14.4.6 Recommended techniques

No single solution can be applied to all children. Several techniques are possible using a main agent and add-on agents. These should be chosen in light of cardiac pathology, age, CPB duration, level of hypothermia, and postoperative forecasts [6]. In cases of children aged over 3 months with a simple form of heart disease and brief CPB (< 90 minutes) without deep hypothermia (T° > 34°C), it is desirable to extubate the child early (fast-track protocol). In such instances, several combinations may be used:
 

Based on current data, propofol infusions cannot be recommended for young children. In children aged under two years, the choice is restricted to a combination of fentanyl-halogenated agent, fentanyl-ketamine, or fentanyl-midazolam. Postoperative analgesia may be provided by a continuous infusion of morphine (20 mcg/kg/hour) or midazolam-sufentanil.

In any complex cases with prolonged intubation (infants or neonates, cyanotic heart disease, pulmonary hypertension, ventricular failure, CPB > 90 min, deep hypothermia), anaesthesia should ideally be based on the use of midazolam (0.05-0.1 mg/kg IV) and fentanyl or sufentanil at doses inhibiting the stress response (approximately 50-100 mcg/kg and 5-10 mcg/kg respectively). These doses are split between induction, sternotomy, CPB and sternum closure. Anaesthesia is maintained by adding sevoflurane (≤ 1 MAC), isoflurane (≤ 1 MAC if reduced SVR is advantageous), midazolam (0.1 mg/kg/h, sympathicolysis), ketamine (1 mg/kg/h, sympathetic stimulation) or propofol (2-10 mg/kg/h, reduced preload) depending on the required haemodynamic impact. Sevoflurane and fentanyl offer the greatest haemodynamic stability in very young children. A muscle relaxant (generally vecuronium or rocuronium) is chosen based on whether heart rate and arterial pressure must be maintained and on the required duration of action. The vagolytic effect of pancuronium bromide can be useful for limiting opiate-related bradycardia, which is poorly tolerated by young children.

Induction

Three induction methods are recommended depending on the child’s condition:
 

In the event of bradycardia, atropine (0.01 mg/kg IV – minimum dose 0.1 mg) is recommended before manipulating the airways of children aged up to two years.

Intubation

Young children’s pulmonary functional residual capacity (FRC) and oxygen reserve are reduced while metabolic requirements are high. The apnoea period imposed by the intubation procedure should be kept as brief as possible and preceded by a period of hyperventilation using a mask. Moreover, it is essential to reduce PaCO₂ in the event of pulmonary hypertension. However, high intrathoracic pressure (forced positive-pressure ventilation, Valsalva manoeuvre) forces a bidirectional shunt in the R-to-L direction (P_RA > P_LA). Children with predominantly L-to-R ASDs may suddenly desaturate or embolise at arterial level when ventilated at an excessive positive pressure. Since hyperventilation renews the alveolar air more quickly than the pulmonary blood flow, the alveolar concentration of anaesthetic gas is increased and therefore its blood concentration also rises. This induction-related acceleration is more pronounced for soluble agents (halothane) than for agents with low solubility (desflurane) – isoflurane and sevoflurane are middle-ranking in this respect [31].

Nasal intubation is preferable for children weighing up to 5 kg or in cases of prolonged intubation for children aged up to 2 years. Given the occasional need to institute hyperventilation and due to low pulmonary compliance, an internal tube diameter that prevents leakage below a pressure of 20 cm H₂O is required. A cuffed tube ensures air-tightness and is recommended in all cases, except for neonates [2]. In 25% of cases, cardiac malformations are combined with other malformations. Some subjects also have facial dysmorphism, which makes intubation difficult. Anaesthetists must plan for this in the preoperative examination to avoid being unexpectedly faced with a cyanotic, curarised child who is difficult to ventilate and moreover has an invisible glottis!

High-volume, rapid administration of fentanyl often prompts rigidity and choreoathetotic movements through extrapyramidal stimulation. Mask ventilation without curare is therefore difficult or even impossible. Bradycardia reduces cardiac output. This situation can rapidly become dangerous. The best solution is rapid muscle relaxation with 0.3 mg/kg of vecuronium or 1.2 mg/kg of rocuronium (time required until intubation is possible: 35 seconds), or possibly 1-2 mg/kg of succinylcholine (time to intubation: < 15 seconds) [28].

Ventilation

The reduction in pulmonary compliance and increase in airway resistance in children with L-to-R shunting and increased pulmonary blood flow is proportionate to vascular overload. The correlation with PAP is less evident [29]. The pulmonary vessels may compress bronchi and restrict flow due to their excessive size.

Positive-pressure ventilation negatively impacts the right heart and has a positive effect on the left heart. PVR is minimal if the pulmonary volume matches the functional residual capacity (see Figure 14.19) [9]. Hypoxia and hypercapnia lower local pH and prompt pulmonary vascular vasoconstriction. Hyper- or hypo-ventilation can therefore be used to reduce or increase PVR and adjust the Qp/Qs ratio in the required direction.
 

