Chapter 2: Physiologic Goals of Mechanical Ventilation

Introduction

Many clinical management decisions are designed to return abnormal physiologic function to normal or to return abnormal laboratory data to normal. However, during mechanical ventilation, it may not be prudent to target normal blood gas values irrespective of the tidal volume (V_T) delivered, pressure applied, or Fio₂ used. The inappropriate use of the ventilator may cause lung injury, activate inflammatory mediators, and potentially cause or extend multisystem organ failure. Of particular concern are patients with acute respiratory distress syndrome (ARDS), asthma, or chronic obstructive pulmonary disease (COPD), whose lungs have abnormal mechanics. Regardless of the pathophysiology requiring ventilatory support, the primary goals of mechanical ventilation should be to (1) cause no additional injury, avoiding ventilator-induced lung injury by minimizing lung stress and strain, (2) maintain gas exchange and acid-base balance at a level appropriate for the specific patient, accepting hypercapnia and hypoxemia where indicated, and (3) ensure patient-ventilator synchrony, selecting the mode and ventilator settings that best match the patient's respiratory drive while ensuring lung protection.

Tidal Volume and Alveolar Distending Pressure

Tidal Volume

In the past, approaches to mechanical ventilation suggested V_T of 10 to 15 mL/kg of ideal body weight (IBW). We now know that these V_T are excessive for any patient who requires mechanical ventilation. A V_T of greater than 10 mL/kg IBW should be avoided in all acutely ill patients regardless of their lung mechanics. Since it is impossible to clinically detect localized overdistention, an acceptable V_T in a given patient must be judged relative to alveolar distending pressure.

Alveolar Distending Pressure

Alveolar distending pressure is assessed by measuring end-inspiratory plateau pressure (Pplat), which reflects mean peak alveolar pressure. To measure Pplat, a 0.5- to 2-second end-inspiratory breath-hold is applied. Pplat should be limited to 30 cm H₂O if chest wall compliance is normal. This is generally achieved by using a V_T of 4 to 8 mL/kg IBW for patients with ARDS and a V_T no greater than 10 mL/kg IBW for any patient requiring acute mechanical ventilation. Exceeding this Pplat target should be avoided in the absence of a stiff chest wall.

Positive End-Expiratory Pressure

The recommended level of positive end-expiratory pressure (PEEP) is 8 to 15 cm H₂O for mild ARDS and 10 to 20 cm H₂O for moderate to severe ARDS, which is needed to maintain lung recruitment. If ...