Bedside Evaluation of Pulmonary Embolism by Saline Contrast–enhanced Electrical Impedance Tomography: Considerations for Future Research

To the Editor:

We read with great interest the article by He and colleagues (1) entitled “Bedside Evaluation of Pulmonary Embolism by Saline Contrast Electrical Impedance Tomography Method: A Prospective Observational Study.” The authors found that pulmonary embolism (PE)-invoked regional perfusion defection could be detected with saline-contrasted electrical impedance tomography (EIT) and claimed that the method showed high sensitivity and specificity for diagnosis of PE. However, several factors potentially affecting the reported findings should be discussed.

For measurement of pulmonary perfusion, a short apnea is needed during bolus injection of 10 ml 10% NaCl to eliminate the interruption from cyclic breath. The conscious patients were required to hold their breath at the end of expiration for 8 seconds or longer. Although the shorter the apnea, the more feasible for conscious patients to hold their breath, it needs imperative time to allow blood mixed with saline to travel through the whole pulmonary circulation. Slutsky and colleagues (2) found that mean pulmonary transit time ranged from 4.3 to 12.6 seconds (mean 7.7 ± 1.5 s) in humans. Moreover, acute PE can lead to an increase of pulmonary vascular resistance, which would remarkably prolong pulmonary transit time (3). In this context, it is questionable that a period with a lower level of 8 seconds is enough for saline to pass through the lung in patients with PE.

On the other hand, for those intubated, holding breath for even 8 seconds might be challenging as dyspnea is common among patients with PE; manual expiratory hold is likely to trigger spontaneous breath, which would dramatically impact the intrathoracic electric impedance. To avoid spontaneous breath, sometimes neuromuscular relaxant is needed, which was not detailed in this article.

Recently, Mauri and colleagues published a study exploring the $\dot{\text{V}}$/$\dot{\text{Q}}$ ratio in patients with coronavirus disease (COVID-19), in which a lower concentration (5%) of saline and end-inspiration occlusion for 20 seconds were implemented for determination of pulmonary perfusion (4). Compared with breath hold at the end of expiration, inspiratory hold might be more tolerable for patients with dyspnea and seems more practicable owing to the Hering-Breuer deflation reflex. A maximal inflation of lung during inspiratory breath hold can suppress respiratory drive through activation of the pulmonary stretch receptors. Vice versa while holding breath after an expiration. In addition, CO₂ accumulation contributes more to the urge to breathe than O₂ through chemoreceptors; therefore, a larger lung volume is conducive to dilute the increase in metabolically derived CO₂ levels (5).

Finally, because of the low spatial resolutions, EIT is prone to detect large emboli. On the condition that embolism occurs in segmental branches of pulmonary arteries or lower, EIT might be insensitive to such redistribution of local blood flow. To improve the sensitivity of EIT on PE, Nguyen and colleagues (6) assessed the right lung to left lung perfusion ratio of peak value, maximum uptake, maximum washout, and area under the curve of the averaged contrast dilution curve in each lung. It was concluded that the right lung to left lung perfusion ratios of area under the curve and peak value of the averaged contrast dilution curve are the most promising and reliable in assessing PE, suggesting that EIT might detect the difference between normal and embolized lungs with a unilateral perfusion defect as small as 8% of the lung with a bolus of hypertonic saline.

In general, contrast-enhanced EIT is potentially a promising bedside approach in PE diagnosis. However, numerous issues in regard to feasibility, efficacy, and safety need to be addressed before its clinical application.