I published an abstract somewhere, a while a go, that was called “Volume guarantee does not guarantee volume”. It was a summary of the results of a short-term cross-over trial which showed that tidal volumes stays quite variable when you switch from pressure limited ventilation to volume-guarantee, using the Babylog.
The reason of course is that respiratory efforts vary from breath to breath, so a system designed to measure the tidal volume delivered and then adjust the pressures for the next breath, is bound to be always playing catch-up. Sometimes the variability is small, sometimes, when the baby is irritable or crying, for example, the variability can be quite substantial.
In true volume-controlled ventilation the machines deliver a fixed volume into the ventilator circuit for each breath; there actually was at one time a neonatal ventilator that did this, the old Bourns LS104, which I am old enough to have actually used (in my animal lab). The problem with such machines is that the compressible volume of the ventilator circuit is substantially greater than the volume of a baby’s lungs, so any minor change in lung compliance (the ventilator circuit has, of course, an unchanging compliance) can lead to dramatic changes in the actual pulmonary tidal volume received by the infant. If compliance deteriorates significantly you can end up ventilating just the ventilator circuit, and not the baby. In contrast improvements in compliance can lead to dramatic increases in the volumes delivered to the baby, and excessive, damaging inhalational volumes. The Bourns LS104 was associated with worse pulmonary outcomes and went off the market.
Safe volume ventilation in newborns must be determined by the volume actually entering (or leaving) the lungs of the baby, and measured at the endotracheal tube. Some of the trials of “volume targeted ventilation” (including the 2 of the largest ones included in the Cochrane review) in the newborn have used the Siemens Servo 300c in PRVC mode, which measured the volume delivered at the ventilator end of the circuit. Volumes selected for those trials were determined by watching the babies chest move and selecting a volume which gave good chest movement, which varied between 5 and 15 ml/kg. You really cannot call that volume targeted ventilation, and such studies should be considered separately in systematic reviews of volume targeted ventilation in the newborn. In fact if you take those trials out of the systematic reviews, the evidence base for a clinically significant benefit of volume targeted ventilation is rather weak with not many more than 100 patients on volume ventilation and 100 with pressure limited ventilation (even though I personally think it is likely to be an improvement over pressure limited ventilation).
So a system that adjusts pressures in order to get the target volume, will always have tidal volumes which are variable (unless your patient is apneic or paralyzed). In contrast, a system which targets a particular pressure (standard neonatal ventilators that is) will likely have even higher variations in volume, including potentially damaging breaths with excessive inspiratory volumes.
Overall, volume guarantee systems should therefore have fewer breaths which are seriously excessive (or indeed breaths which are much too small). Is that really true? There are numerous small short-term cross-over studies that demonstrate that indeed there are fewer excessive tidal volumes with the babylog or VN500 systems. In part this is also likely to be due to another safety feature in VG ventilation, at least with the Drager systems, that once the volume passes a particular threshold (for the babylog/VN500 that is 130% of the set tidal volume) the inhalation is terminated, and circuit pressure returns to the set PEEP.
How good are the machines at consistently delivering the desired tidal volume? A new publication from David Tingay and his group from the children’s hospital in Melbourne examined this with the SLE 5000.
They showed with an extensive data collection in 100 babies, with millions of inflations recorded, that tidal volumes were close to the desired tidal volumes with narrow confidence intervals.
One thing I noted that seemed a bit strange however, is from their figure 1.
(A) Bland-Altman plot of VTset and expiratory tidal volume (VTe) . Solid black line denotes the bias, dashed black lines denote the 95% CI of the limits of agreement. (B) Relationship between VTset and VTe; y=0.736x+1.073 (r=0.34, p<0.0001; linear regression). Solid black line represents the line of best fit and dotted black lines represent 95% CI bands. To ease visual interpretation of figures, and after seeking statistical advice, symbols represent the average values for each infant rather than all values analysed (maximum 90 000/infant)
The dots on the graphs represent the average values from individual patients, who had a median of over 80,000 inflations per patient. Therefore over a large number of inflations some babies had average delivered tidal volumes that were up to 3 mL/kg less, or nearly 2 mL/kg more, than the desired tidal volume, if I interpret graph A correctly. I can understand that sometimes the pressure limit is being hit frequently, limiting the administered pressures and therefore the tidal volume, and that this could lead to smaller average tidal volumes than the set volume; but I find it hard to understand why tidal volumes would be consistently larger than the set volume over a large number of inflations, shouldn’t the ventilator have reduced the pressures to lead to smaller tidal volume? I guess it is possible that the ventilator had reduced the peak pressure down to a minimum (PEEP) and the baby was spontaneously generating volumes higher than the set volume. It would be nice to know if these explanations are correct.
Other findings were that it seems that CO2 is probably truly more stable during volume targeted ventilation, and that the latest algorithm for leak adjusting the inflations seemed to work better that the older algorithm, and indeed worked quite well up to an ETT leak of 30%.