Heart-lung interactions with different ventilatory settings during acute lung injury and hypovolaemia: an experimental study

Effects of positive end-expiratory pressure on the predictability of fluid responsiveness in acute respiratory distress syndrome patients

The prediction accuracy of pulse pressure variation (PPV) for fluid responsiveness was suggested to be unreliable in low tidal volume (VT) ventilation. However, high PEEP can cause ARDS patients relatively hypovolemic and more fluid responsive. We hypothesized that high PEEP 15 cmH₂O can offset the disadvantage of low VT and improve the predictive performance of PPV. We prospectively enrolled 27 hypovolemic ARDS patients ventilated with low VT 6 ml/kg and three levels of PEEP (5, 10, 15 cmH₂O) randomly. Each stage lasted for at least 5 min to allow for equilibration of hemodynamics and pulmonary mechanics. Then, fluid expansion was given with 500 ml hydroxyethyl starch (Voluven 130/70). The hemodynamics and PPV were automatically measured with a PiCCO2 monitor. The PPV values were significantly higher during PEEP15 than those during PEEP5 and PEEP10. PPV during PEEP15 precisely predicts fluid responsiveness with a cutoff value 8.8% and AUC (area under the ROC curve) of ROC (receiver operating characteristic curve) 0.847, higher than the AUC during PEEP5 (0.81) and PEEP10 (0.668). Normalizing PPV with driving pressure (PPV/Driving-P) increased the AUC of PPV to 0.875 during PEEP15. In conclusions, high PEEP 15 cmH₂O can counteract the drawback of low VT and preserve the predicting accuracy of PPV in ARDS patients.

Electrical impedance tomography for non-invasive assessment of stroke volume variation in health and experimental lung injury

BACKGROUND

Functional imaging by thoracic electrical impedance tomography (EIT) is a non-invasive approach to continuously assess central stroke volume variation (SVV) for guiding fluid therapy. The early available data were from healthy lungs without injury-related changes in thoracic impedance as a potentially influencing factor. The aim of this study was to evaluate SVV measured by EIT (SVV_EIT) against SVV from pulse contour analysis (SVV_PC) in an experimental animal model of acute lung injury at different lung volumes.

METHODS

We conducted a randomized controlled trial in 30 anaesthetized domestic pigs. SVV_EIT was calculated automatically analysing heart-lung interactions in a set of pixels representing the aorta. Each initial analysis was performed automatically and unsupervised using predefined frequency domain algorithms that had not previously been used in the study population. After baseline measurements in normal lung conditions, lung injury was induced either by repeated broncho-alveolar lavage (n=15) or by intravenous administration of oleic acid (n=15) and SVV_EIT was remeasured.

RESULTS

The protocol was completed in 28 animals. A total of 123 pairs of SVV measurements were acquired. Correlation coefficients (r) between SVV_EIT and SVV_PC were 0.77 in healthy lungs, 0.84 after broncho-alveolar lavage, and 0.48 after lung injury from oleic acid.

CONCLUSIONS

EIT provides automated calculation of a dynamic preload index of fluid responsiveness (SVV_EIT) that is non-invasively derived from a central haemodynamic signal. However, alterations in thoracic impedance induced by lung injury influence this method.

Respiratory variation in aortic blood peak velocity and caudal vena cava diameter can predict fluid responsiveness in anaesthetised and mechanically ventilated dogs

BACKGROUND AND M&MS

Dynamic preload indices, such as systolic pressure variation (SPV), aortic flow peak velocity variation (ΔVpeak) and distensibility index of the caudal vena cava (CVCDI), are reliable indices for predicting fluid responsiveness in humans. This study aimed to investigate the ability of these indices to predict fluid response in 24 healthy dogs undergoing general anaesthesia and mechanical ventilation. Aortic flow peak velocity variation (∆Vpeak), CVCDI, and SPV were calculated before volume expansion (5mL/kg bolus of lactated Ringer's solution). The aortic velocity time integral (VTI) was measured before and after volume expansion as a surrogate of stroke volume. Dogs were considered responders (n=9) when the VTI increase was ≥15% and non-responders (n=15) when the increase was <15%.