In any situations where right-side circulation is impaired (e.g. RV defect, Fontan physiology, etc.), the respirator should be finely adjusted to maintain the lowest possible mean intrathoracic pressure (minimum PEEP to prevent alveolar collapse, I:E ratio of 1:3-4), while hyperventilating the child to reduce local pH, generally with a low tidal volume (< 10 mL/kg) and a high rate. If NO is added even at a low dosage (10 ppm), this reduces PVR by up to 30% [12].

In volume-controlled ventilation, it is possible to administer the same tidal volume (TD) even if conditions change (compliance, relaxation, bronchospasm, etc.), although there is a risk of a very high pressure peak. Alarms and pressure limits prevent the pressure from rising excessively high. Although pressure-controlled ventilation offers the benefit of limiting the risk of barotrauma, it administers variable tidal volume depending on the mechanical conditions of the thorax and lungs. It is the preferred option for young children (< 10 kg), particularly since the TD of anaesthesia ventilators is imprecise if the administered volume is low due to the machine's large compressible volume. In these instances, the pressure-controlled technique is more accurate.

Cyanotic episodes

Some situations may trigger cyanotic episodes (blue spells) between induction and CPB, especially in the event of dynamic stenosis of the RV outflow tract. These may be triggered by two mechanisms.
 

These measures are combined with hyperventilation (respiratory alkalosis), an increase in circulating volume, and monitoring of ventilation (checking for unilateral intubation and pneumothorax, aspirating secretions, etc.). In cases of pulmonary stenosis/hypoplasia, a reduction in PVR is prevented by hyperventilation, since this procedure increases the gradient through the stenosis.

In bidirectional shunts, pulmonary vasodilation overloads the pulmonary circulation and induces a harmful reduction in systemic blood flow.

Pulmonary hypertension

There are numerous PAH-prone medical conditions in which a hypertensive crisis may easily be triggered by a series of factors that must be avoided at all costs: hypoventilation, hypercapnia, hypoxaemia (PaO₂ < 60 mmHg), atelectasis, high intrathoracic pressure, acidosis, hypothermia, sympathetic stimulation and pain. PAH lowers the pulmonary blood flow and intensifies arterial desaturation. It increases RV afterload and results in right ventricular failure. Although anaesthetic agents have a relatively low impact on the pulmonary circulation, they may have an influence in critical situations [10].
 

Treatment consists of hyperventilation (PaCO₂ 30-35 mmHg), hyperoxia (FiO₂ 0.8-1.0), correction of acidosis, inhalation of NO and/or prostaglandin and milrinone (see Pulmonary Hypertension).

Perioperative infusions

Since CPB represents a large fluid input that is often higher than the child’s circulating volume, intravenous administration is minimised.
 

If volume needs to be administered to correct hypotension, a colloid or a 5% (10-15 mL/kg) or 20% (5 mL/kg) albumin solution is used and repeated if necessary. Blood is used in the event of anaemia. Although 6% HES and 5% albumin are equivalent in terms of renal function and morbimortality, HES, whose toxicity has not been demonstrated in paediatrics, leads to a less positive fluid balance than albumin [32]. However, albumin is the safest agent for replacing circulating volume.

Fluid maintenance is ensured by an isotonic balanced solution containing sodium and glucose, since young children are prone to hyponatraemia if free water is administered and to hypoglycaemia in the event of fasting [33]. Sufficient blood glucose levels must be maintained throughout operations on young children, even if the physiological stress of surgery prompts hyperglycaemia, which may result in metabolic acidosis, hyperosmolarity (risk of brain haemorrhage), and osmotic diuresis. Traditionally, hyperglycaemia is avoided due due to its potential harmful effects on the central nervous system in the event of low blood flow or circulatory arrest. However, there is no evidence proving that this notion applies to CPB in children. Indeed, this exacerbation does not appear to affect infants aged under 3 months, since the immaturity of their blood-brain barrier protects them from the effects of hyperglycaemia. In contrast, the harmful cerebral effects of any hypoglycaemia add to those of hypoperfusion. Indeed, neonates are at higher risk of hypoglycaemia due to their limited reserves and low gluconeogenesis.

Blood volume management

Fluid overload is often triggered by haemodilution and capillary permeability caused by CPB – tissue is oedematous, gas exchange is impaired, and the child’s weight rises by 7-15%. Mortality rises in tandem with this weight gain, which generally lasts 2 to 3 days into the postoperative phase [4,33]. The main risk factors are young age, CPB duration, clinical status, and cyanosis [27]. Overload > 8% is an independent predictor of prolonged mechanical ventilation and acute nephropathy [13]. Restrictive fluid administration is the best form of prevention against the risk of fluid and sodium overload [26].

Blood volume indices are generally split into two categories: static indices (CVP, PAWP, LVEDA, etc.) and dynamic indices (respiratory variations during IPPV of systolic arterial pressure, pulse pressure, stroke volume, Vmax of aortic flow, IVC diameter, etc.) (see Chapter 6 - Dynamic Indices and Goal-directed Fluid Management). In adults, the predictive value of the former for hypovolaemia is low, while that of the latter is very high. Unfortunately, both are relatively ineffective in children since their thoracic, pulmonary and arterial compliance is very high [11]. Only a > 15% respiratory variation in maximum aortic blood flow velocity appears to be a reliable indicator of hypovolaemia [17].

