Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 61

This is a bluish discolouration of the skin and mucous membranes that results from an excessive amount of reduced (deoxygenated) haemoglobin (usually >5 g/dL in patients of European descent; the sign may be harder to detect in darker skin). As it relates to the quantity of deoxygenated blood rather than the proportion, it follows that it will be more readily observed in those with polycythaemia than anaemia. Central cyanosis is best seen on the tip of the tongue and is the cardinal sign of hypoxaemia, although it is not a sensitive sign because it is not usually detectable until the oxygen saturation has fallen to well below 85%, corresponding to a PO₂ of <8 kPa (60 mmHg). Because of the poor sensitivity of cyanosis, it is essential to measure oxygenation by oximetry or arterial blood gas sampling in patients at risk for hypoxaemia. Peripheral cyanosis will be present if there is central cyanosis, but it may also be caused by local circulatory slowing in the peripheries, resulting in more complete extraction of oxygen from the blood (e.g. blue hands and ears in cold weather).