Cerebral oxygenation during paediatric cardiac surgery: identification of vulnerable periods using near infrared spectroscopy.

OBJECTIVE: Neurologic sequelae remain a well recognised complication of paediatric cardiac surgery. Monitoring of cerebral oxygenation may be a useful technique for identifying vulnerable periods for the development of neurologic injury. We sought to measure regional cerebral oxygenation in children undergoing cardiac surgery using near infrared spectroscopy to ascertain such vulnerable periods. METHODS: Observational study of 18 children (median age 1.3 years) undergoing cardiac surgery (17 with cardiopulmonary bypass, 8 with circulatory arrest). Regional cerebral oxygenation was monitored using the INVOS 3100 cerebral oximeter and related to haemodynamic parameters at each stage of the procedure. RESULTS: Prior to the onset of bypass, 10 patients had a decrease in regional cerebral oxygenation of > or = 15% points, reaching an absolute haemoglobin saturation less than 35% in 5 cases. The most common cause was handling and dissection around the heart prior to and during caval cannulation. With institution of bypass, regional cerebral oxygenation increased by a mean 18% points to a mean maximum of 75%. During circulatory arrest regional cerebral oxygenation decreased with rate of decay influenced by temperature at onset of arrest (0.25%/min at < 20 degrees C; 2%/min at > 20 degrees C). Reperfusion caused an immediate increase in regional cerebral oxygenation followed by a decrease during rewarming. Discontinuation of bypass caused a precipitous decrease in regional cerebral oxygenation in 5 patients, reaching less than 50% in 3 patients. CONCLUSIONS: These observations suggest that the pre- and early post-bypass periods are vulnerable times for provision of adequate cerebral oxygenation. Near infrared spectroscopy is a promising tool for monitoring O2 supply/demand relationships especially during circulatory arrest.

Evaluation of two warming systems after cardiopulmonary bypass.

We have compared the Thermomat electric undermattress (JMW Systems, Edinburgh, UK) and the Bair Hugger (Augustine Medical, Courtelary, Switzerland) forced-air warming blanket in 30 adult patients after cardiac surgery. All patients were warmed to an oesophageal temperature of 38 degrees C before termination of cardiopulmonary bypass (CPB); those with oesophageal temperatures < 35.5 degrees C at skin closure were allocated randomly to be rewarmed in the intensive care unit either on the Thermomat (n = 15) or under the Bair Hugger blanket (n = 15), at their highest settings. Oesophageal and lateral thigh skin temperatures were recorded every 15 min for 4 h. There was a significantly faster increase in core temperature (0.5 vs 0.75 degrees C h-1; P < 0.0002) and skin temperature (0.86 vs 1.3 degrees C h-1; P < 0.001) in the Bair Hugger group. However, there was no difference in the number of patients who reached a core temperature of 36 degrees C (15 Bair Hugger, 14 Thermomat) or 37 degrees C (11 Bair Hugger, seven Thermomat), or in the number of patients who reached a skin temperature of 37 degrees C in 4 h (four Bair Hugger, one Thermomat). Twelve patients in the Bair Hugger group reached a skin temperature of 36 degrees C compared with two in the Thermomat group (P < 0.001). The Bair Hugger warmed faster than the Thermomat both centrally and peripherally, and warmed more patients to a core temperature of 37 degrees C in 4 h, but did not reduce the time to tracheal extubation or alter important clinical aspects of postoperative course.

Cerebral oxygenation measured by near-infrared spectroscopy: comparison with jugular bulb oximetry.

BACKGROUND: Near-infrared spectroscopy is a potential tool for measuring adequacy of cerebral oxygenation during cardiac operations. The cerebral microcirculation is predominantly venous (by volume) and therefore regional cerebral oxygenation measured by near-infrared spectroscopy should reflect jugular bulb venous saturations. METHODS: We compared simultaneous regional cerebral oxygenation and jugular bulb venous saturation measurements in 40 children (median age, 4.5 years; range 2 weeks to 14.5 years) in the cardiac catheter laboratory (n = 29) and during cardiac operations (n = 11). RESULTS: For all patients combined the correlation between regional cerebral oxygenation and jugular bulb venous saturation was 0.69 (p < 0.0001) and was similar for the two groups. For individual children undergoing cardiac operations excellent correlations were obtained (r = 0.78 to 0.96; median, 0.91). However, at low values of jugular bulb venous saturation, regional cerebral oxygenation tended to run high, whereas the converse was true for high values of jugular bulb venous saturation. CONCLUSIONS: These findings suggest that near-infrared spectroscopy may be a useful tool for assessing intravascular cerebral oxygenation during pediatric cardiac operations. Prospective studies of neurologic outcome will be required to establish the value of this technique for assessing the adequacy of cerebral protection.

Sonoclot analysis.

Assessment of perioperative bleeding disorders, especially those related to platelet dysfunction, remains a clinical challenge. The management of the bleeding patient in the operating theatre or on the postoperative ward is often empirical with little scientific basis. Much of the reason for this is that conventional clotting studies are not immediately available and there is a perceived urgency, particularly among trainee doctors, to treat bleeding disorders without first establishing the exact nature of the coagulopathy. SCT provides useful information on platelet function, particularly in patients after cardiopulmonary bypass, and has enabled practitioners to rationalize the management of bleeding disorders and not expose their patients to the risks of unnecessary transfusion of blood products. Undoubtedly further studies are required before this instrument can be used reliably in the clinical setting but it may prove to be a useful addition to the available techniques for monitoring perioperative bleeding disorders.

The auditory evoked potential and paediatric anaesthesia.

We have examined the raw EEG activity and auditory evoked responses (AER) in 35 children, aged 3 days to 13 yr (median 1.5 yr), undergoing general anaesthesia for routine surgery. Binaural stimuli were presented at a frequency of 6.12 Hz and the EEG was recorded and stored using the Northwick Park auditory evoked response software. AER were generated by averaging 512 sweeps each of 125-ms duration. In children less than 2 yr of age, the AER was often irregular, whereas children older than 2 yr had AER patterns similar to those in adults. In children less than 2 yr, regular artefact activity was superimposed on the background EEG which was at the same frequency as the instantaneous heart rate and which was often identifiable as the ECG. We conclude that the AER may be unreliable in children less than 2 yr of age, and modification of current methodology may be required if this technique is to become useful in paediatric anaesthetic practice.

Effect of temperature and cardiopulmonary bypass on the auditory evoked response.

We have recorded auditory evoked potentials before and during cardiopulmonary bypass in 10 adult patients undergoing cardiac surgery under moderate hypothermia to 27-28 degrees C. The immediate effect of bypass was a small decrease in latency and increase in amplitude of the early cortical response. We also studied two adults and two children during profound hypothermia with circulatory arrest during cardiopulmonary bypass. Reduction in core temperature to 25 degrees C resulted in an increase in latency and amplitude of the brain stem responses; below this temperature the amplitude decreased but latency continued to increase until the auditory evoked response trace became completely flat between 21 and 19 degrees C. These changes were reversible on rewarming.

Auditory evoked responses and near infrared spectroscopy during cardiac arrest.

We describe a patient who had a cardiac arrest during anaesthesia, in whom regional cerebral oxygen saturation was being measured by near infrared spectroscopy and the auditory evoked responses (AER) were being recorded. Both of these monitors provided useful information on cerebral oxygenation during cardiac arrest. Changes in the AER as the result of either reduced circulation or hypothermia are similar, and should these two situations occur simultaneously there could be difficulty in the interpretation of the AER.