Using Echocardiography to Study the Effects of Hypoxia and Altitude on Heart Function
INTRODUCTION: Physiological assessment of military pilots and aircrew is performed annually. This includes a simulation of a flight at maximum altitude (25,000 ft/7620 m) with acute hypoxia, where they can recognize their symptoms. By detecting the symptoms of hypoxia, they will take corrective actions to avoid hypoxia-induced impairment. However, there is little evidence of what happens in the heart under these conditions. Cardiac function can be evaluated noninvasively with transthoracic echocardiography. The objective was to evaluate the effect of hypoxia and altitude during this simulation on systolic and diastolic cardiac function, pulmonary artery systolic pressure, and cardiac output with transthoracic echocardiography.
METHODS: A total of 72 volunteers (33.90 ± 8.49 yr, 73.6% male) were studied. A baseline transthoracic echocardiography assessment was performed, and systolic and diastolic function were assessed in both left and right ventricles. Cardiac output and pulmonary artery systolic pressure were estimated. Measurements were repeated at 25,000 ft, with and without oxygen, when saturation was below 80%.
RESULTS: A significant decrease was observed under hypoxic conditions when evaluating both right ventricular (RV) systole (RV 12.25 ± 3.1 to 8.9 ± 2.3 cm · s−1) and diastole (RV 6.8 ± 3.5 to 4.8 ± 2.8 cm · s−1 and RV 8.5 ± 5.2 to 5.71 ± 4.1 cm · s−1). However, cardiac output remained stable (7.87 ± 0.58 to 7.68 ± 0.49 L · m−2).
DISCUSSION: Echocardiography is a useful tool for evaluating left and right cardiac ventricular function. The right ventricle, both in its systolic and diastolic function, was the most affected during a simulated hypobaric and hypoxic flight.
Cabrera Schulmeyer MC, Patiño-García D, Alvear M, Aravena D, Montiglio C. Using echocardiography to study the effects of hypoxia and altitude on heart function. Aerosp Med Hum Perform. 2025; 96(11):964–968.
The understanding of cardiac physiology in the military population is always of great interest, both in normal conditions and when the myocardium is subjected to extreme conditions such as high altitude and hypoxia. Hypoxia as encountered at high altitude has long been recognized as a cardiac stress due to the inverse relationship between altitude and barometric pressure. Individuals engaged in flight activities always have a latent risk of exposure to low oxygen concentrations in the event of an accident or system malfunction, which could lead to hypoxia and the risk of sudden in-flight incapacitation.
Transthoracic echocardiography is a noninvasive imaging test that provides high quality data. It gives information on the morphological and functional characteristics of the heart and an assessment of left and right ventricular systolic and diastolic function. This information allows accurate diagnoses and patient management decisions to be made. It is important to consider that the use of echocardiography also allows the evaluation of cardiac physiology both in normal and abnormal conditions.1
At the aerospace medicine center in the Clinical Hospital of the Chilean Air Force, medical assessment of all pilots and crews is performed. One of the tests is simulation of flight at maximum altitude (25,000 ft/7620 m) and during acute hypoxia so that the subjects can recognize the symptoms of hypoxia. Thus, training is required in the face of abnormal situations and to avoid flight emergencies or accidents due to low oxygen pressure in real life.
Hypobaric chambers are used to expose aircrews to a high-altitude environment in a controlled manner to allow them to become familiar with the signs and symptoms of hypoxia in a safe environment.2–4
The working hypothesis in this study was that cardiac function would be decreased during acute hypoxia in air crews at 25,000 ft. The objective of this study was to determine the extent of any hypoxia-induced changes in the systolic and diastolic function of both ventricles, cardiac output, and pulmonary artery systolic pressure.
METHODS
This study was a one-group pre-test/post-test design. After ethical approval by the Scientific Ethics Committee of the Clinical Hospital of the Chilean Air Force (Approval No. HF 72,307), each volunteer signed an informed consent after being provided with detailed explanation of the study procedures. The study was performed to the standards set by the Declaration of Helsinki.
Subjects
The inclusion criteria were healthy men and women between 18–60 yr of age who voluntarily signed an informed consent form. Exclusion criteria were as follows: subjects with hypertension, any type of arrhythmia, and poor echocardiographic windows. Subjects with evidence of barotitis were excluded. Subjects with abnormalities on their baseline echocardiogram, such as valvular heart disease or cardiomyopathy, were also excluded. Specifically, pulmonary systolic arterial pressure (PsAP) was measured and, if it was abnormal, the volunteer was excluded.
Equipment
The Physiological Training Program is a process of instruction and simulation in conditions as close to real life as possible that mimic the complex conditions of flight. Physiological variables like heart rate and blood oxygen saturation (Spo2) were measured constantly.
