Measurement of functional residual capacity during mechanical ventilation for acute respiratory failure. A comparison between closed and an open-circuit helium dilution technique.

Expired volume was measured with a Godart pneumotachograph at the expiration port of the ventilator (Fig 1, tracing 5). We measured the concentration of helium with a katharometer (Codart, response time 0.1 see), at the expiration port of a Siemens-Elema-900 ventilator. From equations 2 and 3, it follows that absolute calibration of the helium meter is not necessary; only linearity of the meter is innportant. The linearity of the katharometer has been checked with different concentrations of helium in gas mixtures, comparable to those used in patients. An example of a helium washout curve is shown in tracing 8 of Figure 1. A mixture of 50 percent helium in oxygen was administered to the patient via the ventilator, by temporarily con-