EDITORIAL |
https://doi.org/10.4103/ijrc.ijrc_108_22 |
Ultrasound-Based Weaning Indices: The Need of the Hour?
Department of Critical Care, Kasturba Medical College, Manipal, 1Department of Critical Care Medicine, Narayana Hrudayalaya, Bengaluru, Karnataka, India
E-mail: dr.harishmm@rocketmail.com
How to cite this article: Chaudhuri S, Maheshwarappa HM. Ultrasound-based weaning indices: The need of the hour? Indian J Respir Care 2022;11:195-9.
Received: 02-06-2022
Revised: 28-06-2022
Accepted: 29-06-2022
Published: 28-07-2022
INTRODUCTION
Weaning is the gradual process of withdrawal from mechanical ventilation.[1] The process of weaning is initiated with the resolution of the primary disease process, which had initially required mechanical ventilation, after which a spontaneous breathing trial (SBT) is conducted.[1] The duration of SBT to predict weaning in critically ill patients varies from 30 min to 120 min.[1–3]
Weaning failure is defined as the failure of SBT or the requirement of reintubation within 48 h of extubation.[4] Clinicians often use both subjective criteria (increased respiratory efforts and diaphoresis) and objective criteria (rapid shallow breathing index and arterial blood gas analysis) to predict successful weaning.[1]
The weaning process involves interplay of vital systems, such as the airway, lung, brain, cardiac, diaphragm, and endocrine/electrolytes, as elucidated by Heunks and van der Hoeven in the ABCDE approach for the causes of weaning failure.[4] The ABCDE approach in weaning refers to optimal functioning of the Airway, Brain, Cardiac, Diaphragm, and Endocrine components in the patient. The component “E” may also include the expiratory muscles, especially the abdominal expiratory muscles, which are often overlooked in critically ill patients.[5] The evaluation of lung, cardiac, diaphragm, and expiratory muscles is greatly enhanced by the bedside use of point-of-care ultrasound (POCUS). The ultrasound weaning indices in conjunction with the existing traditional weaning indices can be an asset in the armamentarium of the clinician to predict successful weaning.
Lung ultrasound for weaning
The evaluation of lung ultrasound is crucial to predict weaning success. Lung ultrasound assesses the extent of de-aeration and is an important predictor of weaning success.[6] The lung ultrasound score (LUS) objectively scores lung regions depending on the extent of de-aeration and has been shown to predict weaning failure, with patients having a higher LUS having greater chances of weaning failure.[6,7] A LUS >15 predicted weaning failure, whereas a score <11 predicted weaning success.[6] Lung ultrasound also allows the diagnosis of weaning-induced pulmonary edema (WIPO) post-SBT and therefore is an important bedside POCUS tool to predict successful weaning.[8] An increase in the B-lines by ≥6, post-SBT, as compared to lung ultrasound pattern before initiation of the SBT, indicates WIPO.[8] A lung ultrasound image with coalescent B lines appearance is depicted [Figure 1].
Figure 1: Lung ultrasound image with coalescent B lines appearance
Cardiac ultrasound for weaning
Echocardiography at the bedside during the SBT helps in predicting systolic or diastolic dysfunction during the process.[9] The calculation of the early (E) mitral valve diastolic velocity, the late (A) diastolic velocity, and the tissue Doppler imaging (TDI) of the mitral valve annulus (e'), can all help diagnose diastolic dysfunction during the SBT.[9] The ratio of mitral valve E/A diastolic velocity >0.95 and the TDI estimation of the E/e' >8.5 detects left ventricular diastolic dysfunction accurately.[9,10] SBT puts a patient to stress, and assessment of left ventricular contractility to evaluate systolic dysfunction, the evaluation of diastolic dysfunction along with the lung ultrasound, is extremely vital to predict cardiac failure and the possibility of WIPO during the SBT process.[9]
Figure 2: Diaphragmatic excursion
Diaphragmatic ultrasound for weaning
The diaphragm being the main inspiratory muscle of respiration, its optimal functioning is extremely vital for successful weaning.[4] The assessment of the diaphragm involves three crucial aspects, the diaphragmatic excursion (DE), diaphragmatic thickening fraction (DTF), and the speed of diaphragmatic contraction (DC).[6,11] DE is the distance traversed by the diaphragm during spontaneous breathing and is evaluated in the motion mode (M-mode) of the ultrasound [Figure 2]. DTF is the extent of diaphragmatic thickening during the inspiration and is measured at the zone of apposition of the pleura with the diaphragm [Figure 3]. The DTF is computed using the formula:
Figure 3: Diaphragmatic thickening during the inspiration measured at the zone of apposition of the pleura with the diaphragm
Figure 4: Rectus abdominis muscle
Figure 5: External oblique, internal oblique, and transversus abdominis muscle
Figure 6: Parasternal intercostal muscles. The red arrows depict the rib shadows and the yellow plus signs depict the parasternal intercostal muscles
DTF = (Thickness of diaphragm during end inspiration-Thickness of diaphragm during end-expiration)/Thickness of diaphragm during end-expiration
The DC is evaluated by measuring the slope of the DC process in mm/s.
