Pulmonary vascular adaptations to hypoxia in elite breath-hold divers

Abstract

Introduction: Elite breath-hold divers (BHD) possess several oxygen conserving adaptations to endure long dives similar to diving mammals. During dives, Bottlenose Dolphins may increase the alveolar ventilation (V_A) to perfusion (Q) ratio to increase alveolar oxygen delivery. We hypothesized that BHD possess similar adaptive mechanisms during apnea.

Methods and results: Pulmonary blood volume (PBV) was determined by echocardiography, ¹⁵O-H₂O PET/CT, and cardiac MRi, (n = 6) during and after maximum apneas. Pulmonary function was determined by body box spirometry and compared to matched controls. After 2 min of apnea, the PBV determined by echocardiography and ¹⁵O-H₂O-PET/CT decreased by 26% and 41%, respectively. After 4 min of apnea, the PBV assessed by echocardiography and cardiac MRi decreased by 48% and 67%, respectively (n = 6). Fractional saturation (F)O₂Hb determined by arterial blood-gas-analyses collected after warm-up and a 5-minute pool-apnea (n = 9) decreased by 43%. Compared to matched controls (n = 8), spirometry revealed a higher total and alveolar-lung-capacity in BHD (n = 9), but a lower diffusion-constant.

Conclusion: Our results contrast with previous studies, that demonstrated similar lung gas transfer in BHD and matched controls. We conclude that elite BHD 1) have a lower diffusion constant than matched controls, and 2) gradually decrease PBV during apnea and in turn increase V_A/Q to increase alveolar oxygen delivery during maximum apnea. We suggest that BHD possess pulmonary adaptations similar to diving mammals to tolerate decreasing tissue oxygenation.

New and noteworthy: This manuscript addresses novel knowledge on tolerance to hypoxia during diving, which is shared by elite breath-hold divers and adult diving mammals: Our study indicates that elite breath-hold divers gradually decrease pulmonary blood volume and in turn increase VA/Q, to increase alveolar oxygen delivery during maximum apnea to tolerate decreasing oxygen levels similar to the Bottlenose Dolphin.

Keywords: cardiac MR (CMR); cardiac PET/CT; cardiac output; echocardiagraphy; freediving.

FIGURE 1

Calculations of ¹⁵O-H₂O-PET/CT determined pulmonary blood volume. First-pass time-activity curves extracted from 3 consecutive 4-min ¹⁵O-H₂O/PET-CT scans on the same subject (only the 2 first minutes are shown). The mean transit time from the right ventricle to the left ventricle cavity increases from 13s at rest (A) to 22s during apnea (initiated 2 min prior to the scan) (B) and returns to 13s during recovery (C).

FIGURE 2

Example of cardiac MRi determined pulmonary transit time. X-axis: time in seconds after 4 min of apnea and concomitant contrast infusion. Y-axis signal intensity. RV: Right ventricle. LV: left ventricle.

FIGURE 3

Example of pulmonary transit time determined by echocardiography after 4 min of apnea. RV: Right ventricle. LV: left ventricle. 4D displays ultrasonic determined pulmonary transit time using time-activity curves in RV and LV after a bolus of contrast agent. (A) before contrast; (B) contrast in the RV; (C) contrast in the LV; (D) pulmonary transit time using time-activity curves in the RV and LV.