What is already known on this topic
- Higher PEEP can optimise lung inflation and improve oxygenation in ventilated newborns.
- Higher PEEP can diminish venous return, hence cardiac output and consequently cerebral blood flow.
What this study adds
- A short‐term modest increase in PEEP reduces RVO but does not lead to a remarkable change in systemic blood flow in most infants.
- Infants with improvements in compliance in response to higher PEEP tend to have improvements in SVC flows.
We have shown that an increase of PEEP from 5 cm H2 to 8 cmH2O for 10 min did not lead to a clinically relevant change in upper body and brain blood flow (SVC flow) in most of the infants in the present study. It was associated with a significant decrease in RVO but there were no clinically relevant changes in any of the other haemodynamic measurements. Improvements in SVC flow were associated with improvements in lung compliance, although this analysis is sensitive to the method of analysis.
In contrast to the literature, our study found relatively little consistent change in the haemodynamic measurements overall.13
Potential reasons for this include our heterogeneous study population, or the impact of our intervention. Possibly, an increase of 5 cm H2
O to 8 cmH2
O in PEEP was not large enough to cause consistent negative effects on blood flow as reported in the literature. Few infants had low systemic blood flows. We expected that infants with low baseline flow would show a larger decrease in flow with the intervention, but they were more likely to have an increase in SVC flow at 8 cmH2
O of PEEP. This may be a regression to the mean.
The decrease in RVO of −17 ml/kg/min was significant, but we are not sure if this is of clinical importance considering the normal baseline RVO in this population. In six of the seven babies with low baseline RVO (<120 ml/kg/min) the intervention resulted in no detectable change, but a potential adverse effect of higher PEEP should be evaluated more thoroughly with further research, especially in infants with low flow.
Probably the most important haemodynamic effect of PEEP in preterm infants with respiratory distress syndrome is through the direct effect on alveoli. When PEEP is set too low, blood will be shunted away from collapsed alveoli which produce a regional increase in pulmonary vascular resistance. With optimal lung volume, lung vascular resistance will be at its lowest point, thus maximising RVO and cardiac input. PEEP is capable of optimising lung volume by keeping open (partially) collapsed alveoli. The intervention performed in this study is not considered a lung recruitment manoeuvre, and we can only speculate about lung volumes in the studied infants.15
Lung volume is also related to airway resistance. At low lung volumes due to insufficient PEEP, airway resistance and the work of breathing are high.
In this study compliance increased with the maximum increase after 10 min of PEEP of 8 cmH2
O. It can take up to 14 min for the alveoli to stablise after a change in PEEP.16
However, the evidence on the effect of PEEP on compliance is conflicting.17
Compliance can only be used as a measurement to optimise ventilation without decreasing blood flow, if it is combined with information on lung volume. Two studies investigating lung compliance and blood flow simultaneously in newborn infants showed a progressive decrease in cardiac output with less decrease in the infants with low compliance.13
We found a reduction in RVO, but we did not find a significant positive association between an increase in compliance and an increase in RVO as we did for SVC flow. This could be caused by differences in flow measurements. In RVO, we looked at the change in velocity and assumed the diameter did not change, whereas change in SVC depended on change in diameter. This suggests that RVO, although perhaps not such a consistent measure of systemic blood flow as SVC, is less vulnerable to measurement error if only change in velocity is assessed. It may be a better method to detect short‐term changes such as in this study.20
Although our intervention seems to be only a minor increase in mean airway pressure, modest changes in PEEP can result in a clinically important decrease in tidal volume or functional residual capacity.21
This could explain the increase in Paco2
found in our study. Other studies suggest that the increase in Paco2
is responsible for an increase in cerebral blood flow.22
The relationship between Paco2
and cerebral blood flow is exponential and a normal Paco2
–cerebral blood flow reactivity is about 4% per mm Hg.24
In this study, the increase in Paco2
was associated with an increase in SVC flow, but not with an increase in RVO or MCA mean velocity which makes the Paco2
–SVC flow relationship difficult to interpret. The potential negative effect of PEEP on RVO could have been balanced by the potential positive effect of a higher Paco2
on cerebral blood flow and SVC flow.