Measured Negative Pressure in Syringes Used for Aspiration Biopsy: Volume and Pressure Relationship

Research Article

Annals Thyroid Res. 2019; 5(2): 198-201.

Measured Negative Pressure in Syringes Used for Aspiration Biopsy: Volume and Pressure Relationship

Giordano NJ1, Rosen JE2 and Lee SL3*

1Department of Surgery, Boston Medical Center, USA

2MedStar Washington Hospital Center, USA

3Department of Medicine, Boston Medical Center, USA

*Corresponding author: Lee SL, Section of Endocrinology, Diabetes and Nutrition, Boston Medical Center, 720 Harrison Avenue, Doctor’s Office Building 8117, Boston, MA 02118, USA

Received: April 25, 2019; Accepted: May 20, 2019; Published: May 27, 2019


Background: Fine-Needle Aspiration Biopsy (FNAB) is the diagnostic test of choice for the evaluation of thyroid nodules. This study evaluates the pressurevolume relationship with 3mL and 10mL syringes used for FNAB in an effort to optimize the mechanics of performing a clinical gold standard procedure.

Methods: A gas pressure sensor was attached to 3 or 10mL syringes via a Luer lock connection. Static pressure measurements were analyzed, and compared against theoretical calculations of pressure and volume.

Results: Pressure measurements demonstrated a reduction in pressure with increasing volumes tested. There was no difference in pressure achieved using a 3 and 10mL syringe at the volumetric change of 1 and 2mL. A change of 3 mL produced 79% of the maximum negative pressure attained by fully withdrawing the plunger of a 10mL syringe. Measured negative pressure highly correlated with the theoretical calculations based on Boyle’s gas law.

Conclusion: To optimize the FNAB technique, clinicians should understand the negative pressure-volume relationship that occurs with aspiration with a syringe during a needle biopsy. The negative pressure obtained for each volume was comparable for 3 and 10mL syringes. A volume change of 3mL attains nearly 80% of the maximum negative pressure obtained compared to a 10mL aspiration and is adequate to perform FNAB. This study documents a physical property of FNAB based on Boyle’s gas law that provides insight, and establishes a standard to establish effective methods to achieve an optimal thyroid FNAB.

Keywords: Aspiration; Negative pressure; Needle biopsy; Fine needle aspiration biopsy; Pressure-volume relationship


FNAB: Fine-Needle Aspiration Biopsy; US: Ultrasound; mL: Milliliter; atm: atmosphere of pressure


Thyroid cancer is the most common endocrine malignancy in the United States. Approximately 53,990 new cases are expected to be diagnosed in 2018 [1]. Thyroid nodules are a common clinical finding especially after the increased use of high resolution imaging such us Ultrasound (US) technology, computed tomography and magnetic resonance imaging but thyroid nodules have a low risk of malignancy of between 5 to 10% [2-7]. Clinical evaluation to diagnose malignancy includes a careful clinical evaluation, TSH evaluation, a thyroid US exam and a Fine-Needle Aspiration Biopsy (FNAB) of nodules [8,9]. Thyroid FNAB is the most accurate test for determining malignancy and is the recommended diagnostic test in the initial evaluation of thyroid nodules [3,8-10] by the American Thyroid Association and the American Association of Clinical Endocrinologists [8,9]. The biopsy technique often uses aspiration to obtain cells or fluid from the thyroid nodule using a needle attached to a syringe [2,3,11-14]. The conceptual basis of a fine needle aspiration biopsy is pulling back on the syringe plunger creates a suction (negative pressure) to aspirate cells from a thyroid nodule into the needle for cytological examination. A nonscientific survey of the 10 endocrinologists at Boston Medical Center who perform thyroid fine needle biopsies showed 1 physician use 0-1 milliliter (mL), 4 physicians use 2-3mL, 3 physicians use 4-6mL and 2 physicians use > 6mL of aspiration during a FNAB. When asked why they used this volume, there was no scientific basis or knowledge of expert guidelines to support their clinical habit. The optimal negative pressure for FNAB is not known. Excess negative pressure during a biopsy increases the probability of an blood-contaminated specimen interfering with the cytological analysis that can result in a biopsy that contains inadequate numbers of thyroid follicular cells for analysis (Bethesda I classification) [15] and cause complications of local pain and hematoma [16,17]. Clinicians use many different techniques to perform fine needle biopsy but there have been no studies to date that determine the aspiration volume and the generated vacuum necessary to optimal negative pressure or syringe size to use during FNAB of thyroid nodules. Understanding the pressure-volume relationship when generating negative pressure in the needle and syringe during a FNAB will allow clinicians to optimize their technique for needle biopsies. This study performed pressure measurements generated using 3mL and 10mL syringes at different volumes of aspiration and compared the measured negative pressures with those predicted by Boyles’ law of gases. Boyle’s law is an experimental gas law that describes how the pressure of gas decreases as the volume of the container increases if the temperature and amount of gas remains unchanged within a closed system.

Materials and Methods

Theoretical calculations

Boyle’s law was used to calculate the theoretical difference in pressures generated from incremental 1mL changes in volume in 3mL and 10mL Luer lock syringes. The theoretical calculations assumed an initial pressure of 1 atmosphere of pressure (atm; 101.3 kPa) in Boston, MA (5.8 m above sea level) at a constant room temperature (23oC). In our calculations, the syringe plunger seal started at a resting position of 1mL. The new Pressure (P2) with pulling out the syringe plunger can be calculated knowing the initial pressure (P1 = 101.3 kPa) after a change in volume from V1 to V2. Mathematically this can be described as P2=P1xV1/V2. The equation shows that, as volume in the syringe increases there is a proportional decrease in the pressure of the in the closed system of the syringe at a constant temperature.

Gas pressure sensor system

A gas pressure system (Vierner Software & Technology, Beaverton, OR) was used to measure the changes in gas pressure with a pressure transducer (Honeywell, Morristown, NJ) designed to measure absolute pressure. A membrane inside of the transducer flexes as pressure changes causing a relative alteration in output voltage that is measured by the gas pressure system. The device takes 100 microseconds to generate a response time, and is accurate within 0.25% of the full scale span best fit straight line. The pressure sensor was fitted directly to the syringe with a Luer lock to minimize dead space and insure a proper seal (Figure 1). 10ml syringe and 3ml syringes, the most common size syringes used for the application of a FNAB, were used in these experiments. The syringes were individually sealed in sterile packaging ((Becton Dickinson, Franklin Lakes, NJ). Three different syringes of each size were used to insure reproducibility of the data. Pressures were measured three times at each volume selected. The device was re-calibrated to zero every time the syringe was connected to the sensor. All measurements for both the 10mL and the 3mL syringes were taken within a half hour in an effort to maintain the same barometric pressure and temperature throughout each cycle of testing. Initially the plunger was set in the syringe at 1mL, attached to the gas pressure sensor device by the Luer lock and then the volume in the syringe was increased in 1mL increments by manually pulling back on the plunger. At each volume, static pressure measurement was obtained at least 3-4 seconds after aligning the plunger seal at the desire volume to ensure stabilization of the pressure reading. Between each measurement, the plunger was released, and observed to spontaneously return to the initial plunger position at 1mL to ensure a proper seal between the sensor and the syringe was maintained.