Cardiac Contractility Drives Cardiac Interoception
Authors: Summers Jack , Afonso Luis , Lockette Warren 
Published: May 2024

Introduction: Many of the receptors for the exteroceptive senses (e.g. vision, hearing, smell, touch, pain, etc.) are known. Interoception allows us to sense threats to our “milieu interior.” The mechanisms by which threats induce a feeling of anxiety manifested by heart palpitations, shortness of breath (dyspnea), and gastric awareness (queasiness) are unknown. Using photoplethysmography, we reported that individual variation in cardiac interoception, i.e. the ability to feel a change in one’s heartbeat without taking one’s pulse, correlated with contrasts in heart rate and stroke volumes among the population. Hypothesis: Because increases in heart rate and stroke volume raise cardiac contractility, we postulated that stretch activated piezo channels in the heart are responsible for cardiac interoception. Methods: Interoceptive awareness was determined with a ten point Likert scale in which volunteers rated their ability to feel their heartbeat. Interoceptive accuracy was measured by asking subjects to count their heartbeats without taking their pulse over four time intervals in random order. Simultaneously, heart rates were surreptitiously recorded with a two lead electrocardiogram (EKG). Cardiac interoceptive accuracy was calculated using the formula 1/4 Σ [1 − (|EKG measured heartbeats − subjectively reported heartbeats|) / EKG measured heartbeats]. We used 2D speckle tracking ultrasound to measure the relationship between cardiac strain patterns, normalized for heart rate (HR) and body surface area (BSA), and cardiac interoception in 12 resting men and women. We also measured gastric interoception by determining the percent increase in the volume of water some subjects could drink after reporting water satiety. Results: Cardiac interoceptive awareness correlated with interoceptive accuracy ( r = 0.73, p = 0.009). Speckle tracking of cardiac chambers with echocardiography demonstrated that global longitudinal left ventricular (LV) strain correlated with total left ventricular preload (atrial reservoir strain, r = 0.83, p = 0.0009) and early diastolic filling (atrial conduit strain, r = 0.88, p = 0.0002). LV strain also correlated with interoceptive awareness ( r = 0.58, p = 0.046). When stratified, individuals with the highest interoceptive accuracy (n = 6), compared to those with the lowest interoceptive accuracy (n = 6), had less LV strain, [values expressed as (percent change/BSA)*HR ± S.E.M., 828 ± 81 vs. 582 ± 47, p = 0.026]. Cardiac interoceptive awareness also correlated with gastric interoception accuracy, (n = 6) r = 0.90, p = 0.015. Conclusions: Changes in the activation of cardiac stretch receptors by cardiac filling and contractility could modify interoceptive awareness in men and women at rest. Contrasts in stretch receptor activation in other contractile organs (e.g. stomach) could explain variation in interoceptive awareness in the population. Henry M. Jackson Foundation for the Advancement of Military Medicine. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.