During spontaneous breathing cardiac output lacks major effect on pulmonary shunting in porcine lungs with partial collapse

Introduction

Spontaneous breathing (SB) improves oxygenation compared to mechanical ventilation (MV), and does so even without recruit-ing atelectasis [1,2]. Since it cannot be excluded that cardiac output (CO) impacts on pulmonary shunt, we investigated whether pulmonary shunt correlates with CO in a porcine model of lung collapse.

Methods

In 12 anaesthetized and relaxed supine piglets, lung collapse was induced by negative pressure application to the endotracheal tube during MV. Six animals resumed SB after 15 minutes; the other six were kept on MV at a respiratory rate and tidal volume corresponding to SB. All animals were followed over 120 minutes, and repeated measurements were converted to the area under curve and analysed by Mann-Whitney test and linear regression.

Results

PaO₂/FiO₂ was higher and venous admixture (Qva/Qt) was lower in the SB group. Hemodynamics was stable and CO was similar in both groups. Qva/Qt correlated with CO (r = 0.83, P = 0.04) in the MV group, but not in the SB group (r = 0.08, P = 0.88) (Figure 1).

Figure 1.

Venous admixture (Qva/Qt) plotted against CO (pooled data for each group). Solid circles, mechanical ventilation (MV); open circles, spontaneous breathing (SB).

Conclusion

SB achieves higher PaO₂/FiO₂ and lower Qva/Qt compared to MV. During SB, Qva/Qt seems to be unaffected by CO. This lung collapse model has stable hemodynamics and gas exchange for at least 2 hours irrespective of the mode of ventilation.