Heart Rate Decreases During Head-Out-of-Water Immersion and With Lower-Body Weight Support: A Measure of Vagal Reserve?

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To the Editor:
We read with great interest the article entitled “Using heart rate (HR) to prescribe physical exercise during head-out water immersion” (HOWI) by Kruel et al. (3). They studied HR effects of HOWI in 395 individuals and found HR to decrease with decrements related to water depth. Interestingly, HR lowering was correlated with baseline HR. The authors suggested that Karvonen's maximal HR prediction formula be modified for HOWI-induced HR lowering (predicted HRmax = 220 − age − ΔHR) to assess the intensity of HOWI exercise.
Body weight support (WS) treadmill exercise also induces lower-body compression, intrathoracic blood pooling, and augmented venous return. Although originally developed to simulate microgravity conditions of aerospace, such systems have been used for orthopedic/neurologic patients and are now widely available to the general public (1). Weight support treadmills generally consist of a computerized treadmill equipped with a pressurized air chamber that generates a vertical upward force on the lower body that opposes the force of gravity to effectively decreasing body weight. Variable WS degrees can be achieved by adjusting lower-body positive pressure. Interestingly, waist and chest level HOWI equate to 54% and 35% of body WS (2).
Accordingly, while resting HR reflects vagal tone, do the authors think that a HOWI-related HR decrease is a measure of vagal reserve? Also, could modifying Karvonen's formula be applicable to WS treadmill exercise?
To date, we have prospectively studied 40 healthy males aged 30 ± 8 years. Baseline HR measurements (100% body weight) were recorded using telemetry monitoring after 2 minutes of standing on the WS treadmill, (AlterG Anti-Gravity Treadmill; Alter G, Fremont, CA, USA), and were repeated after 2 minutes of WS to 75, 50, and 25% of body weight, selected in random order (2). On repeated-measures analysis, WS decreased mean HR 81 to 71 b·min−1 progressively with increasing WS (p < 0.001). Changes in HR were correlated with baseline HR (r = 0.37–0.40, all p ≤ 0.02) at 75, 50 and 25% of body weight.
We previously studied chest/waist HOWI effects in 21 subjects with results comparable to Kruel and recently reanalyzed our data (4). ΔHR was significantly correlated with baseline HR at chest/waist levels (both r = 0.68, p = 0.001).
Therefore, HR decreases during WS seem similar to those of HOWI and are also related to resting HR at different body WS levels. Accordingly, if predicted HR responses are validated by direct measurement during HOWI exercises, then a modified Karvonen's formula may also apply to WS treadmill exercise.
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