Immediate and long-lasting effects of recruitment maneuvers on the pressure-volume curve in normal anesthetized animals

Introduction

Recruitment plays a major role in defining the sigmoid shape of the static pressure-volume (PV) curve in ARDS. The injured lung is at risk of atelectasis due to surfactant dysfunction and due to increased superimposed pressure caused by increased lung weight. In normal lungs, however, alveoli are much more stable than in the injured lung. Therefore, it is not clear to what extent recruitment defines the shape of the PV curve in normal lungs. We hypothesized that resolving atelectasis using a recruitment maneuver leads to typical changes in the shape of the PV curve.

Methods

After induction of anesthesia and oral intubation, six sheep were mechanically ventilated in a prone position. Data acquisition was performed using an Evita4Lab measurement system (Draeger Medical, Lübeck, Germany). The protocol consisted of a low flow inflation maneuver (LF1), followed by a recruitment maneuver (RM) and a second low flow inflation (LF2). Before LF1 and between interventions, animals were ventilated at ZEEP for 5 min. The shape of the static PV curves during LF1 and LF2 and the shape of the dynamic PV curves during ventilation at ZEEP were analyzed.

Results

During LF1 a sudden incline (INC) in the static PV curve was observed in all animals. Due to the incline, the curves were not compatible with the sigmoid approximation according to Venegas and colleagues. The sudden incline had disappeared in LF2 (Fig. 1). The shape of the dynamic PV curves and dynamic compliance were changed by LF1. Afterwards dynamic compliance remained unchanged irrespective of 5-min ZEEP and of the RM (Fig. 2).

Figure 1

Figure 2

Conclusion

In normal sheep lungs, recruitment does influence the shape of the static PV curve. Other than in ARDS, the presence of recruitment does not generate a sigmoid PV curve. The changes in dynamic compliance after a RM were preserved for at least 5 min, indicating a long-lasting effect. The effects of recruitment on the shape of the PV curve might serve as diagnostic tool to differentiate normal patients from patients at risk of lung injury.