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Paper text:
high - frequency oscillatory ventilation ( hfov ) , as used by bojan and colleagues , is , at least in theory , an ideal tool for lung - protective ventilation as it allows effective pulmonary gas exchange with the delivery of a very small tidal volume ( vt ) below dead space and diminished risk of atelectrauma .
numerous animal and clinical studies have shown clearly that mechanical ventilation ( mv ) itself can initiate or exacerbate lung injury , termed ventilator - induced lung injury .
the application of a vt and the repetitive opening and closure of alveoli have been identified as important pathophysiological mechanisms . until now
, only two randomized controlled trials investigating the effect of hfov on patient outcome have been performed   and the larger of these included only 58 children .
the main finding of this study was that hfov did not significantly improve survival ( 66% with hfov versus 59% with conventional mv ) or total ventilator days ( 20  22 days with hfov versus 22  17 days with conventional mv ) .
as a consequence , hfov is not universally employed in pediatric critical care . also , it seems very unrealistic that any new pediatric hfov trial will be initiated within the next few years .
this means that well - designed observational studies are needed to shed light on some of the many other aspects of pediatric hfov which remain to be explored , including the identification of patients who are most likely to benefit from hfov , the timing of hfov ( early versus rescue ) , optimal oscillator settings , and monitoring during hfov . in the previous issue of critical care , bojan and
a total of 120 patients were treated with hfov on the day of surgery , thus excluding rescue hfov use .
patients were transitioned to hfov when hypoxemia and acidosis occurred despite increasing alveolar ventilation on conventional mv ( if vt exceeded 10 ml / kg ) or when there was evidence of pulmonary hypertension and right ventricular ( rv ) failure .
the main finding was that the duration of mv was significantly shorter in patients in whom hfov was initiated on the day of surgery .
however , there are some concerns related to the methodology of the study and the practical use of hfov in the authors ' institution . some of the limitations inherent to their work have been properly addressed . but their study was designed as a retrospective one introducing confounding by indication .
hence , the authors have calculated a propensity score to minimize this , although the variables chosen for calculating it raise some concern .
included were all demographic and post - operative variables that yielded a p value of less than 0.10 in the univariate analysis between patients with hfov and those without hfov .
however , it seems more rational to define , beforehand , which variables influence the decision of the attending physician to transition to hfov and therefore which variables should be used for calculating the propensity score . as a consequence of this approach , hemodynamic and
if the authors had taken these into account when calculating their propensity score , the results of the study might have been even more firm .
also , it is not customary in their institution to routinely use an open lung strategy .
arterial partial pressure of oxygen ( pao2 ) increases linearly with lung volume during hfov .
animal work has shown improved lung compliance and less hyaline membrane formation when such a strategy was applied .
the effect of increased lung volume on pao2 is even more profound when oscillating on the deflation limb of the pressure - volume curve [ 9 - 11 ] .
furthermore , there is more simultaneous alveolar recruitment and overstretching on the inspiratory than on the expiratory limb of the pressure - volume curve .
the hysteresis of the lungs provides an additional benefit : when oscillating on the expiratory limb , less pressure is required to maintain lung volume in comparison with when oscillating on the inspiratory limb .
nevertheless , what is especially interesting about the work of bojan and colleagues is that they used hfov when there was evidence of pulmonary hypertension or rv failure in their patients .
this is an interesting approach as it is often assumed that high intra - thoracic pressures increase rv afterload and thus may enhance rv dysfunction .
however , in a small study of 5 children after fontan cardiac surgery , the use of hfov did not result in an increase in pulmonary vascular resistance ( pvr ) .
so how can the positive findings of the study by bojan and colleagues be explained ?
this typically u - shaped curve shows that the pvr is high at both residual volume and total lung capacity .
the curve is at its optimum ( that is , lowest pvr ) at ' normal ' functional residual capacity .
it is not uncommon for ventilated patients after cardiac surgery to suffer from pulmonary dysfunction caused by atelectasis or even acute lung injury .
one of the main approaches in such disease conditions is to maintain sufficient end - expiratory lung volume by the application of positive end - expiratory pressure ( peep ) .
however , the required level of peep is difficult to establish and may coincide with the delivery of high inspiratory pressures ( > 30 cm h2o ) .
hfov is , in fact , nothing more than a continuous positive airway pressure system with superimposed small oscillations delivering the small vt . as a consequence ,
it is important to stress that the findings of bojan and colleagues warrant prospective confirmation in a well - designed randomized controlled trial . in the mean time , the authors are to be congratulated for their work as their study has provided arguments not to rule out the early use of hfov in pediatric cardiac surgery patients .
hfov : high - frequency oscillatory ventilation ; mv : mechanical ventilation ; pao2 : arterial partial pressure of oxygen ; peep : positive end - expiratory pressure ; pvr : pulmonary vascular resistance ; rv : right ventricular ; vt : tidal volume .
in the previous issue of critical care , bojan and colleagues reported their experiences with high - frequency oscillatory ventilation ( hfov ) after pediatric cardiac surgery . 
 a total of 120 patients were treated with hfov on the day of surgery , thus excluding rescue hfov use . 
 the main finding of the authors was that the duration of mechanical ventilation was significantly shorter in patients in whom hfov was initiated on the day of surgery . 
 especially interesting about this work is that the authors used hfov when there was evidence of pulmonary hypertension or right ventricular ( rv ) failure in their patients . 
 this is an interesting approach as it is often assumed that high intra - thoracic pressures increase rv afterload and thus may enhance rv dysfunction . 
 the findings of bojan and colleagues may be explained by the fact that they were able to decrease the pulmonary vascular resistance by finding a proper balance between atelectasis and overdistension of the lung . 
 it can be argued that it is possible to do so by applying positive end - expiratory pressure . 
 but , at the same time , this may coincide with the delivery of high inspiratory pressures ( > 30 cm h2o ) . 
 as hfov is , in fact , a continuous positive airway pressure system , its advantage is that it is possible to maintain sufficient lung volume without large injurious pressure swings . 
 although the observations by bojan and colleagues need to be confirmed in a prospective randomized trial , they have provided arguments not to rule out the early use of hfov in pediatric cardiac surgery patients .