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The Haldane M-value is the ratio of half-saturation pressures for O2 and CO (PO2/PCO) and is a key value of physiological and pathological models exploring CO metabolism or exposure to CO. Nevertheless, the M-values obtained from different studies and different species appear to be inconsistent. A better estimation of M-values and a consideration of interspecies variation will ensure the accuracy of CO study results and allow reliable models to be constructed. We performed a systematic review and meta-analysis in order to determine and compare M-values according to species and experimental conditions. We used electronic libraries (PubMed/Embase/JSTORE) from 1895 to January 2016. For each article, we collected the following parameters: species, age, temperature, pH, atmospheric pressure, concentration of 2,3-diphosphoglycerate (2,3-DPG) or methemoglobin, and type of hemoglobin. The systematic review found 25 articles addressing the M constant, including 135 with individual data and 51 with mean values concerning mammals, laboratory animals, fish, fetal hemoglobin and invertebrates. M-values at a physiological temperature and pH were assessed only for adult (137 [119–153]) and fetal (135 [112–168]) sheep, dogs (215 [193–218]), mice (194 [118–278]) and rats (191 [177–200]). A correlation between M-values and temperature was confirmed for sheep and horses. A correlation between M-values and pH could not be inferred because there was a lack of data. M-values varied according to species. Only the influence of temperature was confirmed. A formal measurement of the M constant in the given experimental conditions should be performed for each animal model study of CO. Further homogeneous studies are necessary to compare the M-values of different animals.

Mucormycosis is an opportunistic fungal infection involving among others the paranasal sinuses, nasal fossa and brain parenchyma. Mucor can invade the brain parenchyma by either contiguous spread from the paranasal sinuses or through vascular invasion. We report a case of fatal rhino-cerebral mucormycosis in whom cytotoxic edema at magnetic resonance diffusion-weighted imaging was symmetrically restricted to both neocortical and paleocortical primary areas of olfactory projection at earliest phase of the disease process. Shortly later tissue damage extended into the whole brain. This undescribed observation raised the hypothesis of preferential way of brain invasion by Mucor through the olfactory tract.