Chapter 83 Acute Respiratory Failure (Case 41)
Patient Care
Clinical Thinking
• It is helpful to take a stepwise approach to support of a patient in respiratory distress:
• Oxygen should be administered while an evaluation is in progress. Oxygenation improves with the delivery of supplemental oxygen or with the delivery of positive pressure, which recruits closed/collapsed airways and alveoli and in so doing decreases mismatch. Oxygen can be delivered by nasal cannula (delivers up to 32%), simple face mask (maximum delivery 35%), and a nonrebreather mask (maximum 60%). Because of poor mask fit and room air entrainment oxygen delivery by these simple devices may not approach these levels. Improvement of ventilation can be accomplished by improving the match so positive pressure systems such as continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) are beneficial. Finally, patients may need the support that can only be delivered via tracheal intubation and use of mechanical ventilation.
• Although it is tempting to treat the symptomatic patient reactively, it is essential, whenever possible, to gather full information, including history and appropriate tests, before interventions.
• Support of a patient in severe respiratory failure will necessitate tracheal intubation and ventilator support. The guiding principle is support with the least morbidity. Ventilators damage lungs and should be used with settings that are as low as can deliver the desired level of support. Mean airway pressures below 20 cm H2O reduce pressure injury to the alveoli (barotrauma). Permissive hypercapnia (allowing increased carbon dioxide) and subsequent respiratory acidosis is safer (if pH >7.25) than using higher airway pressures. Positive end-expiratory pressure (PEEP) should be set at a level that maintains open airways and effectively recruits collapsed lung units without decreasing cardiac output. Oxygen exposure should also be minimized: in general, inspired oxygen less than 60% is considered “safe.”
History
• What are the presenting symptoms?
• Cough indicates irritation in the airways with or without increased mucous (sputum) production. Where the secretions seem to originate is also helpful: copious nasal secretions in young children almost always indicate a viral illness, and a primary bacterial pneumonia usually has a cough producing sputum. The quality of the cough may also suggest a diagnosis
• Agitation occurs in both hypoxemia and in patients with hypercarbia before the onset of lethargy and coma.
• What is the time course of the illness? Was it sudden in onset, or had it been going on for some time and gradually worsened?
Physical Examination
• Evaluation of signs:
• Increased respiratory rate may indicate the degree of respiratory failure, although it may be a sign of increased respiratory demand (e.g., the patient who is acidotic or with fever).
• Decreased respiratory rate may indicate failure of central control that occurs in central nervous system diseases, metabolic diseases, and drug effects, but also occurs in severe status asthmaticus and hypothermia.
• Prolonged exhalation usually indicates lower airways obstruction such as asthma but can be present in congestive heart failure (CHF) as well.