Abstract: An apparatus periodically measures the carbon dioxide content of the breath of a medical patient. The need to recalibrate the apparatus is determined by comparing the carbon dioxide measurements taken at different times during the respiratory cycle to corresponding reference levels. These levels often occur when the apparatus no longer is measuring accurately. When recalibration is indicated the apparatus waits until the patient begins inhaling so that the recalibration does not interfere with measuring the carbon dioxide during exhalation. The apparatus then measures the carbon dioxide content of the ambient air to define an offset value used in patient monitoring.