Printer Friendly

Positive End-expiratory Pressure Titration after Alveolar Recruitment Directed by Electrical Impedance Tomography.

Byline: Yun. Long, Da-Wei. Liu, Huai-Wu. He, Zhan-Qi. Zhao

Background: Electrical impedance tomography (EIT) is a real-time bedside monitoring tool, which can reflect dynamic regional lung ventilation. The aim of the present study was to monitor regional gas distribution in patients with acute respiratory distress syndrome (ARDS) during positive-end-expiratory pressure (PEEP) titration using EIT. Methods: Eighteen ARDS patients under mechanical ventilation in Department of Critical Care Medicine of Peking Union Medical College Hospital from January to April in 2014 were included in this prospective observational study. After recruitment maneuvers (RMs), decremental PEEP titration was performed from 20 cmH [sub]2 O to 5 cmH [sub]2 O in steps of 3 cmH [sub]2 O every 5-10 min. Regional over-distension and recruitment were monitored with EIT. Results: After RMs, patient with arterial blood oxygen partial pressure (PaO [sub]2) + carbon dioxide partial pressure (PaCO [sub]2 ) >400 mmHg with 100% of fractional inspired oxygen concentration were defined as RM responders. Thirteen ARDS patients was diagnosed as responders whose PaO [sub]2 + PaCO [sub]2 were higher than nonresponders (419 [+ or -] 44 mmHg vs. 170 [+ or -] 73 mmHg, P < 0.0001). In responders, PEEP mainly increased recruited pixels in dependent regions and over-distended pixels in nondependent regions. PEEP alleviated global inhomogeneity of tidal volume and end-expiratory lung volume. PEEP levels without significant alveolar derecruitment and over-distension were identified individually. Conclusions: After RMs, PEEP titration significantly affected regional gas distribution in lung, which could be monitored with EIT. EIT has the potential to optimize PEEP titration.

Introduction

Protective lung ventilation limited the plateau pressure (P [sub]plat ) to 30 cmH [sub]2 O in order to avoid alveolar over-distension. [sup][1],[2] However, because of atelectasis, tidal ventilation might be restricted in relative normal lung regions instead of diseased lung regions given that positive-end-expiratory pressure (PEEP) was insufficient. Meanwhile, those aerated regions would become over-distended when a large amount of atelectatic regions were not involved in tidal ventilation. [sup][3],[4] Recruitment maneuvers (RMs), especially achieving full alveolar recruitment, is the possible way to alleviate the inhomogeneity of tidal ventilation when more lung regions participate in tidal ventilation. [sup][5],[6],[7]

Ideal alveolar recruitment is difficult to achieve. Some patients were difficult to be fully recruited because the requested P [sub]plat was too high. [sup][8] Borges et al. [sup][9] suggested to use maximal alveolar recruitment (MAR) instead of full alveolar recruitment. Patients whose arterial blood oxygen partial pressure (PaO [sub]2 ) plus carbon dioxide partial pressure (PaCO [sub]2 ) were higher than 400 mmHg with 100% of fractional inspired oxygen concentration (FiO [sub]2 ) achieved MAR. The optimal PEEP level is hard to decide for an individual because recruitment and over-distension sometimes happen simultaneously during PEEP titration. [sup][5],[10]

Electrical impedance tomography (EIT) is a real-time bedside monitoring tool, which can reflect dynamic regional lung ventilation instead of the static image like computed tomography (CT) scan. [sup][11],[12],[13] In this study, we monitored the regional gas distribution during RMs and consequent PEEP titration with EIT. The aim was to explore if the choice of optimal PEEP could be directed by the optimal gas distribution monitored with EIT for patients with acute respiratory distress syndrome (ARDS) on the bedside.

Methods

Patients and experimental protocol

Consecutive ARDS patients under mechanical ventilation in Department of Critical Care Medicine of Peking Union Medical College Hospital were included in this prospective study from January to April in 2014. Exclusion criteria were: Age <18 years, pregnancy and lactation period, and any contraindication to the use of EIT (pacemaker, automatic implantable cardioverter defibrillator, and implantable pumps). The study was approved by the Ethics Committee of Peking Union Medical College Hospital. Written informed consent was obtained from all patients or their legal representatives prior to the study.