RESULTS AND CONCLUSIONS

Before volume expansion, ΔVpeak, CVCDI and SPV were higher in responders than in non-responders (P=0.0009, P=0.0003, and P=0.0271, respectively). Receiver operating characteristic (ROC) curves were plotted for the three indices. The areas under the ROC curves for SPV, ΔVpeak, and CVCDI were 0.91 (CI 0.73-0.99; P=0.0001), 0.95 (CI 0.77-1; P=0.0001), and 0.78 (CI 0.56-0.92; P=0.015), respectively. The best cut-offs were 6.7% for SPV (sensitivity, 77.78%; specificity, 93.33%), 9.4% for ΔVpeak (sensitivity, 88.89%; specificity, 100%), and 24% for CVCDI (sensitivity, 77.78%; specificity, 73.33). In conclusion, ΔVpeak, CVCDI, and SPV are reliable predictors of fluid responsiveness in healthy dogs undergoing general anaesthesia and mechanical ventilation.

Brazilian recommendations of mechanical ventilation 2013. Part 2

Perspectives on invasive and noninvasive ventilatory support for critically ill patients are evolving, as much evidence indicates that ventilation may have positive effects on patient survival and the quality of the care provided in intensive care units in Brazil. For those reasons, the Brazilian Association of Intensive Care Medicine (Associação de Medicina Intensiva Brasileira - AMIB) and the Brazilian Thoracic Society (Sociedade Brasileira de Pneumologia e Tisiologia - SBPT), represented by the Mechanical Ventilation Committee and the Commission of Intensive Therapy, respectively, decided to review the literature and draft recommendations for mechanical ventilation with the goal of creating a document for bedside guidance as to the best practices on mechanical ventilation available to their members. The document was based on the available evidence regarding 29 subtopics selected as the most relevant for the subject of interest. The project was developed in several stages, during which the selected topics were distributed among experts recommended by both societies with recent publications on the subject of interest and/or significant teaching and research activity in the field of mechanical ventilation in Brazil. The experts were divided into pairs that were charged with performing a thorough review of the international literature on each topic. All the experts met at the Forum on Mechanical Ventilation, which was held at the headquarters of AMIB in São Paulo on August 3 and 4, 2013, to collaboratively draft the final text corresponding to each sub-topic, which was presented to, appraised, discussed and approved in a plenary session that included all 58 participants and aimed to create the final document.

Pulse pressure variation and stroke volume variation under different inhaled concentrations of isoflurane, sevoflurane and desflurane in pigs undergoing hemorrhage

OBJECTIVES

Inhalant anesthesia induces dose-dependent cardiovascular depression, but whether fluid responsiveness is differentially influenced by the inhalant agent and plasma volemia remains unknown. The aim of this study was to compare the effects of isoflurane, sevoflurane and desflurane on pulse pressure variation and stroke volume variation in pigs undergoing hemorrhage.

METHODS

Twenty-five pigs were randomly anesthetized with isoflurane, sevoflurane or desflurane. Hemodynamic and echocardiographic data were registered sequentially at minimum alveolar concentrations of 1.00 (M1), 1.25 (M2), and 1.00 (M3). Then, following withdrawal of 30% of the estimated blood volume, these data were registered at a minimum alveolar concentrations of 1.00 (M4) and 1.25 (M5).

RESULTS

The minimum alveolar concentration increase from 1.00 to 1.25 (M2) decreased the cardiac index and increased the central venous pressure, but only modest changes in mean arterial pressure, pulse pressure variation and stroke volume variation were observed in all groups from M1 to M2. A significant decrease in mean arterial pressure was only observed with desflurane. Following blood loss (M4), pulse pressure variation, stroke volume variation and central venous pressure increased (p < 0.001) and mean arterial pressure decreased in all groups. Under hypovolemia, the cardiac index decreased with the increase of anesthesia depth in a similar manner in all groups.

