Assessment of oxygenation response to prone position ventilation in ARDS by lung ultrasonography

Abstract

Acute respiratory distress syndrome (ARDS) is of major concern for the intensivist, as ARDS can only be treated in the intensive care unit (ICU). A cornerstone treatment is mechanical ventilation in order to buy time while the disease leading to ARDS can be cured. Therefore, the first aim of mechanical ventilation is to maintain a viable gas exchange. Over the years we realized that the cost of mechanical ventilation, as measured by the occurrence of ventilator-induced lung injury (VILI) [1], was too high if the target of the mechanical ventilation was normal blood gases. From there, permissive hypercapnia and oxygen saturation between 85 and 90 % were accepted to decrease the risk of VILI.To better tailor the ventilator settings, particularly to avoid potentially dangerous intratidal collapse and decollapse, increased attention was devoted to the characterization of the pathology underlying ARDS. To date, lung CT scan is the gold standard to assess the distribution of strain and from there to make inferences to minimize the occurrence of VILI. It should be mentioned, however, that no study demonstrated that patient outcome can be influenced from lung CT scan findings, essentially because no study has been done with this objective. Since ARDS lung characteristics cannot be monitored using CT imaging, methods feasible at the bedside are very attractive and lung ultrasound (US) is one of them [2]. One limitation of lung US is that the measurements do not quantify physical findings. Even though an aeration score has been developed, it is not quantitative as the Hounsfield units are for CT [3]. This US aeration score has been shown to correlate with PEEP-induced recruited lung volume obtained from the pressure-volume curve [4] and with weaning failure during spontaneous breathing trial [5]. Recently, however, it was shown that recruitment measured by CT scan (as re-inflation of previously not inflated regions) is not correlated with recruitment measured by the pressure-volume curve [6]. This last method and, by inference the US method, actually measure together both the aeration of previously non-aerated regions and a better inflation of lung regions already aerated. The regional hyperinflation, however, cannot be measured accurately either with CT (because of the mass increase of the ARDS lung) or lung US.Haddam et al.[7] performed a multicenter study in 51 ARDS patients with the aim to predict the oxygenation response to the prone position session from using lung US aeration score. The patients included suffered equally from either primary or secondary lung injury and received appropriate lung-protective mechanical ventilation. The authors found that changes in PaO 2 /F I O 2 ratio and lung aeration score did not correlate in either early or late stage of prone position regardless of the focal or nonfocal nature of ARDS. The same was true for changes in PaCO 2 . For the oxygenation to increase, and for the PaCO 2 to decrease, from increased regional lung aeration the blood should continue to flow towards those rea...

Combined lung and brain ultrasonography for an individualized “brain-protective ventilation strategy” in neurocritical care patients with challenging ventilation needs

“…A simple qualitative LUS evaluation has the potential to predict the occurrence of ARDS in blunt trauma patients [ 23 ], and predict the response to PEEP - induced lung recruitment and PEEP-induced increase in PaO 2 in ARDS patients [ 24 ]. Although the same LUS global reareation score applied to ARDS patients failed to predict the response to pronation in terms of oxygenation improvement, basal ARDS morphology described by means of LUS before pronation (focal vs. diffuse morphology) was predictive of effective reareation of dorsal areas, of immediate PaO 2 /FiO 2 improvement and pCO 2 decrease [ 25 , 26 ]. LUS has also proven utility in discerning different degrees of disease severity at a very early stage of ARDS [ 27 ].…”

The Clinical Effect of an Early, Protocolized Approach to Mechanical Ventilation for Severe and Refractory Hypoxemia

“…Such strategies are consistent with current evidence for early interventions improving outcomes in several studies including early paralysis or prone positioning. [11][12][13][14][15]…”

Lateral position during severe mono-lateral pneumonia: an experimental study

“…The influence of body position on gas distribution is well known in ARDS, where prone positioning is commonly indicated as a rescue therapy in severely-ill patients. In prone position, the anatomical and functional distribution of lung ventilation is remarkably modified 23 , and as a result oxygenation, pulmonary mechanics and mortality improved 24 . Unlike the mechanisms responsible for oxygenation improvement during prone position, Figure 2.…”

“…The influence of body position on gas distribution is well known in ARDS, where prone positioning is commonly indicated as a rescue therapy in severely-ill patients. In prone position, the anatomical and functional distribution of lung ventilation is remarkably modified 23 , and as a result oxygenation, pulmonary mechanics and mortality improved 24 . Unlike the mechanisms responsible for oxygenation improvement during prone position, in which the redistribution of ventilation leads to recruitment of the dorsal zones exceeding de-recruitment of the ventral ones 25 , the most probable mechanism in lateral positioning is amelioration of V A /Q mismatch and reduction of pulmonary shunt, caused by the redistribution of blood flow to more ventilated areas 9 .…”