Impact of cardiac output and alveolar ventilation in estimating ventilation/perfusion mismatch in ARDS using electrical impedance tomography

Fluid responsiveness in acute respiratory distress syndrome patients: a post hoc analysis of the HEMOPRED study

PURPOSE

Optimal fluid management in patients with acute respiratory distress syndrome (ARDS) is challenging due to risks associated with both circulatory failure and fluid overload. The performance of dynamic indices to predict fluid responsiveness (FR) in ARDS patients is uncertain.

METHODS

This post hoc analysis of the HEMOPRED study compared the performance of dynamic indices in mechanically ventilated patients with shock, with and without ARDS, to predict FR, defined as an increase in aortic velocity time integral (VTI)  > 10% after passive leg raising (PLR).

RESULTS

Among 540 patients, 117 (22%) had ARDS and were ventilated with a median tidal volume of 7.6 mL/kg [6.9-8.4] and a median positive end-expiratory pressure of 7 cmH2O [5-9]. FR was observed in 45 ARDS patients (39% vs 44% in non-ARDS patients, p = 0.384). Reliability of dynamic indices to predict FR remained consistent in ARDS patients, though with different thresholds. Collapsibility index of the superior vena cava (ΔSVC) showed the best predictive performance in both ARDS (area under the curve [AUC] = 0.763 [0.659-0.868]) and non-ARDS (AUC = 0.750 [0.698-0.802]) patients. A right to left ventricle end-diastolic area ratio  > 0.8 or paradoxical septal motion were strongly linked to the absence of FR (> 80% specificity). FR was not associated with intensive care unit (ICU) mortality (47% vs. 46%, p = 1). However, hypovolemia, defined as an aortic VTI increase  > 32% during PLR (median increase in patients with a partial SVC collapse) was independently associated with ICU mortality (odds ratio [OR] = 1.355 [1.077-1.705], p = 0.011), as well as pulse pressure variation (OR = 1.014 [1.001-1.026], p = 0.034).

CONCLUSION

Performance of dynamic indices to predict FR appears preserved in ARDS patients, albeit with distinct thresholds. Hypovolemia, indicated by a  > 32% increase in aortic VTI during PLR, rather than FR, was associated with ICU mortality in this population.

Absolute values of regional ventilation-perfusion mismatch in patients with ARDS monitored by electrical impedance tomography and the role of dead space and shunt compensation

BACKGROUND

Assessment of regional ventilation/perfusion (V'/Q) mismatch using electrical impedance tomography (EIT) represents a promising advancement for personalized management of the acute respiratory distress syndrome (ARDS). However, accuracy is still hindered by the need for invasive monitoring to calibrate ventilation and perfusion. Here, we propose a non-invasive correction that uses only EIT data and characterized patients with more pronounced compensation of V'/Q mismatch.

METHODS

We enrolled twenty-one ARDS patients on controlled mechanical ventilation. Cardiac output was measured invasively, and ventilation and perfusion were assessed by EIT. Relative V'/Q maps by EIT were calibrated to absolute values using the minute ventilation to invasive cardiac output (MV/CO) ratio (V'/Q-ABS), left unadjusted (V'/Q-REL), or corrected by MV/CO ratio derived from EIT data (V'/Q-CORR). The ratio between ventilation to dependent regions and perfusion reaching shunted units ( V D ' /QSHUNT) was calculated as an index of more effective hypoxic pulmonary vasoconstriction. The ratio between perfusion to non-dependent regions and ventilation to dead space units (QND/ V DS ' ) was calculated as an index of hypocapnic pneumoconstriction.

RESULTS

Our calibration factor correlated with invasive MV/CO (r = 0.65, p < 0.001), showed good accuracy and no apparent bias. Compared to V'/Q-ABS, V'/Q-REL maps overestimated ventilation (p = 0.013) and perfusion (p = 0.002) to low V'/Q units and underestimated ventilation (p = 0.011) and perfusion (p = 0.008) to high V'/Q units. The heterogeneity of ventilation and perfusion reaching different V'/Q compartments was underestimated. V'/Q-CORR maps eliminated all these differences with V'/Q-ABS (p > 0.05). HigherV D ' / Q SHUNT correlated with higher PaO2/FiO2 (r = 0.49, p = 0.025) and lower shunt fraction (ρ =  - 0.59, p = 0.005). HigherQ ND / V DS ' correlated with lower PEEP (ρ =  - 0.62, p = 0.003) and plateau pressure (ρ =  - 0.59, p = 0.005). Lower values of both indexes were associated with less ventilator-free days (p = 0.05 and p = 0.03, respectively).

CONCLUSIONS

Regional V'/Q maps calibrated with a non-invasive EIT-only method closely approximate the ones obtained with invasive monitoring. Higher efficiency of shunt compensation improves oxygenation while compensation of dead space is less needed at lower airway pressure. Patients with more effective compensation mechanisms could have better outcomes.

Imaging the pulmonary vasculature in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by a redistribution of regional lung perfusion that impairs gas exchange. While speculative, experimental evidence suggests that perfusion redistribution may contribute to regional inflammation and modify disease progression. Unfortunately, tools to visualize and quantify lung perfusion in patients with ARDS are lacking. This review explores recent advances in perfusion imaging techniques that aim to understand the pulmonary circulation in ARDS. Dynamic contrast-enhanced computed tomography captures first-pass kinetics of intravenously injected dye during continuous scan acquisitions. Different contrast characteristics and kinetic modeling have improved its topographic measurement of pulmonary perfusion with high spatial and temporal resolution. Dual-energy computed tomography can map the pulmonary blood volume of the whole lung with limited radiation exposure, enabling its application in clinical research. Electrical impedance tomography can obtain serial topographic assessments of perfusion at the bedside in response to treatments such as inhaled nitric oxide and prone position. Ongoing technological improvements and emerging techniques will enhance lung perfusion imaging and aid its incorporation into the care of patients with ARDS.

Evaluation of adjacent and opposite current injection patterns for a wearable chest electrical impedance tomography system

Objective.Wearable electrical impedance tomography (EIT) can be used to monitor regional lung ventilation and perfusion at the bedside. Due to its special system architecture, the amplitude of the injected current is usually limited compared to stationary EIT system. This study aims to evaluate the performance of current injection patterns with various low-amplitude currents in healthy volunteers.Approach.A total of 96 test sets of EIT measurement was recorded in 12 healthy subjects by employing adjacent and opposite current injection patterns with four amplitudes of small current (i.e. 1 mA, 500 uA, 250 uA and 125 uA). The performance of the two injection patterns with various currents was evaluated in terms of signal-to-noise ratio (SNR) of thorax impedance, EIT image metrics and EIT-based clinical parameters.Main results.Compared with adjacent injection, opposite injection had higher SNR (p< 0.01), less inverse artifacts (p< 0.01), and less boundary artifacts (p< 0.01) with the same current amplitude. In addition, opposite injection exhibited more stable EIT-based clinical parameters (p< 0.01) across the current range. For adjacent injection, significant differences were found for three EIT image metrics (p< 0.05) and four EIT-based clinical parameters (p< 0.01) between the group of 125 uA and the other groups.Significance.For better performance of wearable pulmonary EIT, currents greater than 250 uA should be used in opposite injection, 500 uA in adjacent one, to ensure a high level of SNR, a high quality of reconstructed image as well as a high reliability of clinical parameters.