CONCLUSION

The effects of desflurane, sevoflurane and isoflurane on pulse pressure variation and stroke volume variation were not different during normovolemia or hypovolemia.

Decreased lung compliance increases preload dynamic tests in a pediatric acute lung injury model

BACKGROUND

Preload dynamic tests, pulse pressure variation (PPV) and stroke volume variation (SVV) have emerged as powerful tools to predict response to fluid administration. The influence of factors other than preload in dynamic preload test is currently poorly understood in pediatrics. The aim of our study was to assess the effect of tidal volume (VT) on PPV and SVV in the context of normal and reduced lung compliance in a piglet model.

MATERIAL AND METHOD

Twenty large-white piglets (5.2±0.4kg) were anesthetized, paralyzed and monitored with pulse contour analysis. PPV and SVV were recorded during mechanical ventilation with a VT of 6 and 12mL/kg (low and high VT, respectively), both before and after tracheal instillation of polysorbate 20.

RESULTS

Before acute lung injury (ALI) induction, modifications of VT did not significantly change PPV and SVV readings. After ALI, PPV and SVV were significantly greater during ventilation with a high VT compared to a low VT (PPV increased from 8.9±1.2 to 12.4±1.1%, and SVV from 8.5±1.0 to 12.7±1.2%, both P<0.01).

CONCLUSIONS

This study found that a high VT and reduced lung compliance due to ALI increase preload dynamic tests, with a greater influence of the latter. In subjects with ALI, lung compliance should be considered when interpreting the preload dynamic tests.

Heart lung interactions during mechanical ventilation

PURPOSE OF REVIEW

To survey the recent medical literature examining studies of the hemodynamic effects of mechanical ventilation.

RECENT FINDINGS

Ventilation-induced dynamic changes in arterial pulse pressure and stroke volume variation (PPV and SVV, respectively) identify volume responsiveness. The cause of PPV and SVV are due to intrathoracic pressure-induced variations in right atrial pressure changing intrathoracic blood volume over the ventilatory cycle. This explains why PPV and SVV are inaccurate with smaller tidal volumes used in acute lung injury, but remain useful in one-lung ventilation and prone positioning. Noninvasive measures of PPV and SVV using finger plethysmography and aortic root ultrasound or estimates of intrathoracic blood volume by thoracic impedance also predict volume responsiveness. Finally, the PPV-to-SVV ratio varies with vasomotor tone and can be used to identify vasopressor need in hypotensive patients. The clinical implications of these findings are starting to be realized in recommended management principles.

SUMMARY

PPV and SVV predict volume responsiveness, but like all monitoring approaches, need to be understood within the framework of their physiologic determinations.

Adjusting tidal volume to stress index in an open lung condition optimizes ventilation and prevents overdistension in an experimental model of lung injury and reduced chest wall compliance

INTRODUCTION

The stress index (SI), a parameter derived from the shape of the pressure-time curve, can identify injurious mechanical ventilation. We tested the hypothesis that adjusting tidal volume (VT) to a non-injurious SI in an open lung condition avoids hypoventilation while preventing overdistension in an experimental model of combined lung injury and low chest-wall compliance (Ccw).

METHODS

Lung injury was induced by repeated lung lavages using warm saline solution, and Ccw was reduced by controlled intra-abdominal air-insufflation in 22 anesthetized, paralyzed and mechanically ventilated pigs. After injury animals were recruited and submitted to a positive end-expiratory pressure (PEEP) titration trial to find the PEEP level resulting in maximum compliance. During a subsequent four hours of mechanical ventilation, VT was adjusted to keep a plateau pressure (Pplat) of 30 cmH2O (Pplat-group, n = 11) or to a SI between 0.95 and 1.05 (SI-group, n = 11). Respiratory rate was adjusted to maintain a 'normal' PaCO2 (35 to 65 mmHg). SI, lung mechanics, arterial-blood gases haemodynamics pro-inflammatory cytokines and histopathology were analyzed. In addition Computed Tomography (CT) data were acquired at end expiration and end inspiration in six animals.

