Cardiovascular effects of breath-hold diving at altitude

Cardiovascular effects of breath-hold diving at altitude

RUNNING HEAD: BREATH-HOLD DIVING AT ALTITUDE

ABSTRACT

Hypoxia, centralization of blood in pulmonary vessels, and increased cardiac output during physical exertion are the pathogenetic pathways of acute pulmonary edema observed during exposure to extraordinary environments. This study aimed to evaluate the effects of breath-hold diving at altitude, which exposes simultaneously to several of the stimuli mentioned above.

To this aim, 11 healthy male experienced divers (age 18-52y) were evaluated (by Doppler echocardiography, lung echography to evaluate ultrasound lung B-lines (BL), hemoglobin saturation, arterial blood pressure, fractional NO (Nitrous Oxide) exhalation) in basal condition (altitude 300m asl), at altitude (2507m asl) and after breath-hold diving at altitude.

A significant increase in E/e’ ratio (a Doppler-echocardiographic index of left atrial pressure) was observed at altitude, with no further change after the diving session. The number of BL significantly increased after diving at altitude as compared to basal conditions. Finally, fractional exhaled nitrous oxide was significantly reduced by altitude; no further change was observed after diving.

Our results suggest that exposure to hypoxia may increase left ventricular filling pressure and, in turn, pulmonary capillary pressure. Breath-hold diving at altitude may contribute to interstitial edema (as evaluated by BL score), possibly because of physical efforts made during a diving session. The reduction of exhaled nitrous oxide at altitude confirms previous reports of nitrous oxide reduction after repeated exposure to hypoxic stimuli. This finding should be further investigated since reduced nitrous oxide production in hypoxic conditions has been reported in subjects prone to high-altitude pulmonary edema.

Keywords: acute pulmonary edema; breath-hold diving; nitric oxide

Key points:

• High altitude and breath-hold diving can induce hypoxia and increase pulmonary capillary pressure.
• Signs of interstitial pulmonary edema have been observed in these conditions.
• A reduced nitrous oxide production could facilitate the development of overt pulmonary edema under hypoxic conditions.

INTRODUCTION

Breath-hold (BH) diving, with the rapid onset of hypoxia and increased external pressure, significantly affects cardiovascular pressure. The cardiovascular response to diving is driven by the diving reflex, inducing bradycardia and selective peripheral arterial vasoconstriction in muscle, skin, and other hypoxia-resistant organs (while cerebral blood flow is even increased) and by blood shift from the peripheral circulation to the intrathoracic venous and capillary vessels (1-3). This complex response is finalized to reduce oxygen consumption and guarantee sufficient blood and oxygen support to the peripheral organs, particularly the brain. Cardiovascular response to BH in diving athletes has been studied in different experimental conditions, i.e., during face immersion, during dry apnea, or in a hyperbaric chamber, using different cardiac techniques and parameters of cardiac function (1,4,5). Previous studies using radiographic cinematography showed an increased end-diastolic area of the heart by 30% early after immersion (6,7). More recently, echocardiography in dry conditions and using submersible echocardiographs allowed cardiac function and morphology to be assessed before, during, and after immersion (8-10).

      The studies assessing cardiac function and morphology early after diving showed an improvement in systo-diastolic functions due to favorable loading conditions relative to pre-diving, namely the recruitment of left ventricular preload reserve and the reduction in afterload (8). During immersion, echocardiography performed at 3- and 10-meter depths showed reduced dimensions of the left atrium associated to a left ventricular diastolic pattern resembling that of restrictive/constrictive heart disease, suggesting that the hemodynamic effects of diving could be explained, at least in part, by a constriction due increased external pressure (9,10). Hypoxia is also a determinant factor inducing cardiovascular response to apnea (11,12).  For this reason, we aimed to study the cardiovascular effect of diving at altitude. Therefore, we planned to evaluate the cardiovascular response of expert breath-hold divers simultaneously exposed to altitude, body immersion, and breath-holding during a diving training session at high altitudes. Nivolet Lake is located in the Italian Alps at an altitude of 2503 meters a.m.s.l.

MATERIALS AND METHODS

The study conforms with the principles of the Declaration of Helsinki. The study protocol was approved by the local University-Hospital Ethics Committee (Comitato Etico Azienda Ospedaliero-Universitaria Pisana; approval number 2805). All participants received information about the aims and procedures of the study and gave their written consent.

Subjects.

