peak HR during +GZ and minimum HR during post +Gz in pure +Gz control run was compared using paired ‘t’ test, there was no significant difference (Table-5). This shows similar time period for HR increasing as well as decreasing response. A similar analysis was done in Push-pull run also. However, it was seen that the minimum HR during -Gz phase had reached 11.3 (±2.5) sec which was beyond the duration of - 1.5Gz plateau. Hence it is not sure whether the minimum HR may extend further if the -Gz duration is more than 10 sec. Because of this reason, the minimum HR recorded during -Gz is not considered for the discussion purpose. Nevertheless when the time to attain peak HR during +Gz and minimum HR during post +Gz within Push-pull run were compared (Table-5), there was significant delay in the post +Gz phase in contrary to the observations, of pure +Gz control run. This finding also suggests that delayed sympathetic vasomotor response may be much prolonged than what had been previously thought of.

Push-pull effect is stated to be commonly occurring during training as well as tactical missions. Even though it is classically described in the “bunt then pull” manoeuvre [6], it can also occur after a point roll followed by a pull or sharp turn [16]. Similarly -Gz can be encountered during outside loops and spins, simple inverted flights, unloaded barrel rolls if the pilot is sitting off-centre etc. If such a manoeuvre is followed by rapid build up of +Gz for recovery. Push-pull effect can occur [14]. Most of the trained fighter pilots are aware of such a phenomenon and will be careful to pull higher +Gz immediately after a -Gz exposure. However, what the pilots may not be aware of is the prolonged time period of about 20.6±9.3 sec, even after the Push-pull manoeuvre, during which the decreased tolerance to +Gz is likely to be persisting as evident in this study. If that is the case, such a long period of lowered +Gz tolerance is a significant hazard to flight safety.