RESULTS

PaCO2 was significantly higher in the Pplat-group (82 versus 53 mmHg, P = 0.01), with a resulting lower pH (7.19 versus 7.34, P = 0.01). We observed significant differences in VT (7.3 versus 5.4 mlKg(-1), P = 0.002) and Pplat values (30 versus 35 cmH2O, P = 0.001) between the Pplat-group and SI-group respectively. SI (1.03 versus 0.99, P = 0.42) and end-inspiratory transpulmonary pressure (PTP) (17 versus 18 cmH2O, P = 0.42) were similar in the Pplat- and SI-groups respectively, without differences in overinflated lung areas at end- inspiration in both groups. Cytokines and histopathology showed no differences.

CONCLUSIONS

Setting tidal volume to a non-injurious stress index in an open lung condition improves alveolar ventilation and prevents overdistension without increasing lung injury. This is in comparison with limited Pplat protective ventilation in a model of lung injury with low chest-wall compliance.

Effects of different flow patterns and end-inspiratory pause on oxygenation and ventilation in newborn piglets: an experimental study

BACKGROUND

Historically, the elective ventilatory flow pattern for neonates has been decelerating flow (DF). Decelerating flow waveform has been suggested to improve gas exchange in the neonate when compared with square flow (SF) waveform by improving the ventilation perfusion. However, the superiority of DF compared with SF has not yet been demonstrated during ventilation in small infants. The aim of this study was to compare SF vs. DF, with or without end-inspiratory pause (EIP), in terms of oxygenation and ventilation in an experimental model of newborn piglets.

METHODS

The lungs of 12 newborn Landrace/LargeWhite crossbred piglets were ventilated with SF, DF, SF-EIP and DF-EIP. Tidal volume (VT), inspiratory to expiratory ratio (I/E), respiratory rate (RR), and FiO2 were keep constant during the study. In order to assure an open lung during the study while preventing alveolar collapse, a positive end-expiratory pressure (PEEP) of 6 cmH2O was applied after a single recruitment maneuver. Gas exchange, lung mechanics and hemodynamics were measured.

RESULTS

The inspiratory flow waveform had no effect on arterial oxygenation pressure (PaO2) (276 vs. 278 mmHg, p = 0.77), alveolar dead space to alveolar tidal volume (VDalv/VTalv) (0.21 vs. 0.19 ml, p = 0.33), mean airway pressure (Pawm) (13.1 vs. 14.0 cmH2O, p = 0.69) and compliance (Crs) (3.5 vs. 3.5 ml cmH2O(-1), p = 0.73) when comparing SF and DF. A short EIP (10%) did not produce changes in the results.

CONCLUSION

The present study showed that there are no differences between SF, DF, SF-EIP and DF-EIP in oxygenation, ventilation, lung mechanics, or hemodynamics in this experimental model of newborn piglets with healthy lungs.

Brazilian recommendations of mechanical ventilation 2013. Part 2

Perspectives on invasive and noninvasive ventilatory support for critically ill patients are evolving, as much evidence indicates that ventilation may have positive effects on patient survival and the quality of the care provided in intensive care units in Brazil. For those reasons, the Brazilian Association of Intensive Care Medicine (Associação de Medicina Intensiva Brasileira - AMIB) and the Brazilian Thoracic Society (Sociedade Brasileira de Pneumologia e Tisiologia - SBPT), represented by the Mechanical Ventilation Committee and the Commission of Intensive Therapy, respectively, decided to review the literature and draft recommendations for mechanical ventilation with the goal of creating a document for bedside guidance as to the best practices on mechanical ventilation available to their members. The document was based on the available evidence regarding 29 subtopics selected as the most relevant for the subject of interest. The project was developed in several stages, during which the selected topics were distributed among experts recommended by both societies with recent publications on the subject of interest and/or significant teaching and research activity in the field of mechanical ventilation in Brazil. The experts were divided into pairs that were charged with performing a thorough review of the international literature on each topic. All the experts met at the Forum on Mechanical Ventilation, which was held at the headquarters of AMIB in São Paulo on August 3 and 4, 2013, to collaboratively draft the final text corresponding to each sub-topic, which was presented to, appraised, discussed and approved in a plenary session that included all 58 participants and aimed to create the final document.

