In these pathologies, the pulmonary blood flow is restricted due to an obstruction of the right tract (tricuspid atresia, pulmonary stenosis or atresia, tetralogy of Fallot). It is dependent on supply from the ductus arteriosus or aortopulmonary collaterals (see Figure 14.15). Cynosis is caused by a fall in Qp and the R-to-L shunt. Typically, SaO₂ is 75-80% and haematocrit  ≥ 50%.

In order to reduce the R-to-L shunt and increase the blood flow from the ductus arteriosus, if it is still patent, PVR must be reduced and SVR increased (see Figure 14.9): SaO₂ rises. Cyanotic children can achieve this spontaneously by squatting, which increases central venous return, reduces blood flow to the legs, and raises SVR. It can also be induced by external abdominal aortic compression, arterial vasoconstrictors (phenylephrine, norepinephrin) or infusions (increasing blood volume) [1]. PVR is generally normal downstream of pulmonary stenosis. If the pulmonary blood flow is highly dependent on the ductus arteriosus, a prostaglandin (PGE1) infusion will keep it patent for sufficient time to fit a stent into it or perform a systemic-pulmonary anastomosis (Blalock shunt). Due to limited O₂ transport, it is essential to maintain high Hb levels (> 150 g/L), if necessary through transfusions.

While SpO₂ provides a suitable means of monitoring Qp/Qs (relative variations in Qp), PetCO₂ gives an underestimation of PaCO₂ due to the dead-space effect caused by the volume of blood bypassing the lungs. Hypoxic ventilatory response is considerably reduced, although hypercapnic response is preserved. Hypovolaemia, which is often encountered in the operating theatre, reduces left-side afterload. It is equivalent to systemic arterial vasodilation, increasing the R-to-L shunt and therefore cyanosis. These phenomena are less pronounced if the shunt is restrictive.

Implications for anaesthesia

R-to-L shunts have an impact on pharmacokinetics. Due to the direct route taken by the systemic venous return to the arteries, there is a faster and higher peak in systemic concentration after intravenous injection of a substance. This speeds up intravenous induction (Figure 14.20) [4].



Figure 14.20: Impact of a R-to-L shunt on: 1) the time taken for an intravenously injected substance to appear in the circulation, and 2) its circulating levels in the arterial circulation. Yellow: normal. Blue: patient with a R-to-L shunt.

In real life, this theoretical effect is reduced in children for three reasons: increased circulating volume, reduced effective cardiac output, and increased protein binding [5]. The situation for inhaled agents is the opposite of that observed for intravenous agents: uptake and elimination of halogenated agents are slowed by the low pulmonary blood flow and dilution of arterialised blood by systemic venous blood [2]. Consequently, their negative inotropic effect is less easily reversible. However, these effects’ clinical impact is limited as they are only significant for agents with medium solubility (halothane, isoflurane) or low solubility (sevoflurane, desflurane, N₂O), when the shunt is approximately 30-50% [3]. The significance of this phenomenon is reduced by several further factors: low effective cardiac output, which means that the alveolar concentration of soluble agents rises faster, the high ratio of alveolar ventilation to functional residual capacity observed in young children, and the presence of a restrictive shunt.

The twofold objective of increasing SVR and lowering PVR determines which anaesthetic techniques should ideally be used.
 

SaO₂ can only be improved by reducing the shunt and not by increasing FiO₂. Ideally, SpO₂ should be between 80% and 90%. However, FiO₂ may affect PaO₂ since O₂ leads to pulmonary vasodilatation. If SaO₂ and pulmonary blood flow are low, it is important to maintain Ht > 40% to ensure O₂ transport. The transfusion threshold is 100-120 g/L Hb. After a Fontan procedure (which is performed if pulmonary blood flow is reduced), spontaneous ventilation should ideally be maintained, while avoiding any hypoxia, hypercapnia and atelectasis.

 

 

Right → left shunt and QP ↓ (Qp/Qs < 1.0)

In order to reduce the shunt, increase SVR (alpha stimulation) and reduce PVR (hyperventilation). If dynamic RVOT stenosis: beta-blocker SaO2 proportionate to Qp/Qs Anaesthesia recommendations:     - Sevoflurane or midazolam GA     - High-dose fentanyl/sufentanil     - Ventilation: hypocapnia, FiO2 ineffective     - Increase SVR to ↑ SpO2     - Prevent systemic vasodilation     - Maintain afterload: hypovolaemia increases the cyanotic R → L shunt     - Maintain Hb > 140 g/L (transfusion threshold > 100 g/L)     - In noncardiac surgery: neuraxial blockade not recommended

 

© BETTEX D, BOEGLI Y, CHASSOT PG, June 2008, last update May 2018



References