Respiratory Failure

Definition and Classification

• Defined as failure of gas exchange due to inadequate function of one or more of the essential components of the respiratory system.

• Classified as hypoxemic (PaO _ 60 mmHg), hypercarbic (PaCO _ 45 2 2 mmHg), or combined.

• Also classified in terms of acuity—acute respiratory failure reflects a sudden catastrophic deterioration, chronic respiratory failure reflects long-standing respiratory insufficiency, and acute or chronic respiratory failure is an acute deterioration in a patient with chronic respiratory failure, usually due to chronic obstructive lung disease. 

Pathophysiology

Respiratory failure occurs when one or more components of the respiratory system fails.

• Disorders due to failure of the central control system can be thought of as controller dysfunction, or central apnea.

• Failure of the respiratory pump—the diaphragm and intercostal muscles that move the chest wall—is termed pump dysfunction.

• Respiratory insufficiency attributable to narrowing, collapse, spasm, or plugging of the large or small airways can be termed airway system dysfunction.

• Respiratory failure due to collapse or flooding of or injury to the alveolar network can be considered alveolar network dysfunction.

• Disease resulting from obstruction, inflammation, or hypertrophy of the pulmonary capillary vessels can be termed pulmonary vascular dysfunction.

Many processes will involve more than one of these components of the respiratory system, but assessment of each compartment can provide a basis for differential diagnosis. 

Clinical Evaluation

Initial inspection should assess upper airway patency and signs of distress such as nasal flaring, intercostal retractions, diaphoresis, level of consciousness.Use of sternocleidomastoid muscles and pulsus paradoxus in a patient who is wheezing suggest severe asthma.Asymmetric breath sounds may indicate pneumothorax, atelectasis, or pneumonia.Oximetry permits rapid assessment of oxygenation. An arterial blood-gas measurement is required, however, to determine CO2 level and acid-base status.Because of the potential for rapid, possibly fatal, deterioration, therapy may need to be initiated without a definite diagnosis.

• Controller dysfunction is suggested by medication history, the absence of tachypnea (respiratory rate _ 12 breaths/min) in a patient with hypercarbia, altered level of consciousness.

• Pump dysfunction is suggested by supine abdominal paradox (diaphragmatic paralysis), peripheral muscle weakness, reduced maximal inspiratory pressure generation.

• Upper airway dysfunction is suggested by stridor, and lower airways dysfunction by wheezing.In ventilated patients obstruction can be deduced by inspection of the flow:time curve as displayed on most current ventilators. AutoPEEP (positive end-expiratory pressure), a sign of delayed emptying of the lungs in ventilated patients, is another sign of obstruction.

• Alveolar compartment dysfunction is evident when there are signs of consolidation on auscultation, with tubular breath sounds and dullness.Since alveolar flooding effectively increases the stiffness of the lung, respiratory compliance, as measured on the ventilator [VT/(end-inspiratory plateau pressure _ PEEP)], is reduced to _30 mL/cmH2O.

• Pulmonary vascular dysfunction is reflected indirectly by signs of right heart failure on exam (qP2,qJVP, right-sided heave). 

TREATMENT

• First priority is always to establish adequate oxygenation.If hypercarbia and acidosis coexist, mechanical ventilation should be strongly considered.

• Attention must always be paid to establishing airway patency, even if another cause of respiratory failure is present.This may mean removal of a foreign body, suctioning, or simply a jaw lift.

• With respiratory failure due to alveolar dysfunction, increasing end-expiratory lung volume with extrinsic PEEP may substantially improve arterial oxygenation.