Predictive value of pulse pressure variation for fluid responsiveness in septic patients using lung-protective ventilation strategies

Background

The applicability of pulse pressure variation (ΔPP) to predict fluid responsiveness using lung-protective ventilation strategies is uncertain in clinical practice. We designed this study to evaluate the accuracy of this parameter in predicting the fluid responsiveness of septic patients ventilated with low tidal volumes (TV) (6 ml kg⁻¹).

Methods

Forty patients after the resuscitation phase of severe sepsis and septic shock who were mechanically ventilated with 6 ml kg⁻¹ were included. The ΔPP was obtained automatically at baseline and after a standardized fluid challenge (7 ml kg⁻¹). Patients whose cardiac output increased by more than 15% were considered fluid responders. The predictive values of ΔPP and static variables [right atrial pressure (RAP) and pulmonary artery occlusion pressure (PAOP)] were evaluated through a receiver operating characteristic (ROC) curve analysis.

Results

Thirty-four patients had characteristics consistent with acute lung injury or acute respiratory distress syndrome and were ventilated with high levels of PEEP [median (inter-quartile range) 10.0 (10.0–13.5)]. Nineteen patients were considered fluid responders. The RAP and PAOP significantly increased, and ΔPP significantly decreased after volume expansion. The ΔPP performance [ROC curve area: 0.91 (0.82–1.0)] was better than that of the RAP [ROC curve area: 0.73 (0.59–0.90)] and pulmonary artery occlusion pressure [ROC curve area: 0.58 (0.40–0.76)]. The ROC curve analysis revealed that the best cut-off for ΔPP was 6.5%, with a sensitivity of 0.89, specificity of 0.90, positive predictive value of 0.89, and negative predictive value of 0.90.

Conclusions

Automatized ΔPP accurately predicted fluid responsiveness in septic patients ventilated with low TV.

Influence of pressure control levels on the pulse pressure variations: an animal study using healthy piglets

Pulse pressure variation (PPV) is a promising predictor for volume responsiveness. However, recent studies have criticized its validity during small tidal volume (TV) ventilation. The present study evaluated the influence of pressure control level (PCL) on PPV. Six anesthetized healthy piglets simulating hemorrhagic shock underwent five stages of intravascular volume status change. Each stage comprised four cycles of PCL manipulation. In each cycle, five different PCLs were applied in random order. Among 600 arterial pressure tracings obtained during PCL manipulations, 26 tracings were excluded because of signal artifact or ectopic beats. For the remaining 574 tracings, the percentage of normal beats among total recorded beats in each tracing was 99.80% ± 0.85%. When manipulating PCL causing an abrupt change within -16 ∼ +8 cmH(2)O, the PPV responded rapidly and stabilized within 60 s after manipulation. With higher increment in PCL (+12 ∼ +16 cmH(2)O), it required 90 s for PPV to stabilize. Under each cycle of PCL manipulation, the PPVs are linearly correlated to the PCL (r = 0.84 ± 0.21) and TV (r = 0.83 ± 0.22). The PPV as well as the slopes of the trend lines decreased from hypovolemic stages toward hypervolemic stages. Pulse pressure variation responds rapidly to change in ventilator setting and is linearly correlated with the PCL and TV. These characteristics may have important applications in critical care to improve the interpretation of PPV in accord to different ventilator settings.