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Accuracy of predicting arterial blood gases following mechanical ventilator adjustment


Clinicians adjust mechanical ventilator (MV) in critical care units to optimise arterial blood gases (ABG). The present study evaluates their accuracy in predicting ABG values following MV adjustment.


Thirty-one sets of data were collected from 16 patients requiring mechanical ventilation on the ICU. Each set consisted of two subsets - before and after MV adjustment. Each of subset consisted of the following variables: FIO2, respiratory rate (breaths/minute), tidal volume (ml/kg), body temperature (°C), haemoglobin concentration (g/dl), and ABG values (pH, PaCO2, PaO2, base excess). All data, with the exception of ABG values after MV adjustment, were presented to clinicians. Thirty-three clinicians (11 respiratory therapists (RTs), 11 nurses, and 11 medical staff) were asked to predict the new ABG values as they would in clinical settings. All data are presented as mean ± SD unless stated otherwise.


All datasets include changes in FIO2, while only 13 datasets include changes in minute ventilation. Increases in FIO2 were 0.19 ± 0.04 and decreases were -0.12 ± 0.04. Size of change of minute ventilation was 1.38 ± 0.95 l/minute. The absolute magnitudes of change for measured pH, PaCO2, and PaO2 were 0.024 ± 0.027, 0.40 ± 0.37 kPa and 5.91 ± 3.35 kPa, respectively. The magnitudes of change for predicted pH, PaCO2, and PaO2, respectively, were 0.025 ± 0.033, 0.34 ± 0.23 kPa, and 3.45 ± 2.03 kPa for medical staff ; 0.024 ± 0.021, 0.40 ± 0.41 kPa, and 3.91 ± 1.75 kPa for RTs; and 0.022 ± 0.015, 0.32 ± 0.22 kPa, and 3.21 ± 2.1 kPa for nurses. The 95% limits of agreement between predicted and measured size of changes for pH, PaCO2, and PaO2, respectively, were 0.043 to -0.041, 0.724 to -0.596 kPa, and 3.22 to -8.14 kPa for medical staff ; 0.036 to -0.036, 0.66 to -0.66 kPa, and 2.64 to -6.64 KPa for RTs; and 0.036 to -0.076, 0.74 to -0.9 kPa, and 3.91 to -9.31 kPa for nurses. Correlation between duration of clinicians' clinical experience and errors in predicting size of change for pH, PaCO2, and PaO2 (-0.0225, -0.054, and 0.173, respectively) were weak and not statistically significant.


Clinicians show considerable variation in predicting ABG values. Medical staff and nurses underestimate the magnitude of PaCO2 changes whereas RTs show some accuracy. All clinicians have a large bias in predicting the size of PaO2 changes.

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Correspondence to H Al-Otaibi.

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Al-Otaibi, H., Bedforth, N., Mahajan, R. et al. Accuracy of predicting arterial blood gases following mechanical ventilator adjustment. Crit Care 14, P166 (2010).

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  • Mechanical Ventilator
  • Respiratory Rate
  • Body Temperature
  • Tidal Volume
  • Critical Care