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Capnography
| Capnography is the continuous measurement of the patient's inhaled and exhaled concentrations of carbon dioxide. Display of the waveform is preferable to a digital readout of the values. When an endotracheal tube is inserted, its presence in the trachea must be verified by clinical assessment and identification of carbon dioxide in the exhaled gases. In addition to accidental esophageal intubation, the absence of carbon dioxide in the patient's exhaled gases alerts the anesthesiologist to accidental disconnection of a paralyzed patient from the anesthetic breathing system (also detected by a disconnect alarm) or the sudden cessation of pulmonary blood flow (cardiac arrest). The gradual decrease in exhaled carbon dioxide concentrations over several breaths may reflect a partial leak in the anesthetic breathing system or decreased pulmonary blood flow as may accompany hypotension or pulmonary embolism. Hypoventilation or unexpected increases in carbon dioxide production as associated with malignant hyperthermia or thyroid storm will be promptly reflected by increases in the exhaled concentrations of carbon dioxide. Rebreathing of carbon dioxide due to an exhausted carbon dioxide absorber or malfunctioning inspiratory or expiratory valves will manifest as increased inspired and exhaled concentrations of carbon dioxide. The presence of exhaled carbon dioxide during cardiopulmonary resuscitation is evidence that external cardiac compressions are generating pulmonary blood flow and presumably blood flow to major organs such as the brain and heart. The end tidal carbon dioxide concentration often underestimates the PaCO2 reflecting an alveolar-to-arterial difference for carbon dioxide due to deadspace ventilation. |
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