Crack Cast Show Notes – Cyanosis – March 2016 www.crackcast.org

CrackCast Episode 14 – Cyanosis

Episode overview:

1) What is the differential diagnosis for cyanosis?

2) List the common causes for methemoglobinemia

10 medications

6 toxins

3) Describe the mechanism for methemoglobin formation, treatment, and indications for

methylene blue

Wisecracks:

1) Explain the oxygen-hemoglobin dissociation curve

2) What is the hyperoxia test?

Rosen’s in Perspective:

Key terms:

● oxygenated vs. deoxygenated hemoglobin = saturated vs. desaturated hemoglobin

● SaO2 (arterial oxygen saturation measured/calculated by ABG) vs. SpO2 (peripheral

oxygen saturation measured by pulse oximetry)

● PaO2 vs. PAO2 (partial pressure of O2 in the blood (measured by ABG) vs. partial

pressure in the alveolus)

Cyanosis = imbalance between oxy/deoxy hemoglobin

cyanosis is specific for hypoxia but not sensitive (can be hypoxic without cyanosis!)

● normal adults that is when deoxyhemoglobin >5g/dl

● cyanosis is about absolute amount of deoxygenated hemoglobin - anemic individuals

turn blue only at lower levels of PaO2 and SaO2

1) What is the differential diagnosis for cyanosis?

Critical:

● acute heart failure

● acute coronary syndrome

● hypovolemic or cardiogenic shock

● acute respiratory failure

● massive PE

● congenital heart disease

Crack Cast Show Notes – Cyanosis – March 2016 www.crackcast.org

Emergent:

methemoglobinemia (consider when no hx/physical suggesting underlying

respiratory/CVS disease)

sulfhemoglobinemia

o rare cause of cyanosis

o most common after exposure to hydrogen sulfide from organic sources

o also from medications (sulfonamide derivatives)

o GI sources (bacterial overgrowth)

o consider when cyanotic and methemoglobin on CO-oximetry, but does not

respond to methylene blue treatment

polycythemia (elevated RBC mass)

o three main causes:

1) polycythemia vera – bone marrow stem cell disorder with increase RBC

mass, cyanosis, and splenomegaly

2) secondary polycethmia – increase in erythropoietin (appropriate or not) in

response to chronic hypoxemia

eg. congenital heart disease, cigarette smoking, high altitude

exposure

3) relative polycethemia – result of reduced plasma volume

eg. dehydration

Non-emergent:

Raynaud’s phenomenon

may cause a cyanotic appearance

15% of the population

F>M

2) List the common causes for methemoglobinemia

Common causes of methemoglobinemia. Rosen's 8th Edition. Box 14-1. Chapter 14 - page 130.

Crack Cast Show Notes – Cyanosis – March 2016 www.crackcast.org

3) Describe the mechanism for methemoglobin formation, treatment, and

indications for methylene blue

Pathophysiology:

normally hemoglobin binds O2 via iron in its reduced state (Fe2+)

if iron is oxidized it forms methemoglobin (Fe3+) - “ferric state”

this new oxidized state cannot bind O2 and thus cannot transport O2 to tissues or remove

CO2 leading to hypoxia and acidosis

normally methemoglobin is only ~1% of total hemoglobin stores (cyanosis when greater

than 10-25%)

the body can use NADH to reduce methemoglobin back to Fe2+ (major pathway)

PEARL: pathognomonic sign of methemoglobinemia is dark-purple-brown or chocolate looking

blood when exposed to room air

PEARL 2: second pathway to reduce methemoglobin exists using glutathione and G6PD. This is

the MOA of methylene blue.

Treatment:

If cutaneous exposure, first don appropriate PPE and then decontaminate patient

Urgent treatment with oxygen and methylene blue indicated for:

symptomatic hypoxia (dysrhythmias, angina, respiratory distress, altered LOC, seizures)

OR methemoglobin levels >30%

If patient does not respond to methylene blue but has elevated methemoglobinemia consider

sulfhemoglobinemia

Crack Cast Show Notes – Cyanosis – March 2016 www.crackcast.org

Wisecracks:

1) Explain the oxygen-hemoglobin dissociation curve

2) What is the hyperoxia test?

Bedside test to determine the cause of cyanosis (broad categories): poor oxygenation from lung

dysfunction vs. presence of R to L shunt

Official test is using a baseline ABG, and then repeating ABG after high flow O2 for 10min

if PaO2 increases to above 150mmHg problem is with their lungs

if PaO2 stays below 100mHg problem is likely a R to L shunt

lots of caveats and imprecise, but potentially useful tool

Can use SpO2 instead of PaO2 and see whether patients sat improves from 88% to 100% after

10 min of high flow oxygen. If so then likely V/Q cause of hypoxia.