How Much Negative Pressure Are We Generating During Thoracentesis?

TO THE EDITOR

Handheld digital pleural manometers (DPMs) are used to measure pleural pressure during thoracentesis. Lee et al demonstrated concordance between pleural pressure measurements done with a DPM and with an electronic transducer manometer.1

Only static intrathoracic pressure can be measured with the DPM. Pleural pressure is usually measured at the beginning of the thoracentesis (opening pressure), after removal of 240 cc of fluid, and at the end of the thoracentesis (closing pressure). To our knowledge, no published studies document the negative pressure that occurs during the actual drainage. This letter focuses on the difference between the negative pressures generated with manual intermittent suction (MIS) vs the negative pressure bottle (NPB).

CASE REPORT

A 73-year-old male with a history of stage IV adenocarcinoma of the lung presented with progressive shortness of breath. Thoracic ultrasound examination demonstrated a large right pleural effusion. Ultrasound-assisted right-side thoracentesis was performed, and a DPM was used to measure pressures during the procedure. Three static pleural pressure measurements were assessed: the opening pressure, the pressure after the removal of 240 cc of pleural fluid, and the closing pressure. An NPB was used, and a total of 800 cc of pleural fluid was removed. No signs of lung entrapment were evident. Thoracic ultrasound confirmed the total removal of the fluid and the presence of pleural sliding at the conclusion of the procedure. The patient tolerated the procedure, confirmed improvement of his respiratory symptoms, and denied any chest pain or cough. A week later, the patient presented with dyspnea on exertion and reported the feeling of pleural fluid accumulation. Thoracic ultrasound confirmed reformation of the pleural fluid, and 760 cc of fluid was removed using a manual 60-cc suction syringe. As during the previous procedure, 3 pleural pressure measurements were performed and revealed the absence of lung entrapment. On both occasions, we measured the negative pressure generated by the suction device. The negative pressure generated by the NPB started at approximately –115 cm H₂O. Following the removal of 800 cc of fluid, the bottle pressure was approximately –75 cm H₂O (Figure 1). On the other hand, when MIS was performed with a 60-cc syringe, the negative pressure was approximately –250 cm H₂O each time with a fully pulled syringe (Figure 2). Despite the large difference between the negative pressures generated in each method, we saw no difference in the outcomes (Table).

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Figure 1.

Graph demonstrates the difference in negative pressure generated by a negative pressure bottle in relation to the volume of fluid removed while performing thoracentesis. On the right, a digital pleural manometer shows the negative pressure measurement at the start and at the end of the procedure.

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Figure 2.

Graph demonstrates the difference in negative pressure generated by manual intermittent suction with a 60-cc syringe (13 full 60-cc syringe suctions) in relation to the volume of fluid removed while performing thoracentesis. On the right, a digital pleural manometer shows the measurement of negative pressure at each full syringe suction.

DISCUSSION

The DPM is a new tool that can measure pleural pressure during thoracentesis.1,2 The DPM is especially useful for patients with a high risk of expansion pulmonary edema or with lung entrapment.3 Young patients with large pleural effusion (>3 L) and more than 7 days' duration of lung collapse are at risk for developing expansion pulmonary edema.4,5 Obtaining static measurements of the pleural pressure while performing thoracentesis can provide information about the intrathoracic pressure. The measurements identify any sudden drop in the intrathoracic pressure and provide an early sign of lung entrapment. Our patient did not have any signs of lung entrapment during either procedure.

The amount of negative pressure generated by the NPB and MIS during the fluid removal process is significantly different. Although we saw no difference in the outcomes at the end of each procedure, the amount of negative pressure generated with MIS exceeded the negative pressure generated by the NPB by approximately 100 cm H₂O with each syringe filling. On the other hand, the negative pressure generated by the NPB is a continuous negative pressure throughout fluid drainage that is inversely reduced by the volume removed. One of the factors that can contribute to the development of expansion pulmonary edema is the speed of the fluid removal. Considering both methods, the negative pressure generated by MIS can reach approximately –250 cm H₂O with full syringe expansion and can cause more rapid fluid removal. Also, MIS can cause intermittent fluctuations in the intrathoracic pressure. To avoid the rapid removal of fluid and pressure fluctuations while using MIS, a gentle manual negative pressure can be applied.

The outcomes of both procedures did not show that one method was superior to the other. However, the NPB method is less cost effective than MIS because of the price of the bottles.

Our findings are an eye-opener about the amount of pressure generated with manual fluid removal.6 However, the real negative pressure generated by each method cannot be confirmed by static intermittent measurement of the pleural pressure. Intrathoracic pressure should be continuously measured to obtain the real correlation between the negative pressure generated and the pleural pressure.