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Oxygen Delivery and Acute Hypoxia: Physiological and Clinical Considerations
Published in Anthony N. Nicholson, The Neurosciences and the Practice of Aviation Medicine, 2017
Carbamino compounds are formed when CO2 reacts with proteins at the terminal amino (NH2) groups or the amino groups on the side chains of amino acids like lysine and arginine (RNH2 + CO2 ⇔ RNHCOOH). Most carbamino compounds in the blood are formed by combination of CO2 with the amino groups on the globin chains of haemoglobin; haemoglobin has a lot of available NH2 groups and there is a high concentration of haemoglobin in the blood. The amount of carbaminohaemoglobin formed is little affected by PCO2 but very affected by the state of oxygenation of the haemoglobin. Deoxygenated haemoglobin forms carbamino compounds more readily than oxyhaemoglobin and it is also a better buffer, which enhances bicarbonate formation. Both these changes contribute to the Haldane effect – at any given PCO2, the quantity of carbon dioxide carried is greater in deoxygenated than oxygenated blood. Figure 6.5 gives the CO2 dissociation curve for blood with high and low oxygen content. It can be seen that there is no plateau and the relationship is fairly linear and steep over the range of values for PCO2 seen in health and disease.
Oxygen: a new look at an old therapy
Published in Journal of the Royal Society of New Zealand, 2019
Richard Beasley, Diane Mackle, Paul Young
Hyperoxaemia also leads to both acute and chronic physiological responses, which may cause harm, and which are the counterbalance of those resulting from hypoxaemia (O’Driscoll et al. 2017) (Table 1). High FiO2 and hyperoxaemia lead to complex respiratory effects, which, depending on the underlying chronic and/or acute respiratory disorders, may include a reduction in respiratory drive, the reversal of hypoxic pulmonary vasoconstriction and absorption atelectasis, resulting in reduced alveolar ventilation (Aubier et al. 1980; Robinson et al. 2000). Together with the Haldane effect (the unloading of CO2 by haemoglobin in hyperoxic conditions), these respiratory effects lead to an increase in the arterial partial pressure of carbon dioxide (PaCO2) which may be harmful if marked (O’Driscoll et al. 2017). In response to hyperoxaemia, the cardiovascular systemic reduces oxygen delivery to the tissues by decreasing cardiac output, increasing systemic vascular resistance and blood pressure, and reducing coronary arterial blood flow, resulting in a paradoxical reduction in myocardial oxygen consumption (Waring et al. 2003; Beasley et al. 2007a; Farquhar et al. 2009). Not all the effects are potentially harmful, as the liberal provision of oxygen intraoperatively and in the early post-operative period may reduce the risk of post-operative infection (Allegranzi et al. 2016).