Cerebral vasomotor reactivity in brain dead patients
Background: Cerebral vasomotor reactivity (CVMR) is a measure of the changes in arteriolar resistance in response to changes in arterial C02 concentration. CMVR is easily performed at the bedside using transcranial Doppler ultrasound (TCD). CVMR has not been studied in brain death. Methods: Patients with a clinical diagnosis of brain death, confirmed by a neurointensivist, were studied. CVMR was determined at the time of the apnea test. An experienced sonographer used a Pioneer TC 2020 (Nicolette) with a 2 MHz probe to insonate a middle cerebral artery of these patients. Continuous middle cerebral artery blood flow velocity (MCAV), systolic blood pressure (SBP), heart rate (HR), body temperature, oxygen saturation, and end-tidal C02 was performed during the procedure.A confirmatory arterial blood gas was obtained at baseline, five and ten minutes into the apnea test, and three minutes after hyperventilation. CMVR was calculated as follows: (MCAV at hypercapnia/MCAV at baseline)x100 - (MCAV at hypocapnia/MCAV at baseline)x100.(Normal in adults: 86±16%) CMVR was then divided by the absolute change in C02 to yield the percentage change (%Δ) in MCAV per mmHg C02 (Normal in adults: 2–4%). Results:Ten patients were studied (8 men) with a mean age 47±16 years.Their underlying disease varied: 3 ischemic strokes, 3 ICH, 2 SAH and 2 anoxic encephalopathy. Values of parameters monitored during the apnea test for this population: SBP:140±25 mmHg, HR:90.6±31/min, temperature: 35.8±2, PC02 at baseline: 40.6±9, at end of test: 74.6±13, at hyperventilation: 33.3±5. CMVR was 43.5±20% and %Δ was 1.3±0.4. All these patients had MCAV patterns compatible with brain death. Conclusion: CMVR is severely reduced or exhausted in brain dead patients. This test should be considered when TCD is used as a confirmatory tool in clinically brain dead patients.