Cyanosis is linked to three phenomena:
- R-to-L shunt with mixture of arterialised and venous blood before systemic ejection;
- Low pulmonary blood flow;
- Low cardiac output.
When venous blood massively contaminates the arterial blood, SaO2 is no longer solely dependent on ventilation and O2 transport, but also on factors that modify SvO2 such as tissue consumption of O2 (VO2) and systemic cardiac output. In R-to-L shunts, the ratio between pulmonary impedance and systemic impedance (PVR/SVR) determines the Qp/Qs ratio and determines the degree of cyanosis. When arterial blood desaturation is caused by a shunt, the increase in FiO2 has very little influence on the SaO2. If systemic blood flow is too low, as in the case of aortic hypoplasia, SvO2 plummets and the pulmonary blood flow is unable to saturate the venous blood.
In R-to-L shunts, the process of blood bypassing the lungs is similar to an increase in the venous mixture (dead-space effect) at gas exchange. End tidal CO2 (PetCO2) therefore gives an underestimation of PaCO2. These patients retain a normal ventilatory response to hypercapnia, but exhibit a reduced response to hypoxemia [2]. They hyperventilate chronically to compensate for low CO2 clearance. Their maximum exercise capacity is proportional to the pulmonary blood flow; their VO2 increases only very gradually during exercise.
Haematocrit (Ht) increases, sometimes beyond 65%, to enable O2 delivery (DO2) to keep pace with VO2. Unfortunately, oxygen delivery decreases once the haematocrit exceeds 60% due to increased blood viscosity and cardiac workload. The risk of thrombosis is high. Tissue perfusion is slow, depleted and highly sensitive to dehydration and cold.
The level of cyanosis is dependent on the total desaturated haemoglobin concentration and occurs when the quantity of deoxyhaemoglobin is greater than 50 g/L. The more concentrated the haemoglobin is, the earlier cyanosis occurs during desaturation. Anaesthetists are sometimes misled by its late onset in cases of anaemia or haemorrhage.
In R-to-L shunts, the process of blood bypassing the lungs is similar to an increase in the venous mixture (dead-space effect) at gas exchange. End tidal CO2 (PetCO2) therefore gives an underestimation of PaCO2. These patients retain a normal ventilatory response to hypercapnia, but exhibit a reduced response to hypoxemia [2]. They hyperventilate chronically to compensate for low CO2 clearance. Their maximum exercise capacity is proportional to the pulmonary blood flow; their VO2 increases only very gradually during exercise.
Haematocrit (Ht) increases, sometimes beyond 65%, to enable O2 delivery (DO2) to keep pace with VO2. Unfortunately, oxygen delivery decreases once the haematocrit exceeds 60% due to increased blood viscosity and cardiac workload. The risk of thrombosis is high. Tissue perfusion is slow, depleted and highly sensitive to dehydration and cold.
The level of cyanosis is dependent on the total desaturated haemoglobin concentration and occurs when the quantity of deoxyhaemoglobin is greater than 50 g/L. The more concentrated the haemoglobin is, the earlier cyanosis occurs during desaturation. Anaesthetists are sometimes misled by its late onset in cases of anaemia or haemorrhage.
- For Hb = 100 g/L: cyanosis if SaO2< 50%
- For Hb = 150 g/L: cyanosis if SaO2< 67%
- For Hb = 200 g/L: cyanosis if SaO2< 75%
On the other hand, cyanosis occurs later in neonates because foetal haemoglobin is more saturated than adult Hb at the same PaO2. If preductal coarctation is present, cyanosis is confined to the lower part of the body.
Cyanotic diseases have significant multisystem effects.
Cyanotic diseases have significant multisystem effects.
- Haematological effects: increased mass and rigidity of the erythrocytes (microspherocytosis due to the relative lack of iron), hyperviscosity, microvascular occlusions, cholelithiasis secondary to an excess of heme ring to metabolise. Thromboembolic risk is high.