During SBT, the DE ≥1.65 cm, DTF ≥49.48%, and DC >10 mm/s have been found to predict weaning success, as against DE <0.80 cm, DTF <28%, and DC <8 mm/s, all of which was found in patients with failed weaning.[6,11] With
regards to a high predictive ability for successful weaning, a recent study has shown that the incorporation of the three ultrasound-based weaning indices with their cut-offs, namely the DE >1.3 cm, DTF ≥30%, and LUS <11, along with the traditional RSBI 102 showed the highest area under curve AUC (0.919), to predict successful weaning with a sensitivity of 96.0% and a specificity of 89%.[6] Thus, a comprehensive weaning strategy using the traditional RSBI with the ultrasound-based weaning indices is the need of the hour.
Abdominal expiratory muscles ultrasound for weaning Diaphragmatic weakness is an inevitable occurrence in most critically ill patients.[5] Therefore, it is in these patients that the abdominal expiratory muscles assume greater significance at the time of SBT.[5] The abdominal expiratory muscles help in better cough, clearing of secretions, and ensure higher chances of successful weaning.[5,12] The abdominal expiratory muscles- rectus abdominis, external oblique, internal oblique and transversus abdominis play an important role in successful weaning [Figures 4 and 5]. The greater thickness of these muscles have been shown to predict successful weaning.[12] Furthermore, the thickness of the abdominal expiratory muscles in otherwise healthy individuals follows the particular pattern: rectus abdominis > internal oblique > external oblique > transversus abdominis.[5] The maintenance of this pattern of thickness has been shown to generate better pressure for expiration and hence has been postulated to predict weaning success.[5] Thus, clinicians must remember to assess these easily accessible muscles of expiration which have an ever-increasing role in aiding the weaning process in the critically ill patient, where the diaphragm has already weakened and is functioning at a suboptimal level.
Figure 7: Algorithm of ultrasound.based weaning indices
Parasternal intercostal muscle ultrasound
Diaphragmatic weakness is prevalent in critically ill patients.[5,13]
Figure 8: Echocardiography depicting the mitral valve E and A velocity measurements
Figure 9: Echocardiography depicting the mitral tissue Doppler E' and A'
Furthermore, in patients with abdominal trauma or postsurgical patients, with dressing over the abdomen, the diaphragmatic ultrasound may not be feasible for the clinician. Thus, the need for other extra-diaphragmatic muscles which could be easily evaluated using bedside ultrasound was imperative. The parasternal intercostal muscles which aid in the generation of tidal volume were the extra-diaphragmatic muscle that assumes tremendous significance, in case the diaphragm could not be evaluated.[13] The parasternal muscles, evaluated at the second intercostal space, assume a higher role only if the diaphragm is significantly weak [Figure 6].[13] Normally, patients should not have a higher thickening fraction of the parasternal muscles during the SBT. It has been shown that if the parasternal thickening fraction was ≥ 17% during SBT, it predicted weaning failure, as compared to those with a 5% thickening fraction.[13]
The POCUS has become an extremely vital tool for the clinician working in the intensive care set-ups. Ultrasound has its own advantages, being nonradiating, available at the bedside, may be repeated whenever essential, and also has a fast-learning curve. The incorporation of the objective assessment of the systems at interplay, using POCUS during the weaning process will definitely enhance the predictive ability for weaning success.
Algorithm depicting the various ultrasound-based weaning indices which may be useful during the weaning process and aid in clinical decisions [Figure 7].
The objective evaluation of the lung (LUS), cardiac dysfunction (measurement of the mitral valve E velocity and mitral valve tissue Doppler e' for assessing diastolic dysfunction) [Figures 8 and 9], diaphragm (DE, DTF, and DC) along with extra-diaphragmatic muscles such as parasternal intercostals (parasternal muscle thickening fraction) and abdominal expiratory muscles (thickness pattern) complete the entire spectrum of ultrasound evaluation required for the quintessential assessment of respiratory load during the weaning process. The combination of all these with the traditional weaning indices is the need of the hour.
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