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 the neural control of them. 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 Graphic Jump Location Table 14-1Causes of Hypercapnic Respiratory Failure ||Download (.pdf) Table 14-1 Causes of Hypercapnic Respiratory Failure
Inadequate ventilatory muscle function
• Electrolyte imbalance
• Pharmacologic agents
– Long-term corticosteroids
– Aminoglycoside antibiotics
– Calcium channel blocking agents
• Inherited myopathies and muscular dystrophies
• Mechanical disadvantage
– Flattened diaphragm
– Thoracic deformity
Impaired neural transmission
• Spinal cord injury
• Motor neuron disease
• Neuromuscular blockade
Excessive ventilatory load
• Mucosal edema
• Increased dead space
• Increased carbon dioxide production
• Dynamic hyperinflation (auto-PEEP)
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. Long term use of corticosteroids and aminoglycoside antibiotics or calcium channel blockers can impair neuromuscular transmission. 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 ...