Chapter 2: Physiologic Goals of Mechanical Ventilation

Objectives

1. Discuss the pressure and volume targets to be used when ventilating patients.

2. List the guidelines for the selection of tidal volume, plateau pressure, and driving pressure during ventilation of acutely ill patients.

3. Define permissive hypercapnia, discuss when it should be employed, and discuss problems with its use.

4. Discuss concerns regarding the use of high oxygen concentrations in critically ill patients.

5. List the gas exchange and acid-base targets for critically ill patients.

6. Discuss concerns regarding patient-ventilator synchrony.

Introduction

Most 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 is not prudent to target normal blood gas values irrespective of the tidal volume (V_T), pressure applied, or FIO₂. The inappropriate application of the ventilator causes lung injury, activates inflammatory mediators, and potentially causes or extends multisystem organ failure. Of particular concern are patients whose lungs have abnormal mechanics. Regardless of the pathophysiology requiring ventilatory support, the primary goals of mechanical ventilation are to (1) cause no additional injury, avoiding ventilator-induced lung injury by minimizing lung stress, strain, and FIO₂; (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 predicted body weight (PBW). We now know that this V_T is excessive for any patient who requires mechanical ventilation. A V_T of greater than 8 mL/kg PBW should be avoided in all acutely ill patients regardless of their lung mechanics. The only time that a V_T of greater than 8 mL/kg PBW might be acceptable is during the brief transition from invasive ventilation to spontaneous breathing, and even then, the V_T should not be greater than 10 mL/kg PBW. 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 28 cm H₂O if chest wall compliance is normal. This is achieved by using a V_T of 4 to 8 mL/kg PBW for all patients requiring mechanical ventilation for acute respiratory failure. Exceeding this Pplat target should be avoided in the absence of increased pleural pressure such as abdominal hypertension or morbid obesity.