The anesthesia breathing system is a gas pathway that connects the patient’s airway to the anesthesia machine; it extends from the fresh gas inlet to the point where gas escapes, either into the atmosphere or into a scavenging system. The breathing system functions to deliver gas from the anesthesia machine to the patient and to remove carbon dioxide by washout or chemical neutralization. Throughout this circuit, there are many factors that have an impact on the delivery and exit of gases.
Breathing circuits have some degree of resistance to flow that causes a pressure drop as gases pass through the tube. This is illustrated through Ohm’s and Hagen–Poiseuille’s laws. In Ohm’s law, flow (Q) is directly proportional to the pressure (P) difference and inversely related to resistance (R). In Poiseuille’s law, the pressure gradient is directly proportional to the length (L), viscosity (ν), and flow rate (V), and inversely proportional to the radius (r) to the fourth power.
Two important factors affecting the “airway” resistance are flow rate and the type of flow. Flow can be laminar, turbulent, or, clinically, it is more often a combination of both.
Laminar flow illustrates particles that flow in one direction, parallel to the wall, and down a pressure gradient. Looking at the diameter, the flow is fastest in the center and decreases parabolically due to friction. Resistance is directly related to the flow rate. Poiseuille’s law follows the laminar flow.
In turbulent flow, particles move in all directions, and the flow rate is the same across the diameter of the tube. The pressure difference will increase to maintain flow, and this, in turn, increases resistance. For turbulent flow, gas density is more important than viscosity, and resistance is directly related to the flow rate squared. Turbulent flow can either be generalized or localized. When laminar flow exceeds a critical flow rate, it becomes generalized, turbulent flow. Localized turbulent flow occurs below the critical flow rate, at constrictions, curves, or other irregularities in the tube.
To reduce resistance, the circuit length should be minimized, diameter maximized, and constrictions, or areas likely to generate turbulent flow should be avoided.
Resistance will foist strain on the patient if he or she is required to do some, or all, of the respiratory work when on a ventilator. Consequently, resistance in the anesthesia breathing system parallels the work of breathing.
Rebreathing involves inhaling previously respired gases that may or may not have carbon dioxide removed; inspired gas is a combination of fresh gas and rebreathed gas. The effect rebreathing has on a patient will depend on: (1) fresh gas flow and (2) mechanical dead space.