Measuring interstitial fluid pressure with fiberoptic pressure transducers

Abstract

In this report we describe a practical procedure for measuring interstitial fluid pressure (IFP) using fiberoptic pressure transducers based on optical interferometry. Eight mice were used for subcutaneous IFP measurements and four mice for intramuscular IFP measurements with a FOBPS-18 fiberoptic pressure transducer. We used four mice for subcutaneous IFP measurements with a SAMBA-420 MR fiberoptic pressure transducer. One measurement was made for each mouse simultaneously by using a fiberoptic system and an established approach, either transducer-tipped catheter or wick-in-needle technique. The mean IFP values obtained in subcutaneous tissues were -3.00 mm Hg (SEM-/+0.462, n=