Anaesthetists cannot take a break during CPB, because they have many data to monitor. Indeed, they must watch the child's face and fontanelle (if not closed) after the pump is activated, in order to check for any conjunctival oedema or haemorrhagic suffusion caused by an obstruction of the superior vena cava by a venous cannula. This results in jugular venous stasis and cerebral oedema. It is possible to detect this phenomenon quickly by constantly measuring CVP and ScO₂. Moreover, obstruction of the hepatic venous flow causes hepatic and mesenteric stasis with ascites and fluid sequestration. Malformations of the systemic and/or pulmonary venous return may be the cause of poor drainage by the CPB venous cannula. Extracardiac L-to-R shunting prevents systemic arterial pressure from being maintained at an appropriate value – they must be surgically occluded prior to initiation of CPB.

Although hypothermia below 30°C ensures unconsciousness, steps must also be taken to alleviate stress and ensure analgesia, anaesthesia and muscle relaxation. The initiation of CPB often causes children to recover suddenly for three reasons [1].
 

Anaesthesia must therefore be ensured by means of pharmaceutical agents: midazolam (0.3 mg/kg) or fentanyl (10 mcg/kg) are used in most cases to that end. Fentanyl is the safest agent in the event of pulmonary hypertensive crises. In instances where it is chosen as the anaesthetic agent (simple pathology, brief CPB, no hypothermia, age > 3 years), propofol infusion is continued throughout CPB.

Deep curarisation is necessary when the oxygen requirement falls during hypothermia. In contrast, at normothermia, spontaneous movements are the only means of determining whether patients are awake or asleep. It is also advised to apply this type of monitoring with older children, except in cases of shivering (increased oxygen requirement), involuntary diaphragm movements, or opening of the left chambers (risk of gas embolism).

The uniformity of cooling and rewarming can be improved and the temperature gradient reduced (<10°C) between the rectal and oesophageal temperatures by inducing vasodilation during CPB in young children – isoflurane (1-2%) through the oxygenator, phentolamine (10 mcg/kg), phenoxybenzamine (0.5-1.0 mg/kg). Indeed, the oxygen-haemoglobin dissociation curve shift to the left during hypothermia means that very cold blood traversing tissue that is still warm is delivering insufficient O₂ to meet tissue requirements [2]. The pump flow rate is generally given priority – peripheral resistance is manipulated to maintain an appropriate flow rate based on age and temperature.

Although unnecessary while the lungs are not being perfused, ventilation is beneficial once the heart is ejecting blood. During partial CPB, blood perfusing the coronary arteries is mostly blood pumped by the LV, which has therefore been retuned from the lungs by the pulmonary veins. Its O₂ content rises if the lungs are ventilated [3].

If the heart slows and stops due to low temperatures and cardioplegia, the anaesthetist must monitor the TEE to check for any ventricular dilation. This is always possible given the multiple anatomical combinations involved in congenital heart diseases and the potential for incorrectly positioned cannulas. Venting the ventricle is often necessary.
 

 

CPB procedure

The priority is to maintain the pump flow rate. SVR should be manipulated as required. Risk of recovery and movement at the start of CPB (acute haemodilution) Ensure anaesthesia (midazolam, propofol, halogenated agent) if T° > 30°C

 

 

© BETTEX D, BOEGLI Y, CHASSOT PG, June 2008, last update February 2020
 

 

References