A group of 11 healthy male subjects (age range: 18 - 52; average + SD: 34.4+9.6 ) was studied. All subjects were experienced active breath-hold divers; no subject did any diving training in the two days before the experiment. No subject had history, clinical or instrumental (resting ECG, Doppler Echocardiography) evidence of arterial hypertension, cardiac or pulmonary disease. All subjects were non-smokers, were not taking medications, and had been fasting for at least two hours before evaluation in each condition.

Experimental protocol

Subjects were studied in three different conditions: 

-           Basal condition (BAS): at an altitude of 300m asl

-           Altitude condition (ALT): at the altitude of 2503m ASL (in a lodge on the shore of Nivolet Lake, Piedmont - Italy) before the breath-hold diving training session.

-           After diving at altitude (DIV): after a session of breath-hold diving training at 2503m asl.

Breath-hold diving training consisted of two hours of repeated dives at an average depth of 22 meters under constant weight (i.e., without the use of ballast). Divers were free to organize their activity within the two hours of the study.

Basal evaluation was performed the evening before the ascent to Nivolet Lake and the diving training session.

In each condition, subjects were evaluated by the following procedures:

-           Measurement of systolic and diastolic blood pressure (SBP and DBP, respectively) and heart rate (HR). Blood pressure and heart rate were calculated as the average of three measures after five minutes of sitting rest. Mean blood pressure (MBP) was calculated as DBP + 1/3*(SBP-DBP).

-           Measurement of Hemoglobin saturation by a pulse oximeter (Intermed Sat 100, Intermed Srl; Milano - Italy) placed on the right forefinger.

-           The nitrous oxide (NO) breath analyzer measured fractional exhaled NO (Bedfont Scientific Ltd; Harriersham – UK). Subjects were asked to exhale in the analyzer mouthpiece at a constant rate until the instrument confirmed the correctness of the measurement.

Thoracic echography and Doppler echocardiography

Doppler echocardiographic files were recorded and stored for a following off-line analysis. Measurements were made according to the American Society of Echocardiography recommendations (13) by an expert in Doppler–echocardiography, unaware of the identity of the subjects and the condition of recording.

The following parameters were obtained as the average value of three measurements on consecutive cardiac cycles:

-           End-diastolic and end-systolic left ventricular volumes (EDV and ESV, respectively), measured from an apical four-chamber view by area-length method (14).

-           Right ventricular diastolic dimension.

-           Tricuspid annular plane systolic excursion (TAPSE).

-           Early (E) and late (A) peak transmitral diastolic flow velocities, their ratio (E/A), and the deceleration time of E velocity (DTE) were obtained from pulsed-wave Doppler tracings by sampling blood velocities at the level of mitral valve tips.

-           Velocity of mitral valve annular linear displacement in early diastole (e’) by Tissue Doppler Imaging.

-           The E/e’ ratio was calculated as an index of left ventricular filling pressure.

-           Left ventricular stroke volume (SV) was calculated as the difference between diastolic and systolic left ventricular volumes. The ejection fraction (EF) was calculated as 100*(SV/EDV). Finally, cardiac output (CO) was obtained from stroke volume and heart rate.

-           Total peripheral resistances were calculated as the ratio MBP/CO.

Lung echography: quantitative evaluation of ultrasound lung B-lines (BL) (an index of pulmonary interstitial edema previously called “ultrasound lung comets”) was performed following the methodology previously described by Gargani et al. (15)

Statistical analysis

All the variables were tested for the normality and homogeneity of variances, using Lilliefors and Mauchly's tests, respectively. Since all the variables showed a normal distribution with homogenous variances, a Repeated measures ANOVA with Scheffé's correction procedure was used to identify significant differences between the three different conditions of our study (i.e., BAS, ALT, and DIV conditions) for each variable. For all statistical tests, p-values of less than 0.05 (two-tailed) were considered statistically significant.

Statistical analysis was performed with MATLAB for Windows (MATLAB and Statistics Toolbox Release 2020a, The MathWorks, Inc., Natick, Massachusetts, United States).

RESULTS

Cardiac anatomy and function

Both diastolic and systolic left ventricular volumes showed a significant reduction along the three steps of the experiment (Fig. 1) (EDV in DIV condition: p<0.05 as compared to BAS; ESV in DIV condition: p<0.01 as compared to both BAS and ALT). No significant change was observed in the right ventricular diastolic dimension, even if an increase at the limits of statistical significance was observed in the ALT condition (P=0.07) compared to the other conditions.