All the patients received 2-4 mg/h intravenous midazolam and 1-2 mg/h vecuronium bromide to assure no spontaneous breaths. Every patient was ventilated with volume control mode with Drager Evita 4 (Drager Medical, Lubeck, Germany). The tidal volume (V [sub]T ) was set to 6 ml/kg ideal weight, and FiO [sub]2 and PEEP were adjusted accordingly to maintain peripheral capillary oxygen saturation (SpO [sub]2 ) over 90%. If P [sub]plat was >30 cmH [sub]2 O, V [sub]T was decreased 1 ml/kg gradually until P [sub]plat was <30 cmH [sub]2 O or V [sub]T <4 ml/kg ideal weight. After 10-15 min baseline ventilation, PEEP was switched to zero end-expiratory pressure (ZEEP), and FiO [sub]2 was increased to 100% for 3-5 min. Subsequently, PEEP was increased to 15 cmH [sub]2 O for 2 min. If the P [sub]plat was <40 cmH [sub]2 O, PEEP was further increased to 20 cmH [sub]2 O for another 2 min. Decremental PEEP trial started after the RM. And FiO [sub]2 was adjusted back to previous level before RMs, then PEEP was decreased from 20 cmH [sub]2 O or 14 cmH [sub]2 O to 5 cmH [sub]2 O in steps of 3 cmH [sub]2 O every 5-10 min unless SpO [sub]2 less than 90% and PEEP would be never decreased further.

Measurements and data analysis

Blood gases measurements were measured by radiometer 600 series blood gas analyzers, respiratory system mechanics was measured by bedside ventilators. An EIT electrode belt with 16 electrodes was placed around the thorax in the fifth intercostal space, and one reference electrode was placed on the patients' abdomen (PulmoVista 500, Drager Medical, Lubeck, Germany). Electrical alternating currents were applied in a sequential rotating process through adjacent electrode pairs. The resulting surface potential differences between neighboring electrode pairs were measured. The stimulation frequency and amplitude were adjusted automatically by the EIT device to minimize the influence of background noises. EIT measurements were continuously performed at 20 Hz from baseline through RM to decremental PEEP trial. Corresponding EIT data were recorded. EIT data reconstructed uses a finite element method based linearized Newton-Raphson reconstruction algorithm. Baseline of the images was referred to end-expiration of ZEEP.

Five consecutive breaths at the end of each PEEP step were selected. EIT images at end-inspiration ( I [sub]I,P ) and end-expiration ( I [sub]E,P ) were identified, where P denoted arbitrary PEEP levels ( P ϵ {20, 17, 14,…, 5} cmH [sub]2 O). Corresponding images were averaged to minimize noise. We defined tidal image I [sub]TV,P = I [sub]I,P − I [sub]E,P . Assuming Z [sub]k were pixels in images with impedance value of Z ( k ϵ K , K = {1, 2,…, 1024}). Lung regions at end-expiration included pixels m ϵ M where Z [sub]m,E ≥ 25% x max ( Z [sub]k,E ). Lung regions for tidal breathing included pixel n ϵ N where Z [sub]n,TV ≥ 20% x max ( Z [sub]k,TV ). Regions o were considered to be overinflated, if they belong to lung regions at end-expiration but are not or minimally ventilated during tidal breathing ( o ϵ O , O = M − N ). Regions r are considered to be recruited compared to reference PEEP P1, if they belong to lung regions at end-expiration at current PEEP step but not at P1 ( r ϵ R , R = M [sub]Pn − M [sub]P1 , n ≠ 1). Since the EIT images were reconstructed with zero PEEP as baseline, the amplitude of noise at low PEEPs may have the same level as impedance values at end-expiration. Therefore, we selected the end of decremental PEEP trial as P1 (2 cmH [sub]2 O) and calculated the regions o , and r at PEEP = 20, 17,…, and 5 cmH [sub]2 O. The regions o and r were further divided into four anteroposterior segments with equal height and number of recruited and over-distended pixels were calculated (denoted as 4 regions of interests [ROI], where ROI1 corresponds to most nondependent regions and ROI4 corresponds to most dependent regions).

Patients were diagnosed responders after RMs whose PaO [sub]2 + PaCO [sub]2 were more than 400 mmHg with 100% FiO [sub]2 . Recruited pixels were defined new aerated pixels when compared with ZEEP. Over-distended pixels were defined aerated pixels that did not join in tidal ventilation under the same PEEP. For studying intrapulmonary gas distribution, we separated perpendicularly EIT image into 4 equal zones from ventral to dorsal. We had still used globe inhomogeneity (GI) to evaluate gas distribution in tidal ventilation and functional residual capacity, with the nomination of GI-TV and GI-FRC individually. [sup][14] The optimal PEEP was considered which could prevent significant derecruitment without obvious over-distention.

