Respiratory mechanics during general anaesthesia

Abstract

Intraoperative mechanical ventilation is mandatory during many surgical procedures. Knowledge in this field has been widely derived from the experience in the treatment of patients with acute respiratory distress syndrome in the intensive care unit. However, also in surgical patients without lung injury, mechanical ventilation settings affect the clinical outcome, and in particular the occurrence of postoperative pulmonary complications (PPCs). A deep understanding of respiratory physiology is mandatory for the clinician, in order to tailor ventilation settings based on the specific characteristics of each patient. In this paper we will discuss the basis of lung physiology applied to the mechanical ventilation in the operating room. The role of compliance, tidal volume, positive end-expiratory pressure (PEEP), plateau pressure, driving pressure, stress index, mechanical power and other ventilator-derived parameters will be discussed. The above-mentioned physiological parameters are easy to measure and can guide the clinician to assess and titrate mechanical ventilation parameters, but the clinical impact of guiding mechanical ventilation based on these parameters has yet to be determined.

Keywords: Surgery; mechanical ventilation; pulmonary physiology.

Figure 1

Visual inspection of ventilator curves. (A) Measurement of ∆P using an end-inspiratory pause during VCV; (B) visual assessment of the stress index (see text); (C) flow-volume loop in a patient with normal resistance (continuous line), airway obstruction (red dashed line) and airway collapse (maroon dotted line). Note that in both airway obstruction and collapse the flow does not reach zero at end expiration. VCV, volume-controlled ventilation.