Discuss the difference between hypercapnic and hypoxemic respiratory failure and list the causes of each.
Describe the indications for mechanical ventilation.
Discuss concerns and approaches to the initiation of mechanical ventilation.
Discuss the criteria used to initially set the mechanical ventilator for patients with normal lungs, and with obstructive and restrictive diseases.
Discuss the ethical considerations related to initiation of mechanical ventilation.
Ventilatory support should be instituted when a patient’s ability to maintain gas exchange has failed to the level that death is imminent if support is not provided. Respiratory failure is categorized as hypercapnic or hypoxemic. Once the decision is made to initiate mechanical ventilation, selection of the initial ventilator settings is based on the patient’s physiologic status and the best available evidence. Whenever mechanical ventilation is considered, the ethical consequences of the decision must also be addressed.
Hypercapnic Versus Hypoxemic Respiratory Failure
Hypoxemic respiratory failure is characterized by a failure to oxygenate. Hypercapnic respiratory failure is a failure of the ventilatory pump (ventilatory muscles). Frequently, respiratory failure is a result of both hypoxemic and hypercapnic failure, and can be classified as compensated or uncompensated.
Hypercapnic Respiratory Failure
The ventilatory pump comprises the diaphragm and chest wall muscles, as well as their neural control. This is responsible for ensuring adequate alveolar ventilation. Four aspects of the ventilatory pump, either alone or in combination, can result in pump failure: weak muscles, excessive load, impaired neuromuscular transmission/motor neuron disease, or decreased respiratory drive (Table 14-1). Hypercapnic respiratory failure results in an elevated PaCO2.
Table 14-1Causes of Hypercapnic Respiratory Failure ||Download (.pdf) Table 14-1 Causes of Hypercapnic Respiratory Failure
Inadequate ventilatory muscle function
Impaired neural transmission
Spinal cord injury
Motor neuron disease
Excessive ventilatory load
Decreased central ventilatory drive
Pharmacologic agents (sedatives and narcotics)
Idiopathic central alveolar hyperventilation
Severe medullary brainstem injury
Weak respiratory muscles may occur as a result of inherited myopathies and muscular dystrophies, malnutrition, electrolyte imbalance, inadequate peripheral nerve function, or compromised substrate delivery. Chronic pulmonary disease and neuromuscular disease may precipitate pump failure because of a decrease in the force-velocity relationship of the muscle, decreasing maximal muscular contraction. Ventilatory muscle force may also be decreased by the mechanical disadvantage caused by a flattening of the diaphragm as in severe chronic obstructive pulmonary disease or a deformed thoracic cage as in kyphoscoliosis. Patients in the ICU who are mechanically ventilated, especially those ...