No significant change was observed in left ventricular systolic indices (SV, EF) nor hemodynamic parameters derived from echocardiographic study (CO, TPR).

TAPSE was significantly increased at altitude (pre-diving) as compared to basal conditions (P<0.01) (Fig. 2).

Since only four subjects had a tricuspid regurgitation suitable to calculate the systolic pressure gradient between the right ventricle and right atrium, we could not estimate the systolic pulmonary arterial pressure.

As concerns left ventricular diastolic function indices, a significant reduction in E velocity was observed in the DIV condition as compared to pre-diving at altitude (ALT) (P<0.05). Moreover, a significant decrease in e’ was observed after diving at altitude (DIV) as compared to basal conditions (P<0.05). At the same time, E/e’ ratio was significantly increased in ALT as compared to BAS (P<0.05) (Fig. 3).

Hemodynamic parameters

No significant change was observed in the different experimental conditions for blood pressure (systolic, diastolic, and mean), cardiac output, and total peripheral resistances.

Hemoglobin saturation

A significant reduction was observed in both ALT and DIV as compared to the basal condition (P<0.001 and P<0.05, respectively) (Fig. 4, left panel).

NO concentration in exhaled air

A significant reduction of fractional exhaled NO was observed at ALT compared to basal conditions (12.4+9.6 vs. 20.0+11.5 ppb). A breath-hold diving session (DIV) did not further reduce fractional exhaled NO.

Ultrasound lung B-lines

Lung echography performed after breath-hold diving at altitude (DIV) showed significantly higher B-lines counts in comparison to basal evaluation (P<0.05) (Fig. 4, right panel).

DISCUSSION

Healthy individuals may suffer acute pulmonary edema when exposed to extraordinary environments (high altitudes or deep diving) or during strenuous effort (16-18). In these conditions, pulmonary capillary stress failure can be induced by three stimuli: hypoxia-mediated increase in pulmonary arterial pressure, intrathoracic blood shift while diving, and cardiac output increase during intense physical activity (19). Overlapping these basic stimuli may trigger acute pulmonary edema more predictably (20).

Breath-hold diving at altitude may represent the sum of several stressors for pulmonary capillary circulation since, in this condition, hypoxic pulmonary vasoconstriction may be due to the combined effect of both reduced atmospheric pressure and prolonged breath-holding. Moreover, centralization of blood due to immersion and, in the case of swimming, exercise-induced cardiac output increase may foster mechanical stress on the pulmonary capillary wall.

The present study aimed to evaluate the acute cardiovascular effects of two consecutive acute stimuli: reduction of Oxygen partial pressure (altitude) and repetitive breath-hold diving (inducing further hypoxic spikes and abrupt changes in environmental pressure and physical effort). Tests were done a few hours after reaching the altitude to avoid the effects of acclimatization.

We were able to document a significant increase of E/e’ at altitude (pre-diving), with no further increase after breath-hold diving, suggesting an increase in left ventricular filling pressure (21). This finding indicates that acute exposure to hypoxia at altitude is associated not only with the well-known increase in pulmonary pre-capillary resistances but also to an increase of blood pressure in the pulmonary capillary network, potentially contributing to capillary stress failure and high-altitude pulmonary edema (HAPE). It is interesting to note that the increase in E/e’ is almost entirely sustained by the reduction of e’ (diastolic velocities of the myocardium, most likely hypoxia-mediated), while left ventricular filling dynamics (as assessed by transmitral flow velocities) does not seem to further contribute to the mechanisms leading to pulmonary edema.

Observing a significant reduction in exhaled NO at altitude (but not after diving) may add further insight to understanding the pathophysiology of high-altitude acute pulmonary edema (HAPE). Conflicting results about the relationship between exhaled NO and hypoxic stimuli have been reported. Busch et al. (22) showed that HAPE-prone subjects showed a reduction in exhaled NO. In contrast, non-HAPE-prone subjects had no significant change in NO during acute exposure to normobaric hypoxia (breathing a hypoxic synthetic gas mixture corresponding to an altitude of 4500m ASL). Observation during a real ascent to 4559m ASL showed that HAPE-susceptible subjects did not show an increase in exhaled NO, in contrast with the increase in exhaled NO observed in non-HAPE-prone subjects (23). These two studies are hardly comparable since they followed two very different protocols (sudden exposure to hypoxia with no change in environmental pressure vs. progressive ascent in a hypoxic-hypobaric environment during a 22h transfer). However, they converge on a model in which disorders in NO synthesis during acute hypoxia may play a fundamental role in the pathophysiological chain leading to HAPE.