To know what changes occur in the myocardium both at high altitude and in the face of acute hypoxia, its systolic-diastolic function, cardiac output, and pulmonary artery systolic pressure were evaluated using transthoracic echocardiography as a noninvasive monitoring system during training (Ultrasound SonoScape X5 Exp/X5/X5 Pro, Medical Corp., China 20,054,866). The echocardiography operator was highly qualified with postgraduate level specialist training. The operator also underwent aeromedical training at the Clinical Hospital of the Chilean Air Force in its aerospace medicine center (CEMAE) by attending theoretical courses and participating in the activities at altitude and during hypoxia as a student. The echocardiographer learned how to diagnose her own hypoxia symptoms and passed this training prior to starting the study. A volunteer, part of the research group, was evaluated with the same echocardiography equipment at the same altitude and conditions to which the volunteers were to be subjected to demonstrate that the images and the Doppler worked with the same quality and that the equipment would not suffer alterations or damage due to changes in altitude.
Procedure
It seemed interesting to explore the systolic and diastolic function of both ventricles, as well as cardiac output and systolic pulmonary artery pressure. Tissue Doppler imaging is an accurate Doppler technique that is largely independent of the operator and preload. It is also quick to measure. Therefore, it was chosen and used for this purpose, positioning the transducer at the free edge of the mitral (left function) and tricuspid (right function) annulus where e prima (e′) and a prima (a′) waves were measured for evaluating diastolic function and s prima (s′) for systolic function. Measurement techniques do not correspond to the standards currently used in the assessment of left ventricular (LV) systolic function. Measuring LV ejection fraction using the Simpson Biplane method would have been too time consuming. The same criteria for the right ventricle (RV) were applied. RV ejection fraction has been demonstrated not to be a good parameter for assessing RV function. Tricuspid annular plane systolic excursion requires changing the system to M mode, which is again time consuming.
Diastolic function was measured using tissue Doppler assessing a′ and e′. The advantages are they are load independent and easy to capture for a video loop. The e′ and a′ waves reflect the pressure gradient at different times during diastole. The ratio between them was not calculated as we assumed that they would follow a trend during normoxia and hypoxia.
Finally, cardiac output was estimated by measuring the area of the outflow tract, the LV velocity integral, and multiplying it by the heart rate. Measurements were taken at three points in time, one at baseline, at 25,000 ft (7620 m) with oxygen, and finally at 25,000 ft without oxygen. For each measurement an echocardiography loop was recorded to do a revision and measurements after finishing the simulation and reinterpreting the data at a later stage. During the Hypoxic period, the time for doing echocardiography was extremely short, so we needed to do a proper selection of each variable. Tissue Doppler was selected because of its accuracy and low time consumption for obtaining the echocardiographic loop.
Statistical Analysis
For the statistical analysis, jamovi (version 2.3)5 was used to analyze the data. The assumption of normality was checked using the Kolmogorov-Smirnov test. The assumption of homogeneity was met given that there were no factors specified between subjects. A descriptive analysis was carried out where the categorical variables are presented as frequency tables, while summary measures were calculated [mean ± SD or median (interquartile range)] for quantitative variables.
The difference between the observed pre-test and post-test values were analyzed using the paired t-test for parametric variables or the paired Wilcoxon rank test for nonparametric variables. P < 0.05 was considered statistically significant.
RESULTS
The characteristics of the subjects studied are shown in Table I. A tendency to have higher BMIs was observed in men.
The pre/post results of the echocardiographic variables studied in both the pre-test and the post-test in the aircrew members are shown in Fig. 1. The baseline echocardiogram of all volunteers was normal. The basal heart rate of female volunteers was significantly higher than that of males.
Citation: Aerospace Medicine and Human Performance 96, 11; 10.3357/AMHP.6627.2025
A significant increase in heart rate, as well as pulmonary artery systolic pressure, was evident (P < 0.0001) (Fig. 2A, C) without modifying cardiac output (P = 0.9427) (Fig. 2B). No significant changes were found in LV systole (P = 0.5919) (Fig. 2D). However, a decrease in LV diastole was observed when evaluating a′ LV (P = 0.0006) without modifying e′ LV (P = 0.1419) (Fig. 2E, F). Furthermore, a significant decrease was observed when evaluating both right ventricular systole (s′ RV) (P < 0.0001) and diastole (e′ RV and a′ RV) (P < 0.0001 and P = 0.0006, respectively) in response to acute hypobaric hypoxia at 25,000 ft (7620 m) (Fig. 2G–I).
Citation: Aerospace Medicine and Human Performance 96, 11; 10.3357/AMHP.6627.2025
DISCUSSION
An impairment of systolic and diastolic function of the RV was detected in this study; however, cardiac output remained stable. When cardiac function was assessed, both heart rate and pulmonary artery systolic pressure increased in response to acute hypoxia, but no changes were observed in LV systolic function or cardiac output, which remained normal during all three echocardiographic assessment times. Acute hypoxia statistically significantly decreased right ventricular function, with a decrease in s′, e′, and a′ values.
