Oxygen therapy for critically ill patients (Proceedings)
Oxygen is an ideal therapeutic agent because it is easy to administer, readily available and if used correctly relatively safe. There are very few contraindications to oxygen supplementation. Oxygen therapy is often used in emergency and critical care medicine. Optimizing oxygen supplied to tissues is imperative in any number of clinical conditions where hypoxia may play a role. During hypoxia, cellular metabolism is less efficient and may lead to organ dysfunction and death. The critical care clinician must understand the physiology of oxygen delivery and recognize cases that will benefit from oxygen therapy. The distinction between hypoxia and hypoxemia is an important one.
Hypoxemia indicates low concentrations of oxygen in arterial blood. This is identified by performing arterial blood gas analysis. Hypoxia is a decreased concentration of oxygen at the level of the tissue or cell. The reason this distinction is important in the clinical setting is there are a limited number of physiologic causes of hypoxemia and if a patient is hypoxemia, hypoxia must be present. However not all patients that are hypoxic are hypoxemic.
Indications• There are five pathophysiologic causes of hypoxemia:
• Low % inspired oxygen
• Diffusion barrier impairment
• V/Q mismatch
It is not difficult to identify these causes in the clinical setting with the use of arterial blood gas analysis. Hypoventilation will be characterized by hypercapnia and if that is not present, hypoventilation is not the cause. The possibility of reduced concentrations of inspired oxygen is remote in most patients and therefore the top 3 causes of hypoxemia are diffusion barrier impairment, V/Q mismatch and shunting. To further simplify this, a shunt is the physiologic extreme of a low V/Q mismatch, so in reality V/Q mismatch and diffusion barrier impairment are causes of hypoxemia. In the clinical setting the patients response to an oxygen challenge may help identify the cause of hypoxemia. Diseases leading to diffusion barrier impairment tend to be oxygen responsive. Diseases leading to V/Q mismatch are variably oxygen responsive with high V/Q mismatches being oxygen responsive and low V/Q mismatches being poorly oxygen responsive. This is the reason diseases leading to intrapulmonary shunting are not oxygen responsive.
In addition to the mentioned causes of hypoxemia, there are many clinical situations in which oxygen therapy may be of benefit including respiratory distress, sepsis, hyperthermia, pleural space disease, congestive heart failure, anemia, shock, pulmonary contusions, pulmonary hypertension, seizures and head trauma.
Clinical signs observed in hypoxemic patients include tachypnea, tachycardia, cyanosis, cardiac arrhythmias and anxiety. Clinical response to oxygen is important, but objective criteria such as blood gas analysis, pulse oximetry, and alveolar-arterial (A-a) gradients should be evaluated. Another useful ratio to assess response to oxygen therapy is the arterial partial pressure of oxygen/fraction of inspired oxygen (PaO2/FIO2).
A-a gradient = [(BP-47) 0.21) - (PaCO2/0.8)] - PaO2
Normal = < 10 mm Hg