The effects of cardiopulmonary bypass and profound hypothermic circulatory arrest on anterior fontanel pressure in infants

0 From the Departmentsof Anaesthesiaand Cardiovascular Surgeryand the Research Institute , The Hospil',dfor Sick Children, Universityof Toronto, Toronto , Ontario. Presented in part at the Annual Meeting of the AmericanSociety of Anesthesiologists,Las Vegas, October 1986. of Anaesthesia,The Hospital for Sick Children, 555 University Avenue , Toronto, Ontario, M5G IXg 1 P. J. Stow MBBSFFARCS , F. A. Burrows MD FRCPC, M. E. McLeod MOFRCPC, J. G. Coles MDFRCSC Studies in adults have demonstrated significant decreases in cerebral perfusion pressure (CPP) during cardiopulmonary bypass (CPB). 1-~ Similar data in infants undergoing CPD with profound hypothermic circulatory arrcsl (PHCA) are lacking. Inadequate CPP during CPB may be associated with postoperative neurological deficit, and a knowledge of changes in perfusion pressure is important for the management of these infants. Mean arterial blood pressure alone is a poor indicator of CPP,' which is defined as mean arterial blood pressure minus intracranial pressure (MAP - ICP). Intracranial pressure in the infant can be estimated noninvasively using the Ladd transducer to measure anterior fontanel pressure. This device consists of a pressure sensitive membrane on which is mounted a mirror, a fibreoptic light source and a pneumatic system. Distortion of the membrane by a change in pressure alters the reflection of light back to the monitor which responds by altering the air pressure in the system to equalize pressures across the membrane. The pressure required to balance the system is constantly displayed on the monitor as anterior fontanel pressure, s Although affected by application pressure, the measured anterior fontanel pressure (AFP) correlates well with intracranial pressure (ICP) in infants. 6 - The Ladd transducer was used to measure anteriorfot~nel pressure in 23 infants undergoing cardiopulmonary bypass and profound hypothermic circulatory arrest for surgical correction of congenital heart disease. Mean (+- SD) minimum oesophageal and rectal temperatures of 11.3 -~ 1.5~ and 18.1 +- 2.2~ respectively were achieved with a mean duration of arrest of 53.4 + 13.9 minutes. During reperfusion cardiopulmonary bypass after circulatory arrest, mean anterior fontanel pressure (18.3 ~- 6.4 mmHg) lncreased above basetinepre-bypass values (10.6 +-2.9 mmHg) (p < 0005) Mean arterial bloodpress~re decreased slgnificantly from pre-bypass values (57.0 +- 11.8 mmHg) during both cooling 0 8 . 8 +--8.4 mmHg) and rewarming cardiopulmomwy bypass (45.8 +- 8.9 mmHg) (p < 0.005). These changes were associated with a Mgnificam decrea~v in cerebral perfusion pressure during cooling (27.3 + t!l,O mmHg) and rewnrming cardiopulmonary bypass (27.5 +- 10.6 mmItg), compared with baseline pre-bypass values (46.5 +- 12.3 mmHg) (p < 0.005). The data demonstrate significant but transient decreaavs in cerebral perfusion pressure during cooling and re. warming bypass. Methods With the approval of the Human Subject Review Committee, 23 infants (< 10 months of age) admitted to The Hospital for Sick Children for surgical repair of congenital cardiac defects requiring cardiopulmonary bypass and profound hypothermic circulatory arrest were studied. Infants over the age of ten months, who may have a fibrous anterior fontanel, were excluded. Premedication of intramuscular atropine 20 ~g'kg -t was administered 30-60 minutes preoperatively. On arrival in the operaling room, the hair over the anterior fontanel was shaved and tincture of benzoin was applied to the scalp. Following calibration of the Ladd transducer and single-channel S t o w e / a L : ANTERIOR F O N T A N E L PRESSURE DURING C A R D I O P U L M O N A R Y BYPASS chart recorder, the transducer was laid over ihe fontanel and I~eld in place using a 3 cm by 3 cm piece of adhesive foam (3M Canada Inc., North York, Ontario). Excessive application pressure was avoided and the response to Queckenstcdt's manoeuvre noted. 7 Intravenous access and routine monitoring (electrocardiograph, doppler pulse monitor, sphygmomanometer cuff, pulse oximeter and precordial stethoscope) were established. Infants were anaesthetized according to the preference of the anaesthetist, by intravenous induction with fentanyl 10-30 i~g.kg -~ (n = 17) or morphine 2 m g ' k g -l (n = 2), by inhalational induction with halothane (n = 1) or isoflurane (n = 1), or by intramuscular kctamine 10mgkg-i (n = 2). Neuromuscular blockade was achieved with pancuronium bromide 0.15 mg. kg-i given intravenously. The trachea was intubated either nasally or orally, and intermittent positive pressure ventilation was commenced to maintain PaCOz in the range 3 5 - 4 0 m m H g , with an F I n 2 of 0.7-1.0. During stable general anaesthesia a cannula was inserted into a peripheral artery for measurement of systemic blood pressure, and in ten patients a cannula was inserted transcutaneously into the superior vena cava via the internal or external jugular vein, to monitor central venous pressure (CVP). Anaesthesia was maintained with a high-dose narcotic technique using either fentanyl (n = 21) or morphine (n = 2). No inhalational agents were given after the induction period and patients were ventilated with an air/oxygen mixture. Following anticoagulation with heparin 300 lu.kg -l , nonpulsatile CPB was established using a standard roller pump (Cobe Canada Limited, Scarborough, Ontario) and a 0.8 or 1.5 m 2 membrane oxygenator (Kolobow Scimed, Minneapolis, Minnesota). The CPB circuit was primed with blood and Plasmalyte (Travenol, Mississauga, Ontario) solution plus mannito125 per cent 4 ml.kg -] to maintain a haematocrit of 30 per cent. CPB flows were 2 . 4 - 3 . 2 L . m 2. Cardioplegia solution consisted of modified Roe's solution with 20mEq.L -~ of sodium bicarbonate solution in am initial dose of 300 ml' m-t body surface area, and 150 ml-m-2 given at approximately 30-minute intervals. Profound hypothermia with circulatory arrest was used, with mean (__. SD) minimum oesopbageal and rectal temperatures of 11.3  1.5 ~C and 18.1  2.2 ~C, respectively. Vasodilators were administered as necessary to assist rewarming during CPB (Table I). Throughout the study, AFP, MAP and CVP were recorded continuously. CPP was calculated as MAP AFP. MAP, CVP, AFP and CPP were analyzed at five-minute intervals to give mean values during four periods: pre-bypass, cooling bypass, rewarming bypass (immediately after hypothermic circulatory arrest), and post-bypass. During each period oesophageal and rectal TABLE I Vasodilatorsadmiaisleredto assistrewarmingduringCPB temperatures were recorded, and PaCO2 was measured. Arterial blood samples were analyzed at 37~ and values of PaCO2 were not corrected for temperture. The duration of each phase of CPB and hypothermic circulatory arrest was noted. Time taken for AFP to return to cooling bypass levels during rewamfing bypass was also noted. To determine whether there was any baseline drift during hypothernaia, the perfo (...truncated)