Carbon monoxide can be generated from a malfunctioning space heater, from car exhaust, or from a burning building. Individuals exposed to carbon monoxide experience headache, nausea, and dizziness, and if it is not recognized, such individuals may die. They often have a cherry-red appearance, and oxygen saturation as measured with an oximeter is high (approaching 100%). Even on an arterial blood gas, the PAO2 may be normal. Nevertheless, the tissues are depleted of O2. Thus it is imperative that the clinician recognize a potential case of carbon monoxide poisoning and order an oxygen saturation measurement with the use of a carbon monoxide oximeter. If a patient has carbon monoxide poisoning, there will be a marked difference between the measurement of oxygen saturation by oximetry and that measured with a carbon monoxide oximeter.
A useful way to examine the relationship between ventila tion and perfusion is with the two–lung unit model (Fig.
23.8 ). Two alveoli are ventilated, each of which is supplied by blood from the heart. When ventilation is uniform, half the inspired gas goes to each alveolus, and when perfusion is uniform, half the cardiac output goes to each alveolus. In this normal unit, the ventilation/perfusion ratio in each of the alveoli is the same and is equal to 1. The alveoli are perfused by mixed venous blood that is deoxygenated and contains increased PaCO2. PAO2 is higher than mixed venous O2, and this provides a gradient for movement of O2 into blood. In contrast, mixed venous CO2 is greater than PACO2, and this provides a gradient for movement • Fig. 23.8 Simplified Lung Model of Two Normal Parallel Lung Units. Both units receive equal volumes of air and blood flow for their size. The blood and alveolar gas partial pressures are normal values in a resting person at sea level. PACO2, partial pressure of alveolar carbon dioxide; PAO2, partial pressure of alveolar oxygen; PiCO2, partial pressure of inspired carbon dioxide; PiO2, partial pressure of inspired oxygen; PpvCO2, partial pressure of carbon dioxide in portal venous blood; PpvO2, partial pressure of oxygen in portal venous blood; PvCO2, partial pressure of carbon dioxide in mixed venous blood; PvO2, partial pressure of oxygen in mixed venous blood.
of CO2 into the alveolus. Note that in this ideal model, alveolar-arterial O2 values do not differ.
An anatomical shunt occurs when mixed venous blood bypasses the gas-exchange unit and goes directly into the arterial circulation (
Fig. 23.9 ). Alveolar ventilation, the distribution of alveolar gas, and the composition of alveolar gas are normal, but the distribution of cardiac output is changed. Some of the cardiac output goes through the pulmonary capillary bed that supplies the gas-exchange units, but the rest of it bypasses the gas-exchange units and goes directly into the arterial circulation. The blood that bypasses the gas-exchange unit is thus shunted, and because the blood is deoxygenated, this type of bypass is called a right-to-left shunt. Most anatomical shunts develop within the heart, and they develop when deoxygenated blood from the right atrium or ventricle crosses the septum and mixes with blood from the left atrium or ventricle. The effect of this right-to-left shunt is to mix deoxygenated blood with oxygenated blood, and it results in varying degrees of arterial hypoxemia.