Respiratory mechanics in the intensive care unit

Abstract

Listen

Translate article

Assessment of respiratory mechanics can play a central role in the management of critically ill patients undergoing artificial ventilation because of acute respiratory failure (ARF) [1]. This is a condition defined by a rapid deterioration in pulmonary gas exchange that may be due either to alterations in the mechanical properties of the respiratory system leading to ventilation‐perfusion mismatching or shunt, or to neuromuscular insufficiency causing alveolar hypoventilation. Assessment of respiratory function and mechanics is of crucial importance: . to understand the pathophysiology of the disease underlying ARF; . to assess the status and progress of the disease; . to provide guidelines for therapeutic measures (positive end-expiratory pressure, bronchodilators, fluids); . to improve patient–ventilator interaction; . to prevent ventilator-related complications; . to plan the discontinuation of mechanical ventilation. Despite the great importance of monitoring lung mechanics in ventilator-dependent patients, these measurements are not regularly performed. This is probably due to the general prejudice that these measurements are difficult to obtain in the intensive care unit (ICU). This chapter will be focused on respiratory mechanics rather than on general respiratory physiology reflecting the attitude and expertise of the authors. The purpose of this chapter is to review briefly the most common methods and techniques for measuring and monitoring respiratory mechanics at the bedside of the patient in the ICU. We will go through a three point analysis: 1) measurements during controlled mechanical ventilation, i.e. in the relaxed, passive patients; 2) evaluation of respiratory mechanics during assisted mechanical ventilation; 3) the issue of the patient’s evaluation in the weaning process from mechanical ventilation. Before discussing the monitoring techniques, it is necessary to provide a brief overview of the mechanical properties of the respiratory system. The act of breathing is performed against several impediments: elastic, resistive, viscoelastic, plastoelastic, inertial and gravitational forces, compressibility of intrathoracic gas, and distortion of the chest wall from its relaxed configuration. Despite its apparent complexity, the dynamics of breathing have been satisfactory represented, at least for clinical purposes, by a single-compartment model consisting of a rigid tube and a compliant balloon [2, 3].