Respiratory failure is the point at which the lungs can no longer meet the body's demand for gas exchange. It is the final common pathway of an enormous range of illness — pneumonia, heart failure, asthma, COPD, opioid overdose, neuromuscular disease, trauma — and it is one of the most common reasons a ward patient ends up in intensive care. Two things make it a priority topic for the RN. The first is that respiratory deterioration is almost always visible before it is measurable. Respiratory rate is the earliest and most reliably abnormal vital sign in a deteriorating patient, and it is also the one most often eyeballed rather than counted. A patient breathing at 28 with a normal blood pressure and a saturation of 94 percent is in more danger than the numbers suggest. The second is that oxygen saturation is a late and forgiving signal. Because of the shape of the oxyhemoglobin dissociation curve, saturation stays near normal while the partial pressure of oxygen falls substantially, then drops steeply once it starts. By the time the pulse oximeter...