Anesthesia Equipment, Monitoring, and Technology
Machine, circuit, and monitors
The anesthesia machine must deliver a known oxygen concentration, ventilate, and scavenge waste gas. Oxygen supply includes a pipeline inlet, cylinder backup, pressure regulators, a fail-safe valve, and an oxygen failure alarm; the fail-safe reduces nitrous oxide flow when oxygen pressure falls, but it does not guarantee oxygen delivery. Vaporizers are agent-specific and should be serviced; filling a modern agent-specific vaporizer with the wrong agent or overfilling can produce a dangerous concentration. The breathing circuit may be Mapleson, circle, or nonrebreathing. Circle systems use unidirectional valves and a CO2 absorbent; exhausted absorbent causes inspired CO2, hypercapnia, and a rising capnograph baseline. Check for leaks, obstruction, and proper scavenging before each case.
Monitoring is multimodal: pulse oximetry, capnography, ECG, blood pressure, temperature, and inspired/expired gas analysis. Pulse oximetry estimates arterial saturation but is unreliable with poor perfusion, motion, nail polish, methemoglobin, and carboxyhemoglobin; carboxyhemoglobin can read falsely normal or high. Capnography is the standard for confirming tracheal intubation and detecting disconnection, obstruction, hypoventilation, and low cardiac output. A sudden loss of waveform suggests disconnection, obstruction, or cardiac arrest; a gradual rise suggests hypoventilation or absorbed CO2. Neuromuscular monitoring with train-of-four guides reversal and detects residual blockade.
Traps and a rule
Trap: treating an oxygen analyzer as optional or assuming a fail-safe provides oxygen. Another is interpreting a normal pulse oximeter as normal oxygenation in carbon monoxide poisoning. Also, an esophageal intubation may produce a transient waveform, so confirm with sustained capnography and clinical signs.
Rule of thumb: if the capnogram suddenly disappears, disconnect the patient from the ventilator, manually ventilate with 100% oxygen, and check tube position, circuit connections, and vital signs. If the baseline rises, check the CO2 absorbent and ventilation.