Chapter 20

Blood gases can be sampled at the arterial level, venous level, and mixed or central venous level and provide key information needed for management of the critically ill patient. This chapter reviews the application and interpretation of blood gases.

At sea level, at 0°C (32°F), the average atmospheric pressure is 760 mm Hg. Dry room air contains 21% o2, 79% nitrogen, and 0.04% co2. Based on the universal gas law, the Po2 is: 0.21 × 760 mm Hg = 159 mm Hg at these conditions.

The Fio2 is the fraction (%) of inspired o2. At room air, it is 21% (20% often used in round out). As Fio2 increases, the Po2 proportionately increases. Unless a patient is in a closed system like a ventilator, the Fio2 is only estimated. The Fio2 depends on the rate of o2 flow and the patient’s minute ventilation. Each liter per minute of o2 flow within a nasal cannula increases the Fio2 by about 4%. So, if room air is 21% o2, then 1 L/min provides 25%, 2 L/min provides 29%, 3 L/min provides 34%, 4 L/min provides 38%, etc. Flow rates >4 L/min through a nasal cannula are poorly tolerated because of upper airway irritation. A simple o2 mask provides an Fio2 of 35% to 60% at flows of 10 to 15 L/min. A non-rebreather mask with a reservoir can provide 95% o2 at 10 to 12 L/min. A non-rebreather mask has an o2 reservoir and a one-way valve that does not allow the rebreathing of exhaled air. Approximate Pao2 values that are expected in normal persons who are inhaling various concentrations of o2 are listed in Table 20-1. An expected Pao2 when a patient is given o2 can estimated by multiplying the actual delivered percentage of o2 by 6. Thus, a patient getting 60% o2 would be expected to have a Pao2 of about 60 × 6, or 360 mm Hg.

Table 20-1 Expected PaO2 in Patients Inhaling Various Concentrations of Oxygen (mm Hg)

For every 1000 ft (305 m) rise in altitude, barometric pressure drops approximately 25 mm Hg, and the atmospheric Po2 drops about 5 mm Hg. When a person breathes air at 30,000 ft (914 m), where the barometric pressure is about 226 mm Hg, the Po2...

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