Understanding Mechanical Ventilation A Practical Handbook
The care of patients with multiple life-threatening problems is a monumental challenge. Burgeoning information has deluged the generalist and placed increasing reliance on the specialist. Predictably, this has led to the evolution of a team approach, but
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ea level, the partial pressure of O2 is: 160 mmHg 760 mmHg x 0.21 = 160 mmHg Oxygen is available for inspiration at sea level at a partial pressure of about 160 mm Hg Within the respiratory tract the partial pressure of O2 is about 150 mmHg 0.21 x (760-47) x 0.21 = 149 mmHg As it enters the respiratory system, O2 is humidified by the addition of water vapour (partial pressure 47 mm Hg). Humidification serves to make the insired air more breathable; it also results in the drop of the partial pressure of oxygen to about 150 mmHg
Figure 7.1. The oxygen cascade. (Adapted from Hasan23) A. Hasan, Understanding Mechanical Ventilation, DOI: 10.1007/978-1-84882-869-8_7, © Springer-Verlag London Limited 2010
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Chapter 7. Monitoring Gas Exchange In the alveoli the partial pressure of O2 (PAO2) is about 100 mmHg 149-(40/0.8) = 99 mmHg
40 is the normal value of PaCO2 in mmHg. Since CO2 is easily diffusible across the alveolocapillary membrane, arterial CO2 (PaCO2) can be assumed to be the same as alveolar CO2 (PACO2). 0.8 is the respiratory quotient In the alveoli, oxygen diffuses into the alveolar capillaries and carbon dioxide is added to the alveolar air. The result of a complex interaction between three factors [alveolar ventilation, CO2 production (VCO2) and the relative consumption of C2 (VO2)] causes the partial pressure of CO2 in the alveolus to drop to 100 mmHg. this is the pressure of oxygen that equates with the pressure of oxygen in the pulmonary veins, and therefore, with the pressuire of oxygen in the systemic arteries. VCO2 = 250 mL of CO2 /min VO2 = 300 mL of O2 /min
In the systemic arteries the partial pressure of O2 (PaO2) is about 95 mmHg A small amount of deoxygenated blood is added to the systemic arteries (because of a small physiological shunt that normally exists in the body). This is due to unoxygenated blood emptied by the bronchial and thesbesian veins back into the pulmonary veins and the left side of the heart. This “shunt fraction” which represents about 2−5% of the cardiac output, causes the systemic arterial oxygen to ‘‘fall fractionally’’ from 100 mmHg, to about 95 mmHg or less. Thus, in spite of normal gas exchange, the PaO2 may be 5−10 mmHg lower than the PAO2. In the mitochondrion the partial pressure of O2 is unknown Due to substantial diffusion barriers, the amount of oxygen made available to the oxygen-processing unit of the cell (the mitochondrion) is a relatively tiny amount. The mitochondrion appears to continue in its normal state of aerobic metabolism with minimal oxygen requirements. In hypoxia, a fall in the PaO2 within mitochondria (to possible less than 1 mmHg), is required to shift the energy producing pathways towards the much less efficient anaerobic metabolism
Figure 7.1. (continued)
mitochondrion appears to continue in its normal state of aerobic metabolism with minimal oxygen requirements. In hypoxia, with a fall in the PaO2 within mitochondria (to possibly less
7.1 The Arterial Oxygen Tension
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than 1 mmHg), aerobic metabolism is considerably reduced,
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