Work of Breathing

The work of breathing is the energy the respiratory muscles expend to move air in and out of the lungs against the elastic and resistive loads of the respiratory system. Mechanically it is the integral of pressure over the volume moved; physiologically it is reflected in the oxygen the respiratory muscles consume to sustain ventilation.

Definition

The work of breathing is the mechanical work done by the respiratory muscles on the respiratory system during ventilation, calculated as the integral of the applied pressure with respect to the volume change, and partitioned into the work needed to overcome elastic recoil and the work needed to overcome resistance to airflow and tissue deformation.

Scope

This topic covers how respiratory work is defined and measured, its division into elastic and resistive (and within resistive, airway and tissue) components, the optimization of breathing pattern to minimize work, and the oxygen cost of breathing. It is a reference account of a mechanical and energetic quantity and offers no clinical management advice.

Core questions

• How is work of breathing computed as the area under a pressure-volume loop?
• How is total work partitioned into elastic and resistive components?
• Why does the body select a breathing frequency that minimizes total work?
• What is the oxygen cost of breathing, and when does it become significant?

Key concepts

• Pressure-volume work loop
• Elastic work
• Resistive work
• Oxygen cost of breathing
• Optimal breathing frequency
• Respiratory muscle energetics

Key theories

Minimum-work optimization of breathing pattern

For a required alveolar ventilation, slow deep breaths increase elastic work while rapid shallow breaths increase resistive work; the respiratory controller tends to settle on a frequency and tidal volume that minimize the sum, the total mechanical work of breathing.

Mechanisms

Mechanical work equals pressure multiplied by the volume it moves; for breathing it is found by integrating the transpulmonary or transrespiratory pressure over the inspired volume, which corresponds to the area enclosed by the pressure-volume loop of a breath. This work has two parts: elastic work, stored in stretching the lung and chest wall and largely recovered as recoil during expiration, and resistive work, dissipated as heat in driving air through the airways and deforming tissue. Because elastic work rises with larger tidal volumes while resistive work rises with faster flows, there is, for any required ventilation, a breathing frequency at which total work is least, and quiet breathing tends toward it. The energy supplied by the respiratory muscles is reflected in their oxygen consumption — the oxygen cost of breathing — which is small at rest but can rise steeply when the elastic or resistive loads are high.

Clinical relevance

An increased work of breathing — from stiff lungs, narrowed airways, or high ventilatory demand — raises the metabolic cost of breathing and can contribute to respiratory muscle fatigue, and reducing this load is part of the physiological rationale for assisted ventilation. The concept also frames concern that mechanical ventilation itself can impose injurious pressures and volumes. This entry describes physiology and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The mechanical partitioning of respiratory work and the optimization of breathing pattern derive from classic physiological studies and are summarized in standard texts; the clinical importance of mechanical loads on the lung, including iatrogenic injury, is treated in the critical-care literature.

History

The energetics of breathing were quantified in the mid-twentieth century, when Otis, Fenn and Rahn analysed the mechanical work of breathing and showed that breathing pattern tends to minimize it, while DuBois and others developed methods to measure the underlying resistances and compliances. The later recognition that excessive mechanical loading can injure the lung extended the relevance of these mechanics to intensive care.

Key figures

• Arthur B. DuBois
• Arthur Otis
• Wallace Fenn
• Hermann Rahn

Seminal works

• dubois-1956
• otis-1954

Frequently asked questions

What are the two main components of the work of breathing?

Elastic work, done to stretch the lung and chest wall and largely returned as recoil during expiration, and resistive work, dissipated as heat in driving air through the airways and deforming tissue.

Why is there an optimal breathing frequency?

For a given required ventilation, breathing slowly and deeply increases elastic work while breathing rapidly and shallowly increases resistive work; an intermediate frequency minimizes the sum, so quiet breathing tends to settle near that least-work pattern.