CFRN Exam

The questions are rarely pure recall. A typical item describes a patient with a small pneumothorax, an air-filled endotracheal tube cuff or a ventilated patient on 50% oxygen, gives a cruising or cabin altitude, and asks for the expected change or the best nursing action. If you understand why each gas law matters at the bedside, you can reason through any variation.

This chapter reviews how the atmosphere is built, how barometric pressure falls with altitude, and the six gas laws the CFRN exam expects you to apply. Learn the pressure numbers well enough to estimate expansion and oxygen availability without a calculator.

Air has weight. At sea level the column of air above us exerts about 760 mmHg (29.92 inches of mercury, 14.7 psi, one atmosphere). As altitude increases there is less air above, so pressure falls, but not in a straight line: pressure falls fastest at low altitude because air near the surface is compressed by the air above it. By about 18,000 feet the pressure is half of sea level, which means a given mass of gas has doubled in volume.

Approximate Barometric Pressure by Altitude

Altitude is described in two ways. Mean sea level (MSL) altitude is height above sea level; above ground level (AGL) altitude is height above the terrain below. A helicopter cruising at 1,500 feet AGL over a plateau that sits at 6,000 feet MSL exposes the patient to about 7,500 feet of physiologic altitude. The body responds to MSL pressure, not to height above the ground, so always ask what the true pressure altitude is.

Rotor-wing aircraft are unpressurized and usually fly a few thousand feet above the ground, so the patient experiences the actual atmospheric pressure at that altitude. Fixed-wing aircraft are usually pressurized, and the pressure inside is described as cabin altitude. A jet cruising at 35,000 feet typically maintains a cabin altitude of about 8,000 feet. A pilot can lower the cabin altitude (a sea-level cabin restriction) by flying lower, which costs time and fuel but protects patients who cannot tolerate gas expansion or lower oxygen pressure.

Each gas law describes one relationship. The exam rewards knowing which law explains which clinical problem.

The Gas Laws and Their Transport Applications

Boyle's law is written P1V1 = P2V2. If the pressure falls, volume rises by the same proportion. Gas trapped in a closed space either expands, if the wall can stretch, or raises the pressure inside, if the wall cannot. Either result can harm the patient.

Charles' and Gay-Lussac's laws are about temperature. They matter most for oxygen cylinders and for gas volume in cold aircraft. A cylinder that reads 2,000 psi in a heated hangar will read lower after sitting in a cold aircraft overnight even though no oxygen has been used, and a cylinder left in a hot ambulance will read higher. Gas temperature also changes cuff and splint volumes slightly, although pressure change from altitude has a much larger effect.

Henry's law is familiar from opening a carbonated drink: lowering the pressure lets dissolved gas form bubbles. A diver who flies soon after diving can develop decompression sickness because dissolved nitrogen comes out of solution when ambient pressure falls. Divers with suspected decompression illness are transported at the lowest safe altitude or in a sea-level cabin, on 100% oxygen. The CFRN outline no longer emphasizes evolved-gas disorders, so know the principle rather than the details.

Graham's and Fick's laws describe diffusion. Fick's law is the clinically useful one: oxygen moves from alveolus to blood in proportion to the surface area and the pressure gradient, and in inverse proportion to membrane thickness. At altitude the alveolar oxygen pressure falls, shrinking the gradient. A patient whose membrane is already thickened by edema, pneumonia or ARDS loses oxygenation faster than a healthy person.

Barometric pressure is about 760 mmHg at sea level, 565 at 8,000 feet, 523 at 10,000 feet and 380 at 18,000 feet, where trapped gas doubles in volume.

The patient responds to pressure altitude (MSL or cabin altitude), not height above the ground.

Rotor-wing cabins are unpressurized; fixed-wing cabins usually sit near 8,000 feet unless a sea-level cabin is requested.

Boyle explains trapped gas, Dalton explains altitude hypoxia, Charles and Gay-Lussac explain temperature effects, Henry explains dissolved gas.

Fick's law explains why patients with diseased lungs desaturate first at altitude.