MEDizzy - A 150-lb patient scheduled for abdominal surgery is sent for

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Respiratory Physiology

A 150-lb patient scheduled for abdominal surgery is sent for pre-op evaluation and testing. His chest x-ray is normal, and pulmonary function results on room air show: Tidal volume = 600 mL Respiratory rate = 12/min Vital capacity = 5000 mL PaO2 = 90 mmHg PaCO2 = 40 mmHg PECO2 = 28 mmHg The volume of the patient's physiological dead space, determined by applying Bohr’s equation, equals which of the following?

ExplanationPhysiological deadspace is the volume of the respiratory tract, that is ventilated but not perfused by the pulmonary circulation. Bohr’s equation for determination of the ratio of the physiologic deadspace (VD) to the tidal volume (VT) is VD/VT = PaCO2 − PECO2/PaCO2 VD/VT = 40 – 28/40 = 0.3 VD/VT × VT = VD 0.3 × 600 mL = 180 mL Physiologic Deadspace Volume = Anatomic + Alveolar Deadspace Anatomic deadspace can be measured by Fowler’s technique, but is often estimated as 1 mL per pound of body weight. Because there is normally no alveolar deadspace, physiologic deadspace volume approximates anatomic deadspace volume in persons with normal lung function.