The lower inflection point of the inspiratory pressure-volume curve overestimates optimal PEEP in surfactant-treated immature lambs

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Introduction

Static inspiratory pressure-volume (P-V) curves are typically S-shaped with a lower (LIP) and an upper inflection point (UIP), probably because of successive recruitment of lung units. It is often said that PEEP should be set above LIP to avoid derecruitment during expiration. However, as shown in a rabbit model [1], the effect of PEEP on lung volume is determined by the expiratory part of the P-V loop, which has a markedly different shape.

Methods

Eleven lambs with gestational age 119–141 days (term =145 days) delivered by cesarean section were given surfactant and subsequently mechanically ventilated. Using an interrupter technique, static P-V loops were recorded between an end-expiratory pressure of a) 0 cmH₂O (ZEEP) or b) 5 cmH₂O and an inspiratory pressure of 35 cmH₂O.

Results

Loops obtained during insufflation from ZEEP had a large hysteresis; LIP was 16-21 cmH₂O, while UIP was above 30 cmH₂O, if seen at all (Fig. 1 loop A) and the expiratory limbs showed a high deflation stability down to a pressure of 6-11 cmH₂O with a steep segment at lower pressures. Loops from 5 cmH₂O had a nearly linear inspiratory limb, and much less hysteresis (Fig. 1 loop B).

Conclusion

In preterm lambs with fully-recruited lung volume, derecruitment can be avoided by a much lower PEEP than indicated by LIP. During mechanical ventilation of immature subjects, the upper inflection point of the expiratory limb (UIP_exp) is probably a better indicator of optimal PEEP than LIP of the inspiratory curve.

Figure 1

Loops at 15 min of age in a 132 day GA lamb. A: loop from ZEEP; B:loop from positive end-expiratory pressure (5 cmH₂O).