- Effects on coagulation: haemorrhagic diathesis due to a decrease in von Willebrand factor and vitamin K-dependent clotting factors, primary fibrinolysis, and decrease in platelet functionality [8]. Thrombocytopenia is usually only apparent due to the increase in red blood cell mass. PT and PTT measurements are incorrect if Ht is greater than 55%, because of the relative excess of citrate in the sample tubes in comparison to the patient’s reduced plasma volume [3]. The haemorrhagic risk is high.
- Increased risk of infection: endocarditis, brain abscess, pneumonia; antibiotic prophylaxis is recommended in all cyanotic patients [1,7].
- Myocardial effects: chronic ventricular dysfunction (systolic and diastolic) and increased ischemic risk.
- Renal effects: hypoxemia causes cell growth in the glomeruli and thickening of the basement membranes; it results in proteinuria and increased uric acid; the latter is a good marker for renal haemodynamics in cyanotic children [6].
- Neurological effects: the rate of abscess and stroke is increased in cyanotic children [4]. Paradoxical embolisms are always possible with an R-to-L or bidirectional shunt, particularly since several intravenous injections are performed during anaesthesia.
Arterial desaturation and dyspnoea are also symptoms of the impact of congenital heart diseases on the respiratory system [5].
- Interstitial stasis and alveolar oedema if LAP rises: left ventricular failure, left heart obstructive lesions, mitral valve stenosis, pulmonary vein stenosis;
- Pulmonary hypertension: wide L-R shunt with excessive pulmonary blood flow and pulmonary congestion;
- Compression of the tracheobronchial tree: dilation of the LA, double aortic arch, abnormal branching of the left PA from the extrapericardial right PA, excessive dilatation of the pulmonary arteries in pulmonary valve agenesis (severe pulmonary insufficiency);
- Combination of heart diseases involving low pulmonary blood flow and ciliopathies (ciliary epithelium diseases);
- Recurrent sleep apnoea syndrome and broncho-aspiration in Down syndrome patients with patent ductus arteriosus;
- Sequestration and pulmonary hypoplasia combined with anomalous pulmonary venous return.
| Cyanosis and hypoxemia |
|
Causes of cyanosis (SaO2< 85%): R-to-L shunt or low pulmonary blood flow. Increasing FiO2 is ineffective. Consequence of lowered DO2: increase of Ht (55-70%). The severity of cyanosis increases stepwise with rising Ht levels. Major risk: thrombosis in the case of under-hydration.
Systemic effects of cyanosis: - High thromboembolic risk - Impaired coagulation - Ventricular dysfunction - Increased risk of infection - Renal dysfunction Effect of R-to-L or bidirectional shunts: paradoxical embolisms |
© BETTEX D, BOEGLI Y, CHASSOT PG, June 2008, last update February 2020
References
References
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- BURROWS FA. Physiologic dead space, venous admixture, and the arterial end-tidal carbon dioxide difference in infants and children undergoing cardiac surgery. Anesthesiology 1989; 70:219-25
- COLMAN JM. Noncardiac surgery in adult congenital heart disease. In: GATZOULIS MA, et al, Eds. Diagnosis and management of adult congenital heart disease. Edinburgh, Churchill-Livingstone, 2003, 99-104
- PERLOFF JK, MARELLI AJ, MINER PD. Risk of stroke in adults with cyanotic congenital heart disease. Circulation 1993; 87:1954-9
- RIGBY ML, ROSENTHAL M. Cardiorespiratory interactions in paediatrics. "It's (almost always) the circulation stupid!" Paed Respir Rev 2017; 22:60-5
- ROSS EA, PERLOFF JK, DANOVITCH GM, et al. Renal function and urate metabolism in late survivors with cyanotic congenital heart disease. Circulation 1986; 73:396-400
- SILVERSIDES CK, DORE A, POIRIER N, et al. Canadian Cardiovascular Society 2009 Consensus Conference on the management of adults with congenital heart disease: Shunt lesions. Can J Cardiol 2010; 26:e70-e79
- TEMPE DK, VIRMANI S. Coagulation abnormalities in patients with cyanotic congenital heart disease. J Cardiothorac Vasc Anesth 2002; 16:752-65