Effects of acute hemorrhage on intrapulmonary shunt in a pig model of acute respiratory distress-like syndrome.

BACKGROUND: In acute respiratory distress syndrome (ARDS), gas exchange and respiratory system mechanics (compliance) are severely impaired. Besides ventilatory parameters, the degree of respiratory abnormality can be influenced by the circulatory state. This study investigated the influence of acute hypovolemia on the respiratory system. METHODS: We performed a secondary analysis of a previous study including 8 pigs with ARDS-like syndrome induced by lung lavage and surfactant depletion method (ARDS group) and 10 mechanically ventilated pigs with no intervention (CTRL group). Animals of both groups were subjected to hemorrhage and retransfusion successively. We reanalyzed the effect of acute blood volume variations on intrapulmonary shunt (shunt), arterial oxygenation (PaO2:FiO2), global oxygen delivery (DO2) and respiratory system compliance (Crs). RESULTS: In the ARDS group, after hemorrhage, shunt decreased (-28 +/- 3.5 % (p < 0.001)), respiratory system compliance (Crs) increased (+5.1 +/- 1.0 ml/cm H2O (p < 0.001)) moreover, there was a concurrent increase in PaO2:FiO2 (+113 +/- 19.1 mmHg; p < 0.001) but this did not prevent a reduction in DO2 (-317 +/- 49.8 ml/min; p < 0.001). Following retransfusion, shunt and Crs return towards pre-hemorrhage values. Similar changes, but of smaller magnitude were observed in the CTRL group, except that no significant changes in oxygenation occurred. CONCLUSIONS: The present analysis suggests that an acute decrease in blood volume results in a decrease in shunt with a parallel improvement in arterial oxygenation and an increase in Crs during ARDS-like syndrome. Our results strengthen the importance to integrate the circulatory condition in the analysis of the state of the respiratory system. However, the translation of this physiological model in a clinical perspective is not straightforward because our model of acute and severe hemorrhage is not strictly equivalent to a progressive hypovolemia, as could be obtained in ICU by diuretic. Furthermore, the present model does not consider the impact of blood loss induced decrease of DO2 on other vital organs function. TRIAL REGISTRATION: 'Not applicable'.

Influence of tidal volume on pulse pressure variations in hypovolemic ventilated pigs with acute respiratory distress-like syndrome.

BACKGROUND: Sensitivity and specificity of respiratory change in pulse pressure (DeltaPP) to predict preload dependency has been questioned at small tidal volumes (VT) in critically ill patients suffering from acute respiratory distress syndrome (ARDS). We studied DeltaPP in pigs with ARDS-like syndrome during reversible hemorrhagic shock. METHODS: Prospective, observational animal study in a Laboratory Investigation Unit. Sixteen deeply sedated mechanically ventilated pigs were successively ventilated with VT of 10 ml/kg at a respiratory rate of 15 breaths/min (RR15) and VT of 6 ml/kg at RR15 and RR25. ARDS-like syndrome was produced by lung lavage in eight pigs (ARDS group). Severe hemorrhagic shock was induced by removal of 40% of total blood volume followed by restoration. RESULTS: After bleeding, in the control group ventilated with a VT of 10 ml/kg, DeltaPP increased from 8.5 (95% confidence interval [CI], 7.1 to 9.9%) to 18.5% (CI, 15.3 to 21.7%; P<0.05). In the ARDS group, this index increased similarly, from 7.1% (95% CI, 5.3 to 9.0%) to 20.1% (CI, 15.3 to 24.9%; P<0.05). In control lungs, reduction in VT from 10 to 6 ml/kg reduced the DeltaPP reaction by 40%, although it remained a statistically valid indicator of hypovolemia regardless of the RR value. In contrast, in the ARDS group, DeltaPP was an unreliable hypovolemia marker at low VT ventilation, regardless of the RR value (p=not statistically significant). CONCLUSIONS: The present study suggests that DeltaPP is a reliable indicator of severe hypovolemia in pigs with healthy lungs regardless of VT or RR. In contrast, in pigs with ARDS-like syndrome ventilated with small VT, DeltaPP is not a good indicator of severe hemorrhage. However, in this setting, indexing DeltaPP to respiratory changes in transpulmonary pressure allows this marker to significantly indicate the occurrence of hypovolemia.

Misplacement of central vein catheters in patients with hemothorax: a new approach to resolve the problem.

BACKGROUND: In emergency and pre-hospital care, the verification of the correct position of a central venous catheter is based on the observation of blood color reflow as well as pressure changes with respiration. However, in trauma patient with hemothorax, these indices may not always be reliable signs as the catheter is in a blood-filled pleural space. METHODS: A review of reports published describing patients presenting hemothorax and equipped with central venous catheter wrongly assumed to be in the correct position was performed. RESULTS: Over 10 years, seven reports have been published and a last study was found in the references list of one of the reviews. CONCLUSION: In patients with hemothorax due to severe thoracic trauma or other causes, a delay in detection of incorrect placement of a central venous catheter may delay fluid resuscitation and decrease the chances of survival. In this situation, the use of portable ultrasound devices may be an useful method to increase success rate in catheter insertion.

Regulation of regional lung perfusion by nitric oxide.

Improved oxygenation has previously been shown in patients with acute lung injury when ventilated in prone position. We hypothesized that this was due to higher regional production of nitric oxide in dorsocaudal lung regions. We measured nitric oxide synthase mRNA expression and nitric oxide production by citrulline assay in ventral and dorsal lung tissue from patients. In volunteers, regional lung perfusion in prone and supine postures was assessed by single photon emission computed tomography using (99m)Tc macroaggregated albumin before and after inhibition of nitric oxide synthase by N(G)-monomethyl-L-arginine infusion. Nitric oxide synthase mRNA expression and nitric oxide production were significantly higher in dorsal compared with ventral lung regions. In supine posture, lung perfusion was shifted to ventral parts during nitric oxide synthase inhibition, whereas in the prone posture lung perfusion remained unchanged. Our results suggest a role for endogenous nitric oxide in regulation of regional pulmonary perfusion.