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Duration of apnea needed to confirm brain death

Department of Neurology, University of California, San Francisco.

To determine the duration of respiratory arrest needed to attain a PaCO₂ level high enough to provide maximal stimulation of respiration, we evaluated changes in PaCO₂, PaO₂ and apH during periods of apnea lasting as long as 10 minutes in 10 apparently brain-dead subjects. Before apnea, mean PaCO₂ was 33 mm Hg. In seven subjects who did not breathe for 10 minutes, the mean rate of rise of PaCO₂ was 3.2 mm Hg per minute. PaCO₂ at 4 minutes was 50 mm Hg and at 10 minutes was 67 mm Hg. Three subjects breathed, two after less than 2 minutes of apnea, when PaCO₂ was 47 and 54 mm Hg, and one after 4.5 minutes, when PaCO₂ was 47 mm Hg. These data indicate: (1) that the threshold for respiratory stimulation may approach a PaCO₂ of 60 mm Hg in patients with brain damage; (2) that the rate of increase in PaC02 is such that, even in a normocapnic subject after 3minutes of apnea, the PaCO₂ may not be sufficiently high to stimulate respiration; and (3) if a patient is hypocapnic prior to the onset of apnea, PaCO₂ may not reach 60 mm Hg even after 15 minutes. To confirm absolute apnea, then, blood gas monitoring is necessary for verification of normocapnia prior to the beginning of apnea. In the absence of blood gas determinations, no fixed period of apnea, sufficient in all cases to establish absolute apnea, can be ascertained.

Requests for reprints should be addressed to Dr. Caronna, Neurology Service, 4M60, San Francisco General Hospital, San Francisco, CA94110.

Presented at the twenty-ninth annual meeting of the American Academy of Neurology, Atlanta, April 29, 1977.

Dr. Schafer's present address is Section of Neurology, Naval Regional Medical Center, Oakland, California.

Accepted for publication August 26, 1977.

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