Recent validation of 133Xe inhalation.

To the Editor: There have been a number of questions concerning the validity of regional cerebral blood flow from the brain stem and cerebellar regions recorded after inhalation of 133 Xe after placement of probes in the suboccipital region (see p. 203). These posterior fossa measurements have been a relatively new contribution from this laboratory and their validity may be questioned since detectors placed in the suboccipital region may be subject to distortion from counts derived from nasopharynx, air sinuses and extracranial circulation. Brain stem and cerebellar (BSC) rCBF measurements appear to be valid for the following reasons. In the baboon 133 Xe inhalation measurements rCBF(inh) have been alternated with the aortic arch injection method rCBF(inj). The arterial bolus and inhalation measurements are in good agreement for the BSC regions as well as for the hemispheric values. In the baboon, chronic tracheostomy was performed to eliminate 133 Xe contamination from the nasopharynx and the air sinuses. Measurements were made in a primate chair before, during induction, and recovery from different anesthetic or tranquilizing agents. EEG, PEco 2 and blood pressure were recorded. Correlation coefficients between rCBF(inh) and rCBF(inj) were: r = 0.93 (/> < 0.01) for hemispheric flow gray, r = 0.86 for BSC flow gray (P < 0.05). Results obtained with Fg during different stages of responsiveness were: 1) deep barbiturate anesthesia; hemispheric Fg = 52 ± 8 ml/lOOg brain/min, BSC Fg = 48 ± 8.2, 2) light ketamine anesthesia, hemispheric Fg = 79 ± 12, BSC Fg = 73 ± 10, 3) awake after anesthesia, hemispheric Fg = 92 ± 17, BSC Fg = 87 ± 16, 4) unilateral sensory motor stimulation; Fg (stimulated hemisphere) = 101 ± 27, Fg (non-stimulated hemisphere) = 85 ± 9; BSC Fg = 105 ± 29. Finally, during brain death at MABP of 40 mm Hg, brain stem and cerebral flow was almost zero despite continued cardiopulmonary circulation with a satisfactory end-tidal 133 Xe curve. In man, after inhalation of 133 Xe, gamma counts recorded from probes placed over the posterior fossa and directed toward the brain stem-cerebellar regions in the distribution of the vertebral-basilar arterial system, are comparable to those recorded from probes placed over the hemispheres in the distribution of the carotid artery. This is to be expected, since, unlike the carotid bolus method, inhaled 133 Xe passes via the heart to perfuse the entire arterial system of the brain. …