Cyanosis is caused by insufficient pulmonary blood flow (e.g. pulmonary stenosis, tricuspid atresia, Fontan) or contamination of arterialised blood by a R-to-L shunt (TOF, TGA, single ventricle). In R-to-L shunts, blood bypassing the lungs equates to an increase in the venous mixture at gas exchange (dead-space effect). End tidal capnography (ETCO2) therefore gives an underestimation of actual PaCO2. These patients retain a normal ventilatory response to hypercapnia, but exhibit a reduced response to hypoxaemia [3]. They hyperventilate chronically to compensate for low CO2 clearance. Their maximum exercise capacity is proportional to pulmonary blood flow [10]. Their VO2 increases only very gradually during exercise [9]. When arterial blood desaturation is caused by a shunt, the increase in FiO2 has little influence on SaO2. Cyanosis is more quickly evident if Ht is high. Its onset is delayed in the event of anaemia since the depth of cyanosis is dependent on the desaturated Hb concentration (> 50 g/L) [2]. Cyanosis appears at room-air SaO2 levels of 85-90% (depending on Ht).
Haematocrit (Ht) increases, sometimes above 65%, to enable O2 delivery to keep pace with the O2 requirement. This results in blood hyperviscosity and entails a risk of spontaneous thrombosis, especially in the event of dehydration e.g. due to preoperative fasting or insufficient fluid administration in the operating theatre. The transfusion threshold for these patients is evidently much higher than the usual recommended limit (Hb > 100 g/L). A perioperative phlebotomy is only indicated if Ht is > 65% and hypovolaemia, dehydration and iron deficiency are absent [2,13].
Cyanotic diseases have significant multisystem effects.
Haematocrit (Ht) increases, sometimes above 65%, to enable O2 delivery to keep pace with the O2 requirement. This results in blood hyperviscosity and entails a risk of spontaneous thrombosis, especially in the event of dehydration e.g. due to preoperative fasting or insufficient fluid administration in the operating theatre. The transfusion threshold for these patients is evidently much higher than the usual recommended limit (Hb > 100 g/L). A perioperative phlebotomy is only indicated if Ht is > 65% and hypovolaemia, dehydration and iron deficiency are absent [2,13].
Cyanotic diseases have significant multisystem effects.
- Haematological effects: increased erythrocyte mass and rigidity (microspherocytosis due to relative iron deficiency), 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 a decrease in platelet functionality [6,12]. Thrombocytopenia is usually only apparent, due to the increase in red blood cell mass. The haemorrhagic risk is high. 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 [4]. Blood samples must be kept in tubes with an appropriate dosage of citrate.
- Myocardial effects: chronic ventricular dysfunction (systolic and diastolic) and increased ischaemic risk [5].
- Increased risk of infection: endocarditis, brain abscess, pneumonia; antibiotic prophylaxis is recommended for all cyanotic patients [2,11].
- Neurological effects: incidence of brain abscesses is high, but stroke incidence is not connected to hyperviscosity in adults, even though it is increased in children [1,7]. Paradoxical embolisms are always a threat with a R-to-L shunt, particularly while numerous intravenous injections are being performed during anaesthesia.
- Renal effects: hypoxaemia 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 patients [8].
| Cyanosis and hypoxaemia |
| 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 higher the Ht, the more pronounced the cyanosis. Major risk: thrombosis in the event of under-hydration. Systemic effects of cyanosis: - High thromboembolic risk - Impaired coagulation - Ventricular dysfunction - High risk of infection - Renal dysfunction Effect of R-to-L shunting: paradoxical embolisms (stroke) |
© BETTEX D, CHASSOT PG, January 2008, last update May 2018
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