INTRODUCTION

The primary function of the respiratory system is the oxygenation of, and elimination of, carbon dioxide from the mixed venous blood. The two major components of the respiratory system are the lung (the gas exchange organ) and the respiratory muscles (the pump that ventilates the lung); each contributes to the performance of this gas exchange function. Disease leading to dysfunction of either of these two components can result in respiratory failure.

Inadequacy of gas exchange is reflected in the patient’s blood gas values. Respiratory failure is conventionally defined as a partial arterial pressure of oxygen (PaO₂) of less than 60 mm Hg on a fractional concentration of oxygen in inspired gas (FiO₂) of over 0.5 or an arterial partial pressure of carbondioxide (PaCO₂) of more than 50 mm Hg.¹ Clinical signs of impending or existing respiratory failure include an increasing respiratory rate and increased work of breathing as evidenced by the use of accessory muscles of ventilation, and assumption of abnormal body postures.

Failure of each part of the respiratory system results in a distinct clinical picture (Box 15-1). Failure of the ventilatory pump results in alveolar hypoventilation and hypercapnia. Mild oxygen-responsive hypoxemia is seen in hypoventilating patients; however, the hallmark of ventilatory failure is an elevation in PaCO₂. In contrast, failure of the lung leads to hypoxemia with normocapnia or hypocapnia. In hypoxemic respiratory failure, the PaCO₂ remains normal or decreased because hypoxemia stimulates the ventilatory drive.

Although discussed as separate entities, both types of respiratory failure can coexist in a patient. Most of the lung diseases that lead to hypoxemia also increase the work of breathing and therefore the energy demands of the respiratory muscles. Hypoxia itself decreases the amount of energy available to the respiratory muscles, predisposing them to fatigue. In patients with severe or persistent lung failure, the increased ventilatory demands can exceed ventilatory capacity leading to pump failure secondary to respiratory muscle fatigue.²