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Thứ Ba, 5 tháng 11, 2013

CÔNG THỨC TÍNH THỂ TÍCH TRÀN DỊCH MÀNG PHỔI: V (ml)=[16 (mm)x D ]




Abstract
The aim of this study was to establish a practical simplified formula to facilitate the management of a frequently occurring postoperative complication, pleural effusion. Chest ultrasonography with better sensitivity and reliability in the diagnosis of pleural effusions than chest X-ray can be repeated serially at the bedside without any radiation risk. One hundred and fifty patients after cardiac surgery with basal pleural opacity on chest X-ray have been included in our prospective observational study during a two-year period. Effusion was confirmed on postoperative day (POD) 5.9±3.2 per chest ultrasound sonography. Inclusion criteria for subsequent thoracentesis based on clinical grounds alone and were not protocol-driven. Major inclusion criteria were: dyspnea and peripheral oxygen saturation (SpO ) levels ≤92%  and the maximal distance between mid-height of the diaphragm and visceral pleura (DG30 mm). One hundred and thirty-five patients (90%) were drained with a 14-G needle if according to the simplified formula: V (ml)=[16 (mm)x D ] the volume of the pleural effusion was around 500 ml. The success rate of obtaining fluid was 100% without any complications. There is a high accuracy between the estimated and drained pleural effusion. Simple quantification of pleural effusion enables time and cost-effective decision-making for thoracentesis in postoperative patients.

 2010 Published by European Association for Cardio-Thoracic Surgery. All rights reserved.

For ultrasound examination patients were in a sitting position. The ultrasound probe (S5–1, 2.5 MHz, iE33 Philips, Philips, Germany) was moved in a cranial direction in the mid-scapular line. The visceral layer moved during each respiratory cycle with a decrease in interpleural separation during inspiration. The lung behind the pleural effusion appeared either ventilated or consolidated. The maximal distance between mid-height of the diaphragm and visceral pleura (D) was measured after freezing the image in end-expiration (Fig. 2). The diaphragm, liver and spleen had to be clearly visualised before tapping to avoid accidental puncture. An interpleural distance DG30 mm was required to include the patient into the study (Fig. 1). Thoracentesis was performed in the mid-scapular line after previous determination with the probe. All thoracenteses were therapeutic, i.e. aimed at draining the pleural space to a large extent, but to the determined maximum of 2000 ml just to prevent pulmonary oedema. We performed the thoracentesis with a syringe pump system mounted onto the 14 G catheter-over-needle enabling aspiration of the pleural effusion. The volume of fluid (V) was recorded and the tap was terminated when no more fluid could be aspirated. All patients with incomplete aspiration of pleural fluid who had separation of pleural layers of 20 mm on post-puncture ultrasound were excluded from the study. A chest X-ray was performed in all patients after thoracentesis.

Discussion
In the presented study our goal was to establish a practical algorithm by formulating a simplified calculation to facilitate the management of a frequently occurring postoperative complication, pleural effusion. It is generally accepted that chest ultrasonography shows better sensitivity and reliability in the diagnosis of pleural effusions than chest X-ray w1, 2x. Chest ultrasonography can be repeated serially at the bedside without any radiation risk. Modern miniaturized advanced ultrasound systems are portable, allowing physicians to quickly perform rapid diagnostics and thoracentesis right at a patient’s bedside, ideal for emergency situations. The advantage of ultrasound evaluation of pleural effusion is obvious w3, 6, 7x: it helps to quantify the pleural fluid volume using our simplified formula V(ml)sw16=D (mm)x and hence helps in deciding whether or not thoracentesis should be performed. The complication rate in this study was zero, specifically no pneumothorax was noted. The major advantage of thoracentesis per tapping with a 14-G needle is in its minimal invasiveness without a need of skin incision, as being required by aformal chest tube or Seldinger Chest Drainage Kit Portex[1]type. On the other hand, patients are not immobilized after thoracentesis with the method presented in this study.
The authors excluded small pleural collections by excluding patients with pleural separation -30 mm on initial ultrasound examination. It was also suggested that the relationship may not be as linear and clinically important for pleural separations below 20 mm w8–10x. One potential source of error for volume underestimation was lower lobe atelectasis with large effusions over 1000 ml, which may lead to different volume ‘morphology’ not amenable to quantification w2x. Sonographic measurement is also influenced by the size of thoracic cavity. In large thoraces in tall people, the layer measured by ultrasound may cause underestimation of the actual volume of pleural fluid. The results could also be influenced by the examination technique: the transducer must not be angled or tilted, which may result in a scan that is oblique to the transverse plane.
Such measurement may produce overestimation of the effusion width. Finally, few limitations of this study should be mentioned. The small number of the patients could be one of them but on the other side the derived formula is highly accurate, justifying the chosen patient collective. Of course a high intra- and interobserver variability of the performed ultrasonographic measurement may exist accentuating the need for some expertise in ultrasonography.

Conclusion
For bedside decisions practical algorithms, like our presented management of postoperative pleural effusions are beneficial. With our simplified formula we could easily quantify pleural effusion and could decide cost and time effectively whether or not to perform a thoracentesis.
Thoracentesis of pleural effusions G500 ml in patients following cardiac surgery under ultrasound guidance proved to be a safe procedure, and improved postoperative respiration and recovery, and shortened the postoperative stay.

E. Usta et al. / Interactive CardioVascular and Thoracic Surgery 10 (2010) 204–207

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