A later experiment (24) reporting the effects of a very fast ascent on the Mauna Kea volcano (4200m ASL) showed an average reduction in exhaled NO concentration with a wide range of individual variation (26% of subjects showed an increase in exhaled NO) with no apparent relationship between exhaled NO and self-reported symptoms of acute mountain sickness or HAPE. Finally, interesting results were reported by Vinnikov et al. (25), showing that intermittent exposure to hypobaric hypoxia may induce a reduction in exhaled NO after a further ascent to high altitude, suggesting that repeated hypoxic stimuli may blunt NO synthesis in response to hypoxic stress. We also found an average reduction in exhaled NO at altitude in a group of highly trained breath-hold divers; their habit of coping with repeated short hypoxic stimuli could explain this puzzling result.

Ultrasound lung B-lines are a simple and feasible semiquantitative sign of increased extravascular lung water and interstitial lung edema. Previous studies reported increased BL scores during ascent at high altitudes (26,27) and immediately after intensive sea-level breath-hold diving training (28). We observed a significant increase in ULS count, as compared to basal condition, only after diving at altitude. Although the subjects of the present study showed a significant reduction in hemoglobin saturation at altitude (both pre-dive and post-dive), it should be underlined that previous investigations studied the effects of much more severe environmental stress than ours. In the study by Pratali et al. (26), BL were detected at altitudes beyond 3000m ASL, while in the study by Frassi et al. (28) dives were done at remarkably higher depths (31-102m). Nevertheless, our results confirm that multiple overlapping stimuli (i.e., changes in environmental pressure, hypoxia, immersion, physical exercise), even if of mild to moderate intensity, may foster acute pulmonary edema (19,20).

A final comment is due to the observed changes in cardiac anatomy and function, as assessed by Doppler echocardiography. The significant reduction in left ventricular diastolic and systolic dimensions at altitude, both pre-and post-dive, could be explained by both the known diuretic effect of hypoxia (29) and diving (30)  and the possible acute reduction of venous return to left atrium secondary to hypoxic pulmonary vasoconstriction.

The increase in TAPSE (index of right ventricular systolic function) at altitude could be explained by the interplay of several factors. On the one hand, the direct depressive effect of hypoxia on the contractility of myocardial fibers may be outdone by the hypoxia-mediated catecholamines release (31). On the other hand, the increased RV afterload due to hypoxic pulmonary vasoconstriction could induce an increase in RV diastolic dimension and, in turn, an increase in systolic function mediated by the recruitment of the Franck-Starling mechanism. Unfortunately, the lack of subjects with physiological tricuspid regurgitation prevents the assessment of systolic pulmonary arterial pressure and a thorough analysis of pulmonary hemodynamics.

In conclusion, combining environmental hypoxia and breath-hold diving at altitude may contribute to interstitial edema, particularly by enhancing the hypoxic stimuli during diving. The measurement of exhaled NO may be a potential predictor of interstitial edema.

LIMITATIONS OF THE STUDY

The main limitations of our study were the low statistical power and the lack of standardized breath-hold diving activity.

Our paper should, therefore, be considered a generator of hypotheses to be confirmed in future studies on larger series and with standardized diving protocols.

The authors received no financial support for this research.

The authors declare no competing interests.

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FIGURE CAPTIONS

Fig. 1 Left ventricular end-diastolic (EDV) and end-systolic (ESV) volumes in the three conditions.

BAS = basal conditions (300m asl); ALT = altitude (2507m asl); DIV = post breath-hold diving session at altitude

Fig. 2 Tricuspid annulus plane systolic excursion (TAPSE) in the three conditions.

BAS = basal conditions (300m asl); ALT = altitude (2507m asl); DIV = post breath-hold diving session at altitude

Fig. 3 Left ventricular early filling velocity (E) and its ratio to early diastolic mitral valve annular linear displacement (E/e’) in the three conditions.

BAS = basal conditions (300m asl); ALT = altitude (2507m asl); DIV = post breath-hold diving session at altitude

Fig. 4 Haemoglobin (Hb) saturation and ultrasound lung B-Lines (BL) in the three conditions.

BAS = basal conditions (300m asl); ALT = altitude (2507m asl); DIV = post breath-hold diving session at altitude