Statistical analysis

Statistical analyses were performed using SPSS version 21 (IBM, Chicago, IL, USA). Data were tested for normal distribution and homoscedasticity using the Kolmogorov-Smirnov test and the Brown-Forsythe test. Basal data and respiratory mechanisms, had a normal distribution values, were presented as means [+ or -] standard deviation (SD) and analysis of variance test was applied. And recruited and over-distended pixels that had an abnormal distribution were presented as the median and median interval and independent samples Kruskal-Wallis was used. All P < 0.05 were considered to be statistically significant.

Results

Patients and clinical data

A total of 18 ARDS patients under mechanical ventilation in Department of Critical Care Medicine of Peking Union Medical College Hospital were included in this prospective study (10 male, 8 female; age 58 [+ or -] 12 years; acute physiology and chronic health evaluation II 23 [+ or -] 8; ideal weight 62 [+ or -] 10 kg. Twelve patients were diagnosed as pneumonia, other patients were caused by extrapulmonary origins as unknown origin fever, surgery, severe acute pancreatitis, and so on.

Basal data comparison between responders and nonresponders

As shown in [Table 1], 13 ARDS patients with PaO [sub]2 + PaCO [sub]2 >400 mmHg and 100% FiO [sub]2 were diagnosed as responders after RMs, who had received less dose norepinephrine and lower V [sub]T ventilation compared with nonresponders before RMs. In other basal parameters, there were not significant differences between responders and nonresponders. The rate of pneumonia in responders and nonresponders was not significantly different (34.8% vs. 80.0%, P = 0.0615).{Table 1}

Recruitment maneuvers

Three patients received 15 cmH [sub]2 O PEEP of RMs since their P [sub]plat were higher than 40 cmH [sub]2 O. The rest of patients had received both 15 cmH [sub]2 O and 20 cmH [sub]2 O PEEP of RMs. Although there was no significant difference in PaO [sub]2 at ZEEP between two groups, PaO [sub]2 of responders significantly increased at PEEP 15 (302 [+ or -] 87 mmHg vs. 104 [+ or -] 75 mmHg, P < 0.0001) and PEEP 20 (369 [+ or -] 48 mmHg vs. 121 [+ or -] 85 mmHg, P < 0.0001) compared with nonresponders [Table 2].{Table 2}

Image of recruited pixels and over-distended pixels during positive-end-expiratory pressure titration after recruitment maneuvers in responders and nonresponders

As shown in [Figure 1], during PEEP titration, recruited and over-distended pixels under different PEEP were shown from ventral (upper) to dorsal (lower). Recruited and over-distended pixels were marked in purple and white, respectively. Compared with patient no. 3 (nonresponder), much more recruited pixels were observed in dorsal regions and less over-distended pixels in ventral regions of the patient no. 1 (responder).{Figure 1}

Changes of recruited pixels and over-distended pixels during positive-end-expiratory pressure titration after recruitment maneuvers in all patients

As shown in [Table 3], the recruited pixels in all patients decreased grossly along with decremental PEEP, and the over-distended pixels were stable from PEEP 20 to PEEP 14 and decreased gradually from PEEP 14 to PEEP 8.{Table 3}

Recruited and over-distended pixels changed with positive-end-expiratory pressure titration in two groups

As shown in [Table 4], in responders, the recruited pixels were stable grossly but decreased significantly from PEEP 20 to PEEP 14 and from PEEP 8 to PEEP 5, and the over-distended pixels were stable from PEEP 20 to PEEP 14 and decreased from PEEP 14 to PEEP 8. However, in nonresponders, no significant changes of recruited and over-distended pixels were found during PEEP titration.{Table 4}

Difference of recruited and over-distended pixels of four regions of interests with positive-end-expiratory pressure titration

As shown in [Table 5], compared with nonresponders, responders had more recruited pixels in PEEP 8 ( P = 0.037) and PEEP 5 ( P = 0.031) and more over-distended pixels in PEEP 8 ( P = 0.05) of ROI2.{Table 5}

And in responders, the recruited pixels decreased from PEEP 8 to PEEP 5, and the over-distended pixels decreased from PEEP 11 to PEEP 8 in ROI1, the over-distended pixels decreased from PEEP 14 to PEEP 8 in ROI2, and the recruited pixels decreased from PEEP 17 to PEEP 5 in ROI3. However, there was no significant difference of recruited and over-distended pixels during PEEP titration in four ROIs in nonresponders.

