Pathophysiologic Manifestation

• Airway diameter reduction, caused by:
  • smooth muscle contraction
  • vascular congestion
  • bronchial wall edema
  • thick, tenacious secretion

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•  Pulmonary function compromise in acute asthma:
  •  ECG changes indicative of right ventricular hypertrophy and pulmonary hypertension
  •  vital capacity < 50% of normal
  •  1 - second forced expiratory volume (FEV₁): reduced to 30% of expected
  •  maximum/a minimum midexpiratory flow rates: reduced to 20% or less of expected
  •  Significant air trapping
    • in acute illness: residual volume (RV): may approach 400% of normal; functional residual capacity doubles
    • patients report that the attack has ended clinically when RV has fallen to 200% of expected and FEV₁ reaches 50% of expected level

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• Hypoxia:
  • very common finding during acute phase
  • respiratory failure -- relatively uncommon: 10% to 15% of presenting patients
  • hypercapnea: common
  • respiratory alkalosis: common

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• Acute Asthma:
  •  normal arterial carbon dioxide tension but severe airway obstruction present is suggestive of possibly impending respiratory failure
  • Metabolic acidosis in acute asthma with severe obstruction
  •  Clinical presentations may not correlate with blood gas data
    • however, estimating patient's ventilatory state solely on clinical presentation may be dangerous.
    • Patients with possible alveolarhypoventilation should have arterial blood gas tension measurements
  •  Cyanosis: -- very late sign; suggestive that extreme hypoxia may go undetected.

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