A physiological study to determine the mechanism of carbon dioxide clearance during apnoea when using transnasal humidified rapid insufflation ventilatory exchange (THRIVE)

Abstract

Clinical observations suggest that compared with standard apnoeic oxygenation, transnasal humidified rapid-insufflation ventilatory exchange using high-flow nasal oxygenation reduces the rate of carbon dioxide accumulation in patients who are anaesthetised and apnoeic. This suggests that active gas exchange takes place, but the mechanisms by which it may occur have not been described. We used three laboratory airway models to investigate mechanisms of carbon dioxide clearance in apnoeic patients. We determined flow patterns using particle image velocimetry in a two-dimensional model using particle-seeded fluorescent solution; visualised gas clearance in a three-dimensional printed trachea model in air; and measured intra-tracheal turbulence levels and carbon dioxide clearance rates using a three-dimensional printed model in air mounted on a lung simulator. Cardiogenic oscillations were simulated in all experiments. The visualisation experiments indicated that gaseous mixing was occurring in the trachea. With no cardiogenic oscillations applied, mean (SD) carbon dioxide clearance increased from 0.29 (0.04) ml.min-1 to 1.34 (0.14) ml.min-1 as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min-1 to 70 l.min-1 (p = 0.0001). With a cardiogenic oscillation of 20 ml.beat-1 applied, carbon dioxide clearance increased from 11.9 (0.50) ml.min-1 to 17.4 (1.2) ml.min-1 as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min-1 to 70 l.min-1 (p = 0.0014). These findings suggest that enhanced carbon dioxide clearance observed under apnoeic conditions with transnasal humidified rapid-insufflation ventilatory exchange, as compared with classical apnoeic oxygenation, may be explained by an interaction between entrained and highly turbulent supraglottic flow vortices created by high-flow nasal oxygen and cardiogenic oscillations.

Keywords: THRIVE; anaesthesia, general; apnoea; carbon dioxide washout; cardiogenic oscillation; high-flow nasal oxygen; oxygenation, apnoeic.

© 2019 The Authors. Anaesthesia published by John Wiley & Sons Ltd on behalf of Association of Anaesthetists.

Figures

Figure 1

Schematics of the three models used for the experiments: (a) two‐dimensional fluid model used for flow visualisation with particle image velocimetry; (b) three‐dimensional gas model used to visualise transport of gas between carina and mouth; and (c) three‐dimensional model and lung simulator used for turbulence and carbon dioxide clearance measurements. DAQ, data acquisition system.

Figure 2

Fluid flow visualisation using particle image velocimetry in a two‐dimensional water model of the airway with both high‐flow nasal oxygenation and simulated cardiogenic oscillations applied. (a) snapshot of particles in the model; (b) flow vectors at peak cardiogenic inspiration; (c) flow vectors at peak cardiogenic expiration; and (d) flow vectors in the oropharynx.

Figure 3

Video frames showing movement and clearance of nebuliser droplets from the trachea of the three‐dimensional airway visualisation model over one cardiogenic cycle, with a cardiogenic stroke of 20 ml. (a) Without high‐flow nasal oxygen and (b) with high‐flow nasal oxygen.

Figure 4

Mean bulk flow turbulence intensity (RMS velocity variation) measured using hot wire anemometry in the three‐dimensional airway model at different high‐flow nasal oxygen flow rates. (a) during cardiogenic inspiration; and (b) during cardiogenic expiration. Blue = lower trachea; red = mid‐trachea; yellow = subglottis. Error bars = 1 SD.

Figure 5

Clearance of carbon dioxide measured in the carbon dioxide clearance experiment. Error bars show ±3 SD at each combination of high‐flow nasal oxygen (HFNO)/cardiogenic stroke.

Figure 6

Comparison of the rates of rise of carbon dioxide under different oxygenation conditions. AO, apnoeic oxygenation/airway obstruction, combined data mean (95%CI) 11, 46, 47, 48; LFDTI, low‐flow direct tracheal insufflation with intra‐tracheal catheter at a rate of 0.5 l.min−1 35; THRIVE, transnasal humidified rapid‐insufflation ventilatory exchange, slope (95%CI) 8. HFDTI: high‐flow direct tracheal insufflation via tracheal tube at 45 l.min−1, mean (95%CI) 36.