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Abstract
The lung protective strategy is mainly composed of two components. One is minimizing volutrauma by using small tidal volumes. The other strategy is lung recruitment: ”open up the lung and keep the lung open.The most popular way of keeping the lung open is to apply of positive end expiratory pressure (PEEP).PEEP is the cornerstone of hypoxemia treatment in patients with ALI and ARDS, but there is still some controversy over the optimum level to be used and how this should be determined.The present study was carried out to:•Study the value of use of PaCO2-PetCO2 gradient as end point in titration of PEEP, in mechanically ventilated patients with ALI and ARDS.
•Find correlation between PaCO2-PetCO2 gradient, tidal volume /dead space fraction (Vd/Vt) and PEEP titration in ARDS patients.This study included 20 patients fulfilled the diagnostic criteria of ARDS. Thorough history was taken. Full clinical assessment, arterial blood gases and chest X-ray were done. All included cases were mechanically ventilated according to the lung protective strategy. LIS was calculated. Capnography is used as a noninvasive tool for end tidal CO2 measurement which in turn used in the process of PEEP titration using the difference between arterial CO2 and end tidal CO2. With every patient, incremental PEEP titration and determination of the optimal PEEP was done using the PaCO2-PetCO2 gradient and dead space fraction as a guide.
The effect of PEEP titration using the PaCO2-PetCO2 gradient was evaluated using the hypoxic index (PaO2/FiO2), arterial oxygen saturation (SaO2), arterial oxygen tension (PaO2), static compliance (Cst), dead space fraction (Vd/Vt) and other parameters with baseline, pre-optimal, optimal and highest PEEP. Seven out of twenty patients (35%) had pulmonary causes of ARDS while thirteen patients (65%) had extra-pulmonary causes. The PaCO2-PetCO2 gradient decreased with incremental titration of PEEP and minimized till the least value which determined the optimal PEEP. So, we considered that the least value of PaCO2-PetCO2 reached is corresponded to the best PEEP, and the value of the PaCO2-PetCO2 which increased again is corresponded to the highest PEEP. from the above results, there was significant decrease in the PaCO2-PetCO2 gradient between optimal than baseline or pre-optimal PEEP. Also, the increase in PaCO2-PetCO2 with highest PEEP was statistically significant than pre-optimal and optimal PEEP.
The mean of dead space fraction (Vd/Vt) was 0.5000.053 at baseline PEEP, and decreased up to 0.2970.056 with optimal PEEP and increased to 0.3900.067 with highest PEEP. There was significant decrease between optimal than baseline PEEP as regard dead space fraction. Also, the increase in dead space fraction with highest PEEP was statistically significant than optimal PEEP.Lung injury score (LIS) was used in the present study for further evaluation of severity of ARDS. LIS used before the study with the randomly preset PEEP, 6 hours after the study with optimal PEEP and 24hours after the study with optimal PEEP. There was statistical significant difference between LIS after 24hour of the study with optimal PEEP than before the study.Oxygen tension (PaO2), arterial oxygen saturation (SaO2), hypoxic index (PaO2/FiO2) and shunt fraction (Qs/Qt) were used to evaluate effect of PEEP titration on oxygenation. There was statistical significant increase in PaO2 at optimal and highest PEEP than baseline but there was no statistical significant difference at highest PEEP than optimal. Secondly, PEEP titration caused significant increase in SaO2 at optimal and highest PEEP than baseline; also there was statistical significant increase at highest PEEP than optimal.
Thirdly, PEEP titration caused significant improvement of the hypoxic index (PaO2/FiO2) at optimal and highest than baseline PEEP, but there was no statistical significant increase in PaO2/FiO2 at highest than optimal PEEP.Fourthly, PEEP titration caused significant decrease in shunt fraction (Qs/Qt) at optimal and highest than baseline PEEP, also there was statistical significant decrease in Qs/Qt at optimal and highest PEEP than pre-optimal PEEP respectively. As regard optimal and highest PEEP, there was no statistical significant difference in Qs/Qt at the two levels.
Some adverse events occurred during PEEP titration. Four patients experienced a transient hypotension during PEEP titration around pre-optimal, optimal, and highest PEEP, these patients managed by fluids and small dose dopamine during the PEEP titration protocol. Two patients experienced atrial premature beats which resolved spontaneously with no any harm effect on hemodynamics, another patient experienced supra-ventricular tachycardia which managed immediately by verapamil 5mg intravenous injection and resolved immediately within 5 minutes without any effect on hemodynamics .