Management of fluid overload requires a diuretic (furosemide, ethacrynic acid) and haemodialysis in the event of renal failure. Since the severity of fluid excess on initiating dialysis determines the prognosis, the indication for this procedure must be precocious if the diuretics do not provide the anticipated benefit [20].

Transfusion

Young children’s oxygen requirement is higher than that of adults (3 times higher in the case of infants). Whatever the level of saturation, oxygen delivery must be appropriate. It is therefore particularly important to maintain optimal cardiac output and adequate haemoglobin levels if saturation levels are low. Optimal haematocrit is between 50 and 60% for cyanotic children. If this rises above 60% it is advisable to perform haemodilution with exchange of volume for volume as shown in the formula below:
 

As in adults, thresholds for transfusion are restrictive. A value of 80 g/L is a good threshold for non-cyanotic children [23]. In the event of arterial desaturation or severe pulmonary hypertension, it is raised to 100 g/L. Blood administration should ideally be adjusted to SaO₂ and SvO₂, rather than to a specific level of Hb. Transfusions administered to neonates require blood that is compatible with the mother's blood since maternal antibodies passed through the placenta are detectable up to the 3rd month following birth. If the mother’s blood group is unknown, the infant must be given O-negative, Kell-negative blood [23].

Recovery and extubation

Following a long-standing debate on the subject, the current trend is for shorter periods of postoperative mechanical ventilation and earlier extubation, even for neonates or children with complex pathologies [7,19]. For subjects with the same pathology, there is no significant difference in terms of morbimortality if they are extubated on the operating table or in intensive care a few hours later [24]. Rapid extubation is even possible in difficult situations [1]. However, two different situations may be defined based on constraints related to the type of heart disease and the procedure.
 

Rapid extubation (< 2 hours postoperatively) prevents complications related to mechanical ventilation (barotrauma, respiratory infections, tracheal tube obstruction). It allows early mobilisation, reduces respiratory complications, and shortens the period spent in intensive care [1]. It is an indicator that perioperative problems are under control and offers the benefit of facilitating transport to the postoperative monitoring unit. However, since the operating theatre is immobilised while the patient recovers, this represents an organisational and financial loss compared to extubation within an hour of the patient's arrival in intensive care, since an hour in an operating theatre is more expensive than an hour in intensive care. The next issue is to ensure the child's comfort and analgesia without risking hypoventilation or respiratory failure: morphine (bolus 0.1 mg/kg IV at the end of the procedure), dexmedetomidine (0.2-0.7 mcg/kg/h), intravenous paracetamol [15].

Neuraxial anaesthesia

When combined with general anaesthesia, neuraxial blockade offers three significant benefits (see Chapter 4 - Regional Anaesthesia).
 

Arterial hypotension, which is a major drawback of neuraxial blockade in adolescents and adults, does not occur in young children due to their immature sympathetic systems – their heart rate does not change significantly [8]. Respiratory depression occurs in children at morphine doses > 0.05 mg/kg administered epidurally and > 0.02 mg/kg administered intrathecally. The major risk is evidently that of a compressive epidural haematoma, especially in children who are anticoagulated for CPB. In adults, calculations of this risk give an epidural haematoma incidence of 1:12,700 to 1:4,000 in cardiac surgery [5,14,18], with a probable mean value of 1:3,550 [16]. Although no precise data are available, there is every reason to expect that the risk is identical in children. A number of precautions aimed at limiting risk are possible. These include leaving at least an hour between puncture and heparinisation and removing the catheter when coagulation is back to normal, which are both simple to achieve with children. However, it is not possible to place the epidural catheter the day before surgery or perform an intrathecal puncture before putting the patient to sleep, as is recommended for adults. The puncture is always performed post-induction on anaesthetised children [3]. The epidural catheter is inserted through a caudal puncture (in very young children) or T4-T8 thoracic puncture (in children > 4 years). The recommended opiate and local anaesthetic doses are given in Table 14.12. The primary indications for an epidural are procedures involving a thoracotomy as this is more painful than a sternotomy, and procedures without CPB, after which the aim is to establish early spontaneous ventilation (e.g. Glenn or Fontan cavopulmonary anastomosis).

In young children, caudal anaesthesia with morphine (70 mcg/kg in 5-10 mL NaCl 0.9% preservative-free) post-induction provides excellent postoperative analgesia.

Peripheral blocks

Ultrasound-guided deep and superficial blocks are much safer than the conventional blind technique. These blocks are particularly useful in case of thoracotomy. The dosage of systemic opiates can be reduced, the children can be extubated immediately at the end of operation, and the postoperative analgesia is excellent [21,22].
 

Various local blocks and infiltrations performed by the surgeon at the end of surgery are extremely useful as they are both effective and present a low level of risk post-CPB, unlike an epidural block.

 

 

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

 
References