Experimental approaches to the study of the mechanics of breathing during exercise

Abstract

This review describes the methodology and analysis of respiratory mechanics as it pertains to dynamic exercise. Underlying physical principles governing respiratory mechanics and commonly used measuring instruments will be discussed. We explain the physiological basis behind respiration, along with the dynamics of pulmonary ventilation. This review will outline the theoretical framework behind several forms of analysis along with their specific pitfalls, advantages and assumptions. Particular attention will be given to the techniques used to estimate the mechanical work of breathing. Specifically, we detail the different styles of work of breathing analysis and their inherent limitations as well as common sources of error often encountered. Finally, recent technological advancements that contribute to the understanding of respiratory mechanics are explained.

Introduction

The healthy respiratory system, as the body's first and last lines of defence for O₂ and CO₂ transport, faces several major challenges during dynamic exercise. The precise regulation of the partial pressures of arterial O₂ (${\text{Pa}}_{{\text{O}}_2}$) and CO₂ (${\text{Pa}}_{{\text{CO}}_2}$) and during exercise means that alveolar ventilation $({\dot{V}}_{\text{A}})$ increases substantively over resting levels during heavy exercise. The capacity of the healthy respiratory musculature for force development is adequate to meet this task but it is important that the cost of ventilation not be excessive. To this end, the neural networks and reflex mechanisms which control breathing and ensure that ventilation is raised in proportion to metabolic requirements must also control intra- and extra-thoracic airway calibre, the synchronization of neural motor outputs to the upper airway and respiratory pump muscles, along with precise regulation of breathing pattern and breath duty cycle. The mechanical aspects of breathing can be described by the interaction of the lung, the chest-wall, including the abdomen, and the respiratory muscles that act upon them. Understanding and quantifying the complex mechanical features of breathing along with ventilation during exercise has been a focus of physiologists for many years. Not surprisingly, a myriad of technical approaches have been used to address contemporary problems, each with inherent benefits and limitations. Moreover, it is important to appropriately analyze the measure in order to understand what is being inferred.

The purpose of this review is to describe the following areas of respiratory mechanics: (a) How the respiratory system behaves in accordance to the physical principles of fluid dynamics and pressure–volume constants; (b) How pulmonary ventilation occurs and why it is necessary; (c) Instruments used to measure respiratory mechanics, along with their advantages and disadvantages; (d) The many types of analysis that can be undertaken along with their inherent advantages and limitations; (e) How respiratory mechanics can be artificially altered during exercise; and (f) Modern instruments and techniques which can assess respiratory mechanics. Our review does not provide an exhaustive description of all methodologies. Rather, we focus on commonly used instruments and analysis methods as applied to dynamic exercise.