Spirometry and Respiratory Muscle Function During Ascent to Higher Altitudes

Introduction

Humans are increasingly engaged in recreational activities at high altitudes (HA), such as mountain climbing and trekking. Increasing altitude is accompanied by a fall in barometric pressure and progressive decrease in the partial pressure of oxygen. Hypoxia and alterations in lung function play significant roles in morbidity and mortality related to high-altitude exposure [1–4]. Lung function changes at a HA may exacerbate the severity of hypoxemia beyond that expected due to low barometric pressure. These changes affect exercise capacity and promote predisposition to altitude illness. Earlier studies have evaluated the impact of altitude on lung function in high-altitude laboratories and via simulation in hypobaric chambers [5–7]. However, the conditions encountered during actual climbing expeditions differ from those in more controlled experiments, and limited data are available regarding spirometric measurements and respiratory muscle function alterations in an expedition setting [8–15]. Compared with altitude chamber studies, the results of terrestrial field studies may be confounded by several factors including acclimatization, weight loss, dehydration, sleep deprivation, thermal stress, and exposure to pollutants. At a HA, forced vital capacity (FVC) appears to consistently decrease, whereas conflicting results have been reported with regard to forced expiratory volume in 1 second (FEV) and maximal voluntary ventilation (MVV) [6–15]. The ascent to higher altitudes affects lung functions via mechanisms that remain to be completely clarified. A few hypotheses have been proposed to date. As the air density gradually lessens on ascent, expiratory airflow and lung emptying is facilitated [5]. Development of subclinical pulmonary edema is considered a pivotal factor in reducing FVC [6]. Respiratory muscle strength and endurance are extremely important for functioning at HA. However, the effects of HA, excessive ventilatory requirements, harsh environmental factors on maintenance of respiration, and the mechanisms by which these factors affect specific indices are not entirely clear. Several investigators have described diaphragmatic fatigue in exercising healthy humans under hypoxic conditions or HA [16–18], while longitudinal measurements of respiratory muscle pressure and endurance have not been previously performed at a HA. The purpose of this study was to determine the effects of acute and prolonged exposure to HA on lung function and establish the roles of respiratory muscle strength and endurance in mediating lung function changes under these conditions.

Abstract

Alterations in lung function at a high altitude (HA) influence exercise capacity, exacerbate hypoxia, and possibly promote predisposition to high-altitude illness. The effects of the HA on lung function and the mechanisms underlying these alterations are currently under investigation. Here, we present a review of the documented alterations in pulmonary physiology and lung functions at the HA in humans. As the first interface between the environment and the body, the lung plays a vital role in the transfer of oxygen from air to blood. The respiratory system adapts to hypoxic stress at rest and during exercise at a HA when oxygen and carbon dioxide flux from tissues is greater. Hypoxic stimuli induce an increase in ventilatory drive and metabolic cost incurred by the respiratory muscles in addition to substantial dyspnea, which may limit exercise tolerance. The exchange of oxygen and carbon dioxide is significantly affected owing to the lower driving pressure for oxygen from air to blood and more rapid transit time of blood across the pulmonary capillaries. Diffusion limitation and ventilation/perfusion mismatch additionally occur, thereby accentuating hypoxemia. Studies on lung and respiratory muscle functions have disclosed a fluctuating course for spirometric measurements, respiratory muscle strength, and endurance at the HA. Specifically, transient increases in these parameters initially occur on ascent to each new altitude, followed by a gradual decline during the longer stay. Despite these restrictions, humans cope remarkably well in times of considerable stress from the hypoxic environment.

Keywords

high altitude, ventilation, exercise, gas exchange, ventilatory drive, dyspnea, respiratory muscle endurance, spirometry