Going to start with an approach to hemolytic anemias, even though this came up later in the case!
Approach to hemolytic anemias
- A great review by our very own Goop, Pat Cornett, and LT: http://www.aafp.org/afp/2004/0601/p2599.html
- One approach is to think intravascular vs extravascular:
- Intravascular: MAHAs (DIC, TTP, HUS, shear stress from mechanical valves), AIHA, toxin-mediated lysis, complement mediated (eg PNH)
- Extravascular: liver, spleen
- Another approach is intrinsic vs extrinsic:
- Intrinsic: think anatomically!
- membrane defects (eg hereditary spherocytosis) à smear with spherocytes
- enzyme deficiencies (eg G6PD, pyruvate kinase, and a slew more!) à smear with bite cells, Heinz bodies
- hemoglobinopathies (eg sickle cell disease) à smear with sickled cells, target cells
- Extrinsic: usually acquired!
- MAHAs à smear with schistocytes
- AIHA à smear with spherocytes; +direct coombs test
- Infections (eg malaria, babesia)
- Liver disease
- Lab workup:
- For all, expect: indirect hyperbili, elevated retics, high LDH, low haptoglobin
- To differentiate further: get a smear, coags, and other tests depending on clinical suspicion (eg coombs, hgb electrophoresis, infectious w/u, G6PD level, etc)
- Intrinsic: think anatomically!
And, a review of the oxyhemoglobin dissociation curve, co-oximetry, and methemoglobinemia (mostly copied from previous AM report PEARLs, found here!)
Oxyhemoglobin dissociation curve:
*First, remember that hypoxia and hypoxemia are different! Hypoxia = low O2 sat; hypoxemia = low PaO2
Note the curve is sigmoidal, so changes in PaO2 have very little effect on O2 sat when PaO2 is >60 (i.e. PaO2 increase from 60-100 translates to O2 sat going from 90 to 98%), however, for PaO2s from 20-60, there is a more dramatic drop in O2 sat. For O2sats <70%, accuracy is questionable!
- Methemoglobinemia: PaO2 is too high for a given pulse ox reading.
- Carbon monoxide poisoning: falsely high % sat on pulse ox (because pulse ox can’t differentiate carboxyHgb from oxyHgb)
Quick word on co-oximetry: order this separate from your standard ABG/VBG! Measures 4 different hemoglobin derivatives (using different wavelengths): oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin
Patient-related factors for inaccurate pulse ox readings (didn’t discuss this today, but helpful!)
- Abnormal hemoglobin
- Vasoconstriction (hypoperfusion, hypothermia) – move the pulse ox to the earlobe!
- Anemia with Hgb<5
- High A1c (overestimates O2sat)
- Venous congestion (low O2sat because of venous pulsation)
- Nail polish
- Cause: congenital vs acquired (in adults, often the latter)
- For the acquired causes, can be from medication overdose/poisoning, OR normal doses in patients with certain enzyme deficiencies (like G6PD). Major categories of drugs to think of: dapsone, topical anesthetics (e.g. benzocaine), inhaled nitric oxide, aniline and its derivatives (e.g. Aniline dyes)
- Symptoms: can be asymptomatic at low levels. When there are symptoms, they include headache, fatigue, dyspnea, and lethargy. At high levels >40%, can have respiratory depression, AMS, shock, seizures and death (admit to ICU!)
- Pulse ox and PaO2: suspect methemoglobin and order co-oximetry when the pulse ox shows an O2sat <90 but PaO2 on ABG is >70!
- O2 sat on pulse ox decreases by about half the methemoglobin percentage up to a methemoglobin level of 20%. At higher levels, the O2 sat by pulse ox plateaus at about 85%
- 1) remove the offending agent (for acquired methemoglobinemia)
- 2) consider methylene blue when level is >30% (***BUT should not be used in patients with known G6PD deficiency since reduction of methemoglobin by methylene blue depends on NADPH generated by G6PD!). Response is rapid!
- 3) If very severe: give adjunctive blood transfusion, exchange transfusion, and/or hyperbaric oxygen