Hypoventilation in an Anesthetized Saint Bernard
History. A 2-year-old Saint Bernard is brought to you for treatment of a fractured femur. You elect to place an intramedullary pin in the femur, and the dog will require anesthesia.
The dog is anesthetized with a barbiturate, an endotracheal tube is placed, and the dog is allowed to breathe oxygen containing 2% halothane. It is not ventilated but is allowed to breathe the anesthetic mixture spontaneously. The veterinary technician observing the dog notices that the gas reservoir bag on the anesthesia machine is not moving much when the dog breathes. Therefore, she draws an arterial blood sample for measurement of blood gases. Measurement reveals a Pao2 of 480 mm Hg and Paco2 of 90 mm Hg (normal, 40 mm Hg).Comment. This is an example of alveolar hypoventilation. Carbon dioxide is being eliminated by the lungs less quickly than it is produced by the tissues, so the Paco2 is elevated above the normal value of 40 mm Hg. The lungs ability to exchange oxygen is not impaired; the measured Po2 is acceptable for a dog breathing oxygen. Hypoventilation is a common occurrence in anesthetized animals, particularly when anesthesia is induced with a barbiturate drug. Barbiturates are prone to cause apnea. Also, the dog may have been slightly overdosed with barbiturate, resulting in the severe hypoventilation observed in this case. Hypoventilation occurs because the ventilatory response to CO2 is depressed by anesthesia, and therefore a larger increase in Paco2 than usual is required to trigger an increase in ventilation.
Treatment. The dog needs more alveolar ventilation to decrease Paco2 and prevent respiratory acidosis. The additional ventilation can be supplied by squeezing the rebreathing bag on the anesthetic machine. Once the dog recovers from anesthesia, its own respiratory control mechanisms will regulate alveolar ventilation and return Paco2 to normal.