As the RV probably does not achieve good relaxation in acute hypoxia, this prevents the right atrium from generating a good filling of the RV, and thus it is not able to overcome the resistance of the pulmonary artery. This would generate retrograde volume accumulation and dilatation of both right chambers. Because of low pulmonary artery pressure, a secondary effect would be vasoconstriction of the pulmonary artery and not a primary phenomenon as has been proposed in the case of chronic hypoxia.
The basal heart rate was elevated in volunteers, which may have been due to anxiety about the simulated flight. In general, altitude studies only go up to 20,000 ft (6096 m) in the case of aviation and this study reached 25,000 ft (7620 m). Although previous studies have evaluated alterations in cardiac function in response to chronic hypoxia at altitude (more than 10 d of exposure), the present study investigated the effects of acute high altitude on biventricular function.
The physiological response of pulmonary circulation during chronic hypoxia is to increase pulmonary arteriolar resistance, resulting in elevated PsAP. In this study only a mild and nonsignificant elevation of PsAP was seen. Maufrais et al.6 analyzed the left and right ventricles in 24 trekking volunteers, and they found higher volumes in the RV but in chronic hypoxia.
The case of mountaineers where myocardial function has been studied with echocardiography is different because they experience an acclimation process and preconditioning for hypoxia, and subjects have been trained and were not acutely subjected to hypoxia, but rather gradually and sequentially to what became a chronic hypoxia model and not the acute model that is being proposed here.7–9 However, it is important to differentiate acute hypoxia from chronic hypoxia, since during the latter there are adaptive changes in pulmonary and cardiac physiology, as in the case of inhabitants of places located above 13,120 ft (4000 m) or in the case of trained mountaineers.10,11
Acute hypoxia at altitude results in impaired right ventricular diastolic function in aircrews. However, RV diastolic function is rarely quantified in experimental studies, generating a gap in knowledge that needs to be addressed to understand the physiological response of RV systolic and diastolic function in response to different stimuli and/or pathophysiological conditions such as hypoxia, so it was thought that there could be an acute deterioration of RV diastolic function and that this alteration could eventually be the cause of the deterioration in tissue oxygenation.12,13
It is proposed that echocardiography can play an important role during flight simulation, since it would allow the diagnosis and evaluation of some changes in the cardiac cavities and great vessels in their function in the face of acute hypoxia at high altitude.14
The decision about which parameters to study with transthoracic echocardiography was based on the existing literature, the data that would be of interest to evaluate, and the set time for doing the echocardiographic assessment. It will be interesting in the future to explore other variables of myocardial function such as strain (myocyte acceleration and deceleration), strain rate, and speckle tracking. For this purpose, an ultrasound machine with higher technology software is required.15
In the future, we plan to compare the men’s group with the women’s group separately. We believe that there are differences in women due to a greater tachycardia and greater impairment of RV function due to altitude and hypoxia. We are also already working on a protocol where the groups will be separated by sex to look for physiological end echocardiographic differences that probably exist. We will try to integrate more variables to measure such as the ejection fraction with the Simpson technique and E and A waves from transmitral inflow.
We must acknowledge several study limitations. All the subjects were studied in a sitting position, which is not the best position for doing an echo. The time for doing a focused echocardiography was extremely short according to the air force protocols for crews at 25,000 ft (7620 m). The study was carried out on volunteers who knew they would be subjected to special conditions, which probably caused them some anxiety and fear of failure, and that they would be observed and monitored during the entire simulation.
There are few studies that measure the values studied in this sample of aircrews. Thus, it is difficult to determine a precise baseline value for each parameter studied. The use of tissue Doppler can also be controversial and under-explored in the literature given its relative novelty, but obtaining the data is very fast, which was an attractive argument when planning the study. It was chosen due to its greater accuracy and greater operator independence, knowing that there is practically no literature to compare.
In conclusion, this study, carried out in 72 healthy volunteers at 25,000 ft (7620 m) and in hypoxia, shows that the most affected chamber is the RV and that there is a clear tendency to maintain cardiac output. The data suggest that acute hypoxia decreases right ventricular function in the subject studied, with impaired systolic and diastolic RV function as assessed by tissue Doppler. However, the overall trend was toward preservation of LV function.
Study protocol.
Study results. HR: heart rate; CO: cardiac output; PsAP: pulmonary systolic artery pressure; s’ LV: s prima left ventricle; a’ LV: a prima left ventricle; s’ RV: s prima right ventricle; e’ RV: e prima right ventricle; a’ RV: a prima right ventricle.
Contributor Notes
