Page 107 - Physiological based CPAP for preterm infants at birth Tessa Martherus
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High-CPAP does not impede cardiovascular changes at birth in preterm sheep
did not affect CBF, its stability or JVP, compared to low-CPAP, we found no evidence that suggests that high-CPAP may increase the risk of IVH.
Our results are in marked contrast with previous findings showing that end-expiratory pressures above 8 cmH2O cause pulmonary overexpansion and reduce PBF in intubated and mechanically ventilated animals (38-41). Increasing PEEP during iPPV is thought to decrease venous return to the right atrium and increase PVR by increasing alveolar pressures above alveolar capillary pressures, causing capillary closure and a reduction in PBF (39). While these studies were the best available evidence for predicting how high-pressure support affected cardiovascular function at birth, the different PBF responses between studies highlights that the two respiratory support modes are distinctly different. The primary differences being that; (i) superimposing sub-atmospheric intrathoracic pressures (due to spontaneous breathing) on a background of high-CPAP, produces different intrathoracic pressure gradients than intubation and iPPV with PEEP (Figure 7); and (ii) intubation and mechanical ventilation removes the role of the larynx, whereas CPAP and spontaneous breathing includes it.
The differential effects of spontaneous breathing and iPPV on PBF are well-established in
adult physiology and the ability of individual breaths to increase PBF has been shown directly
in fetal sheep (49) and indirectly in newborn infants (50). When infants breathe spontaneously during CPAP, the intrathoracic pressure phasically decreases below the CPAP 4 level. As such, the CPAP level is effectively the highest external pressure applied to the
airways, although pressures in the lower airways must briefly increase above the CPAP level to effect expiration. Nevertheless, the mean airway pressure is lower than the CPAP level measured at the mouth opening. However, during iPPV with PEEP, the PEEP level is the lower limit of the externally applied pressure and pressures phasically increase above this with each inflation (Figure 7). As such, the mean airway pressure is higher than the PEEP level and is substantially higher than occurs with a similar CPAP level during spontaneous breathing. Indeed, we found that during iPPV, lambs had a mean airway pressure of 18.6 cmH2O, despite using a PEEP of 5 cmH2O.
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