Acute Hyperventilation and Hypotension

A 5-ft, 48-kg, elderly woman had a blood pressure of 115/75 mm Hg and heart rate of 70 beats/min. She was receiving no medications and was in otherwise normal health. She was scheduled for a laparotomy and hysterectomy. After induction and orotracheal intubation, controlled ventilation was initiated with a tidal volume of 900 mL and a respiratory rate of 12 breaths/min. Shortly before incision, the arterial blood pressure was 58/35 mm Hg, and the end-tidal PCO₂ was 22 mm Hg. (Blood gases were not tested, but typical values for a patient experiencing such acute hyperventilation are pHa = 7.53, PaCO₂ = 26 mm Hg, bicarbonate = 21 mg/L, and BE = 0 mg/L.) The ventilator was immediately reset to provide 4 breaths/min at a tidal volume of 500 mL. Arterial blood pressure about 2 minutes later was 85/55 mm Hg, and the next reading about 2 minutes later still was 95/65 mm Hg. The patient's end-tidal PCO₂ increased more slowly, and after about 8 minutes, it had returned to 36 mm Hg when the ventilatory rate was increased to 8 breaths/min. (Typically, her blood gas values would be near normal, such as pHa = 7.44, PaCO₂ = 38 mm Hg, bicarbonate = 23.3 mEq/L, and BE = 0 mg/L.)

Hyperventilation after induction is common and frequently contributes to hypotension by various methods, including raised intrathoracic pressure, diminished venous return, and depressed cardiac output (see Chapter 17 ).