Cardiovascular Fitness Improves Cardiac Interoceptive Accuracy

Abstract

Background. Many of the sensory receptors encoding touch, temperature, pain, and visual or auditory cues are known; these exteroceptive receptors allow us to respond to external threats. The interoceptive sensory receptors that identify stimuli within the internal milieu, e.g. changes in our heart rate, gastric fullness, or breathing difficulty are not known. We postulated mechanosensitive piezo channels in the heart are responsible for cardiac interoception. We found that individuals with lower resting heart rates and resting rate‐pressure products, and higher heart rate variability and stroke volumes have greater interoceptive accuracy. These characteristics often result from exercise training.HypothesisWe suggest that individual contrasts in interoceptive accuracy will correlate with genotypic differences in the PIEZO 1 channels in the population and individual variation in fitness levels. Furthermore, activities that increase stroke volume, such as thermal warming, would enhance interoceptive accuracy.MethodsHeart rates, blood pressures, and stroke volumes were measured non‐invasively in 12 healthy men and women using a commercial photoplethysmography device (Finapres,™ Nova Instruments). Fitness levels were assessed using a modified Harvard step test. Cardiac interoceptive accuracy (“IA”) was assessed using a heartbeat tracking task where participants counted their heartbeats during four separate trials after being instructed not to take their pulse or engage in any manipulation that could facilitate heartbeat detection while their EKG was being recorded. “IA” was calculated from 1/4 Σ [1 – (|EKG heartbeats – reported heartbeats|) ÷ EKG heartbeats]; higher scores represent greater interoceptive accuracy. “IA” was assessed while subjects rested supine in a room at 68o F and after warming with thermal blankets for 25‐30 min. Because of confounding effects of menstrual cycle on body temperature, we studied the effect of thermal warming only on men. Because PIEZO 1 channels also mediate regulatory volume changes to osmotic stress, we assessed PIEZO 1 channel‐mediated hemolysis of erythrocytes during hypotonic stress.Results. Cardiac “IA” correlated positively with fitness levels (n = 9, r = 0.59, p < 0.05). Furthermore, the EC50 calculated from an osmotic fragility test on erythrocytes from our subjects correlated with their “IA” (n = 6, r = 0.91, p = 0.01). Thermal heating raised forearm skin temperatures (values expressed as mean oC + SEM) from 36.3 + 0.2 to 37.9 + 0.18 (n = 10, p < 0.0001) which was accompanied by an increase in “IA” scores from (values in arbitrary units + SEM) 83.3 + 3.2 to 89.5 + 2.6 (n = 10, p = 0.002). Surprisingly, mild thermal heating did not affect resting heart rate or vascular resistance, but stroke volume tended to fall (values expressed in ml/beat + SEM) from 102 + 5.6 to 92 + 5.6 (n = 6, p = 0.03).Conclusions. Increasing fitness levels and endogenous PIEZO 1 channel activity correlate with enhanced “IA.” Although the individual’s stroke volume may be a determinant of their “IA,” this self‐awareness of heart beats and other interoceptive clues are sensitive to environmental factors such as mild thermal heating.