As I was preparing to settle in tonight I received a question from a reader on my Linkedin page in regards to the use of sustained inflation (SI) in our units. We don't use it and I think the reasons behind it might be of interest to others. The concept of SI is that by providing a high opening pressure of 20 - 30 cm H2O for anywhere from 5 to 15 seconds one may be able to open the "stiff" lung of a preterm infant with RDS and establish an adequate functional residual capacity. Once the lung is open, it may be possible in theory to keep it open with ongoing peep at a more traditional level of 5 - 8 cm of H20.
The concept was tested 25 years ago by Vyas et al in their article Physiologic responses to prolonged and slow-rise inflation in the resuscitation of the asphyxiated newborn infant. In this study, 9 newborn infants were given a relatively short 5 second sustained inflation and led to earlier and larger lung volumes with good establishment of FRC. Like many trials in Neonatology though sceptics abound and here we are 25 years later still discussing the merits of this approach.
As I have a warm place in my heart for the place that started my professional career whenever I come across a paper published by former colleagues I take a closer look. Such is the case with a systematic review on sustained inflation by Schmolzer et al. The inclusion criteria were studies of infants born at <33 weeks. Their article provides a wonderful assessment of the state of the literature on the topic and I would encourage you to have a look at it if you would like a good reference to keep around on the topic. What it comes down to though is that there are really only four randomized human studies using the technique and in truth they are fairly heterogeneous in their design. They vary in the length of time an SI was performed (5 - 20 seconds), the pressures used (20 - 30 cm H2O), single or multiple SIs and lastly amount of oxygen utilized being 21 - 100%. In fact three of the four studies used either 100% or in one case 50% FiO2 when providing such treatments.
What Did They Show?
This is where things get interesting. SI works in the short term by reducing the likelihood that an infant will need mechanical ventilation at 72 hours with a number needed to treat of only 10! In medicine we normally would embrace such results but sadly the results do not translate into long term benefits as the rate of BPD, mortality and the combined outcome do not remain significant. Interestingly, the incidence of a symptomatic PDA needing treatment with either a medical or surgical approach had a number needed to harm of 11; an equally impressive number but one that gives reason for concern. As the authors speculate, the increased rate of PDA may be in fact related to the good job that the SI does in this early phase. By establishing an open lung and at an earlier time point it may well be that there is an accentuation in the relaxation of the pulmonary vasculature and this leads to a left to right shunt that by being hemodynamically significant helps to stent the ductus open at a time when it might otherwise be tending to close. This outcome in and of itself raises concern in my mind and is the first reason to give me reason to pause before adopting this practice.
Any other concerns?
Although non-significant there was a trend towards increased rates of IVH in the groups randomized to SI. There is real biologic plausibility here. During an SI the increased positive pressure in the chest could well simulate a similar effect to a pneumothorax and impede the passive drainage of blood from the head into the thorax. In particular, longer durations and/or frequent SIs could increase such risk. Given the heterogeneous nature of these studies it is difficult to know if they all had been similar in providing multiple SIs could we have seen this cross over to significant?
I believe the biggest concern in all of this though is that I would have a very hard time applying the results of these studies to our patient population. The systematic review addresses the question about whether SI is better than IPPV as a lung recruitment strategy in the preterm infant with respiratory distress. I have to say though we have moved beyond IPPV as an initial strategy in favour of placement of CPAP on the infant directly after birth. The real question in my mind is whether providing brief periods of SI followed by CPAP of +6 to +8 is better than placement on CPAP alone as a first strategy to establish good lung volumes.
If I am to be swayed by the use of SI someone needs to do this study first. The possibility of increasing the number of hemodynamically significant PDAs and potentially worsening IVH without any clear reduction in BPD is definitely placing me firmly in the camp of favouring the CPAP approach. Having said all that, the work by the Edmonton group is important and gives everyone a glimpse into what the current landscape is for research in this field and opens the door for their group or another to answer my questions and any others that may emerge as this strategy will no doubt be discussed for years to come.