4: Mechanical Ventilation

INTRODUCTION

Mechanical ventilation (MV) is an essential tool for critically ill patients. While emergency physicians are well known for their expertise in emergent airway management, securing the airway is only a fraction of their role. Ventilator management is a crucial facet of emergency medicine, because if it is not applied correctly, it can worsen the clinical course and increase morbidity and mortality.¹ In the past two decades, our understanding of ventilator-induced lung injury (VILI) has resulted in lowering tidal volumes, minimizing barotrauma, and safely using positive end-expiratory pressure (PEEP) to reduce atelectrauma. In addition, we have realized the importance of mitigating patient-ventilator dyssynchrony in order to eliminate biotrauma. Given the current crisis of intensive care unit (ICU) overcrowding and increased critical care volume, critically ill patients have increased lengths of stay in the emergency department (ED) and, at times, are boarded for several hours or even days until a bed is available in the ICU.^2,3 The emergency physician must understand the intricacies of MV for heterogeneous patient populations with dynamic pathologies: No “single setting fits all.” Both patient care and outcomes will improve with special consideration of each patient's needs.

INDICATIONS FOR MECHANICAL VENTILATION

Indications for intubation and institution of MV fall under three basic categories: respiratory failure, airway protection, and anticipation of clinical deterioration. The most common indication for MV is either hypoxic or hypercapnic respiratory failure. These patients frequently have ventilation/perfusion mismatch (a decrease in ventilation or an increase in perfusion to a normally ventilated lung), hypoventilation, shunt, or a decreased ability to manage the work of breathing. These can lead to hypoxemia, hypercapnia, or both. Hypoxemia is often defined as a PO₂ less than 60 mm Hg and hypercapnia is often defined as a PCO₂ greater than 50 mm Hg.

Since hypercapnia is a component of hypoventilation directly and indirectly results in elevation of CO₂, an exact number is less important than the clinical picture. For example, in certain populations, such as patients with COPD who chronically retain CO₂, a higher baseline PCO₂ of 45 to 55 mm Hg can be well tolerated. However, an acute rise in CO₂ from the patient's baseline can cause lethargy, sleepiness, confusion, and altered mental status. In these hypercapnic patients, the primary action of MV is the promotion of appropriate alveolar ventilation with an end goal of enhanced CO₂ clearance.

In the ED, acute intoxication, altered mental status, and massive upper gastrointestinal hemorrhage are commonly seen and are another indication for MV in order to protect the airway from aspiration that could cause significant morbidity and even mortality.

Last, the predicted clinical course will often dictate securing an airway to either facilitate workup, offer definitive treatment, or to stabilize a clinical situation that may be likely to progress to clinical decompensation. For instance, ...