The Effect of Aging and Rapid Saline Infusion on Compensatory Reserve during Whole‐Body Passive Heat Stress

Abstract

The compensatory reserve is defined as the physiological capacity to compensate for reductions in central blood volume. It is measured by a machine learning algorithm, called CRI, which analyzes continuous changes in the features of a photoplethysmographic signal obtained by finger pulse oximetry. The purpose of this study was 2‐fold. First, we examined the effect of healthy aging on compensatory reserve during normothermic and heat stress conditions. Second, we examined the effect of rapid saline infusion during heat stress on compensatory reserve. Eleven young (6 males/5 females; mean ± SD: age, 25 ± 4 years; height, 165 ± 6 cm; weight, 63 ± 9 kg) and ten aged (4 males/6 females; age, 69 ± 4 years; height, 165 ± 9 cm; weight, 68 ± 9 kg) adults were exposed to central hypovolemia via 15 and 30 mmHg of lower‐body negative pressure (LBNP) under normothermic and heat stress (increase in core temperature of 1.5°C) conditions. Approximately 10 min after the heat stress LBNP challenge, warm saline was rapidly infused (15 ml/kg in ~6 min) while subjects remained hyperthermic. Absolute CRI values (Table) range between 0 and 1, where 0 represents little to no reserve for cardiovascular compensation and 1 represents maximal physiological reserve to compensate for central hypovolemia. Under normothermic conditions, an age × LBNP level interaction was observed (P<0.01). Relative to baseline, CRI was reduced during 15 mmHg LBNP in young (P<0.01) but not aged (P=0.21) adults. During 30 mmHg LBNP, CRI was reduced in both groups (P≤0.01). During whole‐body passive heat stress, CRI was lower relative to normothermic baseline in both groups (P<0.01). During subsequent LBNP, CRI was further reduced in young adults during 15 and 30 mmHg (P<0.01). In aged adults, CRI was further reduced at 30 mmHg during heating (P=0.06), but not at 15mmHg (P=0.27). Relative to heat stress, rapid saline infusion did not affect CRI in young (P=0.47) or aged (P=0.99) adults. These results demonstrate that CRI responses at low levels of central hypovolemia (i.e. 15 mmHg ≈ 250 ml) differ between young and aged adults under normothermic and heat stress conditions. Furthermore, CRI is unaffected by rapid saline infusion performed under heat stress conditions in both young and aged adults.Support or Funding InformationFunded by the Department of Defense (W81XWH‐12‐1‐0152). CRI responses in young and aged adults. Normothermic baseline LBNP15 LBNP30 Heat stress LBNP15 LBNP30 Infusion Young 0.88±0.06 0.77±0.11* 0.53±0.17* 0.43±0.27* 0.31±0.20† 0.18±0.13† 0.40±0.18 Aged 0.84±0.20 0.79±0.21 0.67±0.24* 0.45±0.20* 0.40±0.20 0.35±0.23† 0.41±0.15 Values are mean ± SD. different from baseline at P≤0.05; different from heat stress at P≤0.06.