This chapter is probably at least somewhat related to Section F8(ii) from the 2023 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the oxyhaemoglobin dissociation curve and factors that may alter it". The principles underlying the detection of methaemoglobin are discussed elsewhere, as are the causes of methaemoglobinaemia. This chapter focuses on the properties of methaemoglobin, and the situations where it might be undesirable to have too much haem iron in its oxidised Fe3+ state - i.e. essentially any situation where you are not suffering from cyanide toxicity.
The "met" at the front means "changed", as in "meta"; it is not implying that the methaemoglobin is methylated in some way. Oscar Bodansky wrote a splendid monograph on the subject of methaemoglobin chemistry in 1951. Fortunately for the reader, that discussion will wait for another time.
The FMetHb which is reported by the ABG machine is not the result of any sort of idealised biochemical detection method, where each oxidized iron atom is accounted for. Rather, it is a result of the analysis of total haemoglobin absorption-wavelength curves, which are compared to known absorption spectra.
In reality, "methaemoglobin" is the metaphoric representation, the sum influence of several different tetramers on the spectrophotometry reading. Each haemoglobin molecule is a tetramer, and each monomeric component of it can be oxidised individually into one of two states, giving rise to eight distinct populations. These intermediate forms are called "valence hybrids" Yes, "totally oxidised" methaemoglobin can exist (with all four monomers affected) but this would not be the normal state of affairs - normally, the partially oxidised species dominate. "Wintrobe's Clinical Hematology" by Greer et al has a figure (pp 113) demonstrating the changes in the normal oxyhaemoglobin absorption spectrum as more and more haem units are oxidised. Additionally, it is possible to verify the existence of these hybrid species of methemoglobin unequivocally by electrophoretic separation, because each oxidised haem carries one extra positive charge.
The methaemoglobin haem unit is totally useless for oxygen transport. It literally cannot bind oxygen under normal physiological conditions, and therefore there is no such thing as a "methaemoglobin-oxygen dissociation curve". This on its own should not be a problem. Consider: if one has a Hb of 100g/L, and 20% of it is taken out by oxidation, one should still have 80g/L to transport oxygen with, and this would be enough.
However, the lack of carrying capacity is only one of the problems. The presence of an oxidized haem unit in an otherwise normal haemoglobin tetramer leads to the allosteric changes to the haemoglobin molecule. The loss of one haem interferes with "cooperativity", the property of haemoglobin which ensures the sigmoid shape of the normal oxygen-haemoglobin dissociation curve (which is discussed in greater detail elsewhere). The affinity of the unaffected normal haem units in the same molecule increases. The main problem is that the oxidized haem subunits are held fixed in an R-like conformation (R for "Relaxed"), which affects the surrounding normal subunits. The fixed nature of this conformation means that the normal allosteric inhibition of oxygen binding at low pO2 does not occur: the haemoglobin tetramer stubbornly refuses to release oxygen into a hypoxic environment. This phenomenon is known as the Darling-Roughton effect; the net result of this is a shift the oxygen-haemoglobin dissociation curve to the left, and diminished oxygen delivery to the tissues. In this fashion symptoms of methaemoglobinaemia may occur at relatively low FMetHb levels.
Oxidation of haem results in the haemoglobin molecule developing a net positive charge when compared to normal hemoglobin. Thus, methaemoglobin has a higher affinity for negative anions such as
cyanide, fluoride, or chloride, as opposed to the uncharged binding ligands (CO2, CO, and O2) of normal hemoglobin. The avid binding of cyanide to methaemoglobin forms the incredibly stable cyanmethemoglobin, or hemiglobin cyanin, which is used as a marker for haemoglobin in the gold standard haemoglobin concentration assay.
The removal of cyanide from circulation increases the concentration gradient and increases the removal of cyanide from mitochondria, where it is killing you by disabling cytochrome oxidase. Unfortunately, one's situation only improves a little, because now there is this excess of totally useless cyanmethemoglobin in circulation. Oh well, its better than a serum lactate of 60mmol/L. For this reason, in cyanide poisoning haem oxidation is desirable and is usually induced by nitrates (eg. good old amyl nitrate) or by 5-dimethylaminophenol (DMAP).
Weirdly, some animals can exist happily with quite large amounts of methaemoglobin - reptiles and fish especially. The rainbow trout functions happily with a FMetHb of around 17%. Perhaps the rainbow trout is perpetually exposed to toxic levels of cyanide. More likely, it uses the left-shift of the dissociation curve as a reservoir of oxygen; methaemoglobin becomes a useful source of oxygen in situations of extreme hypoxia (such as that experienced by animals adapted to anoxic environments). For this reason, the sea turtle can have up to 65% FMetHb. What is the point of this comment, one might ask? It is difficult to say.