Globe inhomogeneity of two groups

As shown in [Table 6], no significant changes of GI-TV and GI-FRC were found in nonresponders. In responders, GI-TV was improved in all PEEP levels compared with ZEEP, except PEEP 20. GI-FRC was improved in all PEEP levels except PEEP 5. During PEEP titration, there were significant changes of GI-TV from PEEP 8 to 5 and GI-FRC from PEEP 20 to 5. PEEP 11 has the lowest GI-TV of 0.39 (0.33-0.45), and PEEP 17 has the lowest GI-FRC of 0.40 (0.34-0.42).{Table 6}

Discussion

Inhomogeneity of intrapulmonary gas distribution is the main problem for ventilation treatment in ARDS patients. [sup][10],[15] More atelectasis is present under lower PEEP, and more over-distension occurs under higher PEEP. Therefore, tidal ventilation probably happens only in some relative normal lung regions, while more stress would be introduced by involving only small part of alveoli during tidal ventilation. [sup][16],[17]

In order to improve homogeneity of ventilation distribution, RMs for opening the lung tissues would be the right choice. However, alveolar recruitment is a continuous course and behaves a pan-inspiratory phenomenon accompanied by incremental airway opening pressure. Insufficient airway opening pressure cannot achieve full alveolar recruitment and improve gas distribution. Rimensberger et al . [sup][18] recommended that the lung should be maximally recruited and subsequently be maintained opening with small V [sub]T and optimal PEEP. The criteria of "optimal" PEEP are still debatable. Hickling [sup][19] thought the maximal change of tidal compliance during decremental PEEP after full alveolar recruitment predicted optimal PEEP. Ranieri et al . [sup][20] suggested after full alveolar recruitment, the stress index between 0.9 and 1.1 indicated optimal PEEP. In our opinion, optimal PEEP is to achieve the most homogenous ventilation. At first, we had increased PEEP to 15 and 20 cmH [sub]2 O individually to implement RMs for 2 min, and secondly we titrated PEEP and monitor gas distribution dynamically with EIT. [sup][15],[21]

For studying intrapulmonary gas distribution, we divided EIT images into four ROIs perpendicularly from ventral to dorsal. It was found that PEEP mainly increased recruited pixels in ROI3 and over-distended pixels in ROI1 of responders, which coincided to our knowledge. No significant changes were found in all four ROIs of nonresponders, which indicated that RMs had little influences in nonresponders.

We had used GI-TV and GI-FRC individually to evaluate gas distribution in tidal ventilation and FRC. There were significant improvements of GI-TV and GI-FRC when compared with ZEEP and during PEEP titration after RMs in responders, which implied a more homogeneous gas distribution in tidal ventilation and FRC after RMs. Therefore, PEEP levels without significant alveolar derecruitment and over-distension could be identified individually in responders. Few changes found in nonresponders, however, again indicated that RMs had little influences in nonresponders. Hence, an optimal PEEP was difficult to decide.

One of the limitations of this study was that the P [sub]plat was limited to 40 cmH [sub]2 O for safety reason. Some nonresponding patients had poor recruited potential, and for the rest, the P [sub]plat might be not enough to achieve MAR. In further studies, measures should be taken to distinguish these two kinds of nonresponders. Another limitation of the study was that the gold-standard of identifying collapsed lung regions, namely CT scans, was missing. Due to radiation, CT is not a suitable bedside tool and indeed, there is no well-established tool available for measuring recruitment/derecruitment dynamically. The reliability of EIT has already been proven in previous studies [sup][17],[22],[23] and, therefore, the findings of the present study should be reliable.

In conclusion, EIT is a useful tool to monitor regional gas distribution at bedside. PEEP titration after MAR had significantly affected intrapulmonary gas distribution, and the selection of PEEP with most homogeneous air distribution can be guided by EIT.

References

1. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000;342:1301-8.

2. Brower RG, Lanken PN, MacIntyre N, Matthay MA, Morris A, Ancukiewicz M, et al. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome. N Engl J Med 2004;351:327-36.

3. Hess DR. Respiratory mechanics in mechanically ventilated patients. Respir Care 2014;59:1773-94.

4. Gattinoni L, D'Andrea L, Pelosi P, Vitale G, Pesenti A, Fumagalli R. Regional effects and mechanism of positive end-expiratory pressure in early adult respiratory distress syndrome. JAMA 1993;269:2122-7.

5. Rouby JJ, Lu Q, Goldstein I. Selecting the right level of positive end-expiratory pressure in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 2002;165:1182-6.

6. Barbas CS, de Matos GF, Pincelli MP, da Rosa Borges E, Antunes T, de Barros JM, et al. Mechanical ventilation in acute respiratory failure: Recruitment and high positive end-expiratory pressure are necessary. Curr Opin Crit Care 2005;11:18-28.

7. Victorino JA, Borges JB, Okamoto VN, Matos GF, Tucci MR, Caramez MP, et al. Imbalances in regional lung ventilation: A validation study on electrical impedance tomography. Am J Respir Crit Care Med 2004;169:791-800.

8. Gattinoni L, Caironi P, Cressoni M, Chiumello D, Ranieri VM, Quintel M, et al. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:1775-86.

9. Borges JB, Okamoto VN, Matos GF, Caramez MP, Arantes PR, Barros F, et al. Reversibility of lung collapse and hypoxemia in early acute respiratory distress syndrome. Am J Respir Crit Care Med 2006;174:268-78.

10. Cressoni M, Cadringher P, Chiurazzi C, Amini M, Gallazzi E, Marino A, et al. Lung inhomogeneity in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 2014;189:149-58.

11. Wrigge H, Zinserling J, Muders T, Varelmann D, Gunther U, von der Groeben C, et al. Electrical impedance tomography compared with thoracic computed tomography during a slow inflation maneuver in experimental models of lung injury. Crit Care Med 2008;36:903-9.

12. Frerichs I, Dargaville PA, Dudykevych T, Rimensberger PC. Electrical impedance tomography: A method for monitoring regional lung aeration and tidal volume distribution? Intensive Care Med 2003;29:2312-6.

13. Muders T, Luepschen H, Putensen C. Impedance tomography as a new monitoring technique. Curr Opin Crit Care 2010;16:269-75.

14. Zhao Z, Pulletz S, Frerichs I, Muller-Lisse U, Moller K. The EIT-based global inhomogeneity index is highly correlated with regional lung opening in patients with acute respiratory distress syndrome. BMC Res Notes 2014;7:82.

15. Moerer O, Hahn G, Quintel M. Lung impedance measurements to monitor alveolar ventilation. Curr Opin Crit Care 2011;17:260-7.

16. Lowhagen K, Lundin S, Stenqvist O. Regional intratidal gas distribution in acute lung injury and acute respiratory distress syndrome - Assessed by electric impedance tomography. Minerva Anestesiol 2010;76:1024-35.

17. Hinz J, Neumann P, Dudykevych T, Andersson LG, Wrigge H, Burchardi H, et al. Regional ventilation by electrical impedance tomography: A comparison with ventilation scintigraphy in pigs. Chest 2003;124:314-22.

18. Rimensberger PC, Cox PN, Frndova H, Bryan AC. The open lung during small tidal volume ventilation: Concepts of recruitment and "optimal" positive end-expiratory pressure. Crit Care Med 1999;27:1946-52.

19. Hickling KG. Best compliance during a decremental, but not incremental, positive end-expiratory pressure trial is related to open-lung positive end-expiratory pressure: A mathematical model of acute respiratory distress syndrome lungs. Am J Respir Crit Care Med 2001;163:69-78.

20. Ranieri VM, Zhang H, Mascia L, Aubin M, Lin CY, Mullen JB, et al. Pressure-time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model. Anesthesiology 2000;93:1320-8.

21. Blankman P, Hasan D, Erik G, Gommers D. Detection of 'best' positive end-expiratory pressure derived from electrical impedance tomography parameters during a decremental positive end-expiratory pressure trial. Crit Care 2014;18:R95.

22. Frerichs I, Hinz J, Herrmann P, Weisser G, Hahn G, Dudykevych T, et al. Detection of local lung air content by electrical impedance tomography compared with electron beam CT. J Appl Physiol 2002;93:660-6.

23. Marquis F, Coulombe N, Costa R, Gagnon H, Guardo R, Skrobik Y. Electrical impedance tomography's correlation to lung volume is not influenced by anthropometric parameters. J Clin Monit Comput 2006;20:201-7.
COPYRIGHT 2015 Medknow Publications and Media Pvt. Ltd.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2015 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Original Article
Author:Long, Yun; Liu, Da-Wei; He, Huai-Wu; Zhao, Zhan-Qi
Publication:Chinese Medical Journal
Article Type:Report
Date:Nov 1, 2015
Words:3533
Next Article:Fracture Union in Closed Interlocking Nail in Humeral Shaft Fractures.
Topics:

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters