Hemoglobin: structure and function II

Question 1

Describe what methemoglobinemia is

Answer 1

when there is either too much methemoglobin production or because of decreased methemoglobin reduction. Methemoglobin forms when iron within hemoglobin is not reduced from its 2+ (ferrous—oxygen binding form) form to its 3+ (ferric) form. Usually NADPH methemoglobin reductase keeps the iron in its ferrous form in the erythrocyte. When iron is stuck in its +3 ferric form, it can’t bind oxygen, shifts the oxygen dissociation curve to the left and P50 drops

Question 2

What causes methemoglobinemia?

Answer 2

Can by acquired or genetic

• Acquired: exposure to drugs and chemicals that causes the oxidation of the heme by reaction with free radicals of hydrogen peroxide or NO or OH can generate methemoglobin. Examples: use of benzocaine in neonates, well water with nitrates.
• Genetic (hereditary): most commonly due to homozygous deficiency of cytochrome b5 reductase or mutation in hemoglobin resulting in production of hemoglobin M.
• Newborns are susceptible because HbF is more readily oxidized to ferric state, also decreased amount of cytochrome b5 reductase, may become cyanotic with well water, raw spinach, disinfectants, benzocaine.

Question 3

How to dx methemoglobinemia?

Answer 3

person looks cyanotic but arterial partial pressure of O2 is normal, blood looks dark-red/chocolate/brown-blue and doesn’t change with O2 exposure

Question 4

How to tx methemoglobinemia?

Answer 4

• Genetic: no treatment needed for hemoglobin M, cytochrome b5 deficient patients treated with methylene blue (for cosmetic reasons)
• Acquired: methemoglobin levels below 30%=minimal symptoms (light headed, headache), 30-50%= depress CV and CNS, rapid breathing, shortness of breath, 50%-70%= severe, stupor, low HR, respiratory depression, convulsions, 60%+ can be lethal and 70%+ is not compatible with life. Remove drug or chemical causing methemoglobinemia. Methylene blue can be given via IV to provide artificial electron acceptor from the reduction of methemoglobin via the NADPH-dependent pathway

Question 5

Explain the pathophysiology of carbon monoxide poisoning

Answer 5

a. CO binds heme with an affinity 240x that of oxygen, normally people have 3% CO (in smokers its 10-15%), when heme binds CO, allosteric change occurs so that other 3 hemes unload oxygen less well, increasing the affinity of hemoglobin for oxygen and decreasing the delivery of O2 to the tissues (Curve shifts left)

Question 6

Sx of CO poisoning

Answer 6

headache, malaise, nausea, dizziness, NOT cyanotic, “cherry red” appearance, higher levels: seizures, coma, MI, 40% of people have late neurological defects

Question 7

Dx of CO poisoning

Answer 7

co-oximetry

Question 8

Tx of CO poisoning

Answer 8

100% O2 or hyperbaric O2 (competes with CO for binding sites on the heme)

Question 9

Explain in basic terms how a pulse oximeter works.

Answer 9

a. Pulse oximetry is based on Beer-Lamberts law which states the absorption of light of a given wavelength passing through a non-absorbing solvent, which contains an absorbing solute, is proportional to the product of the solute concentration, the light path length and an extinction coefficient.
b. Pulse oximeter probe is a photo detector and 2 light emitting diodes (one at 660nm-red where deoxyhemoglobin absorbs maximally and one at 940nm-infrared where oxyhemoglobin absorbs maximally). The emitter and detector face each other through the tissue (placed on the finger), only PULSATILE FLOW (arterial blood flow) is measured, photodiodes switch on and off several hundred times/second and light absorption is measured. Displayed value is an average of 3-6 seconds. Used to measure oxygen saturation (not partial pressure of oxygen or carrying capacity of oxygen).

Question 10

Describe situations where a pulse oximeter reading may inaccurately reflect a patient’s true oxygenation status

Answer 10

Inaccurate is probe is placed wrong, if only 1 diode is working, shivering/seizing patient, nail polish, deeply pigmented ski, anemia, shock, abnormal hemoglobins (ie: carboxyhemoglobin absorbs at 660nm so will give a falsely high reading, methemoglobin absorbs at 660nm and 940nm so it will also give inaccurate results). Co-oximetry (measure absorbance at 4+ wavelengths) is needed to quantify carboxyhemoglobin and methemoglobin levels