Optimal method of flow and volume monitoring in patients mechanically ventilated with helium–oxygen (He–O₂) mixtures

In mechanically ventilated patients reducing the density of the inspired gas by substituting helium (He) for nitrogen offers several theoretical benefits. However, accurately monitoring tidal volumes of He–oxygen (O₂) mixtures with conventional flowmeters is problematic, as all commonly employed devices are adversely affected by changes in gas density. We tested two widely available flowmeters, to ascertain whether reliable and reproducible correction factors could be obtained.

We used an unadapted Galileo ventilator (Hamilton Medical,Switzerland), to which heliox (21% O₂ 79% He) was connected via the air inlet. This ventilator measures flow/ volume utilising a variable orifice pneumotachogrpah (VOP). The circuit was then connected to a Pitot tube flowmeter (PT) (MCOVX, Datex-Ohmeda, Finland) which in addition, contains a side stream gas sampler to monitor gas composition. In place of a patient, a mechanical lung (BIO-TEK Instruments Inc., USA) was employed, which accurately measures delivered gas volumes independent of gas density. Carbon dioxide (CO₂) was entrained, distal to the flowmeters, just proximal to the mechanical lung, to simulate the clinical setting. Both flowmeters were repeatedly tested across a range of tidal volumes (Vt) (200–1200 ml).

This method proved to be reliable and reproducible. Inspired and expired Vt as measured by the two devices, when plotted against that measured by the mechanical lung, showed excellent linear correlation (R² = 0.99 for all). From the equations of these lines we calculated a set of correction factors (CF) and ordinates (ord). All measurements were repeated five times and means (Av) and standard deviations (SD) derived; results are tabulated.

We conclude that both of these devices can be reliably employed to monitor respiratory variables in patients mechanically ventilated with He-O₂ provided gas composition, temperature and relative humidity are accounted for. However, the consistency of ord values for the VOP are advantageous and make this the preferred device.

Table 1