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When Cyanosis is the Norm
Steven M. Schwartz, MD, FRCPC
Cardiac Critical Care Medicine
The Hospital for Sick Children
Toronto
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• No Disclosures
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When Cyanosis is the Norm
• Physiology of cyanotic congenital heart disease
• Impact on monitoring
• Tips for caring for patients in whom monitoring may be unreliable
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Cyanotic Congenital Heart Disease
• Most cyanotic congenital heart patients cyanotic from birth
– Most cases still represent an increase on available oxygen compared to fetal life
• >5 g/dl reduced Hb
• Multiple physiologic causes
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Cyanotic Congenital Heart Disease
• Physiology associated with intracardiac right-to-left shunts
– Pure right-to-left shunt
– Mixed right-to-left and left-to-right shunts
– Transposition physiology
• Can impact aspects of monitoring and clinical implications of SaO2
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Pure Right-to-Left Shunt
• Anatomic or physiologic obstruction to pulmonary blood flow
• No systemic obstruction
• Atrial, ventricular or great artery communication
• Eisenmenger’s is another example
http://www.lpch.org/DiseaseHealthInfo/HealthLibrary/car
diac/tf.html
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Pure Right-to-Left Shunt
• Clinical implications
– Pulmonary artery saturation < aortic saturation
– All blood diverted from the lungs goes to the body
– Cardiac output is therefore preserved or even increased until hypoxemia is critical
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Mixed Shunts
• Complete mixing of pulmonary and systemic venous return
• Cardiac output is partitioned into pulmonary and systemic flows
• Ratio is determined by presence of anatomic obstruction and relative resistances
http://www.lpch.org/diseaseHealthInfo/HealthLibrary/car
diac/chd.html
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Mixed Shunts
• Clinical implications
– Pulmonary saturation = aortic saturation
– SaO2 above about 75-80% means that there is more than normal pulmonary blood flow despite the presence of cyanosis
– SaO2 therefore has a complex relationship with oxygen delivery
– Unless total cardiac output changes, there is a zero sum game between the pulmonary and systemic flows
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Transposition Physiology
• Entirely parallel pulmonary and systemic circulation in the absence of anatomic connections (ASD, VSD, PDA)
• There is increased pulmonary flow but it is fully saturated already
http://www.lpch.org/diseaseHealthInfo/HealthLibrary
/cardiac/chd.html
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Transposition Physiology
• Clinical Implications
– Pulmonary saturation > aortic saturation
– Pulmonary blood flow cannot be estimated from degree of cyanosis
– Cardiac output is usually preserved until hypoxemia is critical
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Cyanosis in Adult CHD
• Eisenmenger’s syndrome
– Most common but decreasing especially in medically developed countries
• Intrapulmonary shunt secondary to CHD
• Persistent intracardiac right-to-left due to cardiac anatomy
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Monitoring Potentially Affected by CCHD
• Capnography
– Variable pulmonary perfusion independent of cardiac output affects dead space ventilation
– Different physiologic causes of cyanosis means that the relationships between SaO2 and pulmonary blood flow are variable
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R2=0.17 vs. 0.94
PET-CO2=raw PET-CO2–0.36×SpO2+39
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Monitoring Potentially affected by CCHD
• Near infrared spectroscopy
– Dependent on oxygen saturation of both arterial and venous compartments
– Oxygen extraction may be different under conditions of chronic cyanosis
– Has implications for interpretation of absolute values and SpO2-ScO2 difference
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NIRS in CCHD• Difference between SaO2 and ScO2 is less in
CCHD than in acyanotic CHD
• Implies that low ScO2 in cyanosis should not be dismissed as entirely due to low SaO2
• Supported by studies showing absolute value of NIRS may have better association with ND outcomes than SaO2-ScO2 difference
Anesth Analg 2017;125:234–40
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Monitoring Potentially affected by CCHD
• Pulse Oximetry
– Cornerstone for monitoring oxygenation
– Primarily calibrated for normal circulations
– There are newer devices that claim better accuracy and precision in cyanotic patients
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Skin Pigmentation
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Pediatr Crit Care Med 2016; 17:315–320
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Laboratory Monitoring
• Coagulation tests in citrate tubes can be inaccurate– Decreased plasma to whole blood ratio with high
relative citrate concentration
• Falsely high Hct by centrifugation methods– Plasma trapped in larger red cell mass
• Blood glucose may be falsely low– Ongoing in vitro glycolysis. Can be inhibited by
sodium fluoride
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How Do You Assess the Cyanotic Patient if Monitoring is Unreliable?
• Focus on evidence of overall oxygen delivery, not just on specific numbers
• Seek confirmatory evidence where possible
– Mental status, measured SaO2, lactate
• Adjust your expectations
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Summary
• Cyanosis occurs by multiple physiologic mechanisms
• The reliability and precision of standard monitoring can be affected by cyanosis
– Capnography often underestimates PaCO2
– Expected ScO2 is at least somewhat independent of SaO2
– SpO2 is more often overestimated than underestimated at lower SaO2
• Remember that patients are more than their numbers