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A rule for predicting the new equilibrated carbon dioxide partial pressure after changes in the ventilation frequency

Introduction

In mechanical ventilation the arterial carbon dioxide partial pressure (PCO2) is one of the key parameters to control the ventilation frequency. Qualitatively, the effect of changes in the ventilation frequency on the arterial PCO2 level is well known. However, little is known about the time it takes for the PCO2 value to reach a new equilibrium after a change in the ventilation frequency (the period of latency), nor in what way the transition between two states of equilibrium takes place.

Methods

We carried out a clinical study on patients without any history of lung disease or intracranial surgery in order to determine these relations. We collected data for the arterial PCO2 from blood gas analyses at discrete points in time as well as continuous end-tidal CO2 (etPCO2) and transcutaneous CO2 (PtcCO2) data and checked for the accuracy of the latter two. Least-squares fitting and a statistical analysis were carried out.

Results

We determined a general rule to estimate the period of latency after a change in the ventilation frequency. Furthermore, we specified the relation between a change in the ventilation frequency and the change in the PCO2 level. Last, the transition between two PCO2 levels was found to follow an exponential law and the fitting resulted in a formula for the prediction of the new PCO2 level. The new equilibrium can be predicted with high confidence in all cases after only 3 to 4 minutes using four data points while the period of latency lasts much longer, usually between 10 and 20 minutes.

Conclusion

The general rule for the period of latency allows an estimation of the amount of time it takes for the PCO2 value to stabilise again after a disturbance. A quantitative knowledge of the transition between two PCO2 equilibria allows for the prediction of the new PCO2 level long before the period of latency is over. Thus, with our relation between the change in ventilation frequency and the change in PCO2 at hand, an optimal PCO2 level can be aimed for at bedside in the shortest time span possible.

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Correspondence to S Buehler.

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Buehler, S., Jensen, M., Lozano, S. et al. A rule for predicting the new equilibrated carbon dioxide partial pressure after changes in the ventilation frequency. Crit Care 16, P125 (2012). https://doi.org/10.1186/cc10732

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Keywords

  • Lung Disease
  • Discrete Point
  • Short Time Span
  • Arterial Carbon Dioxide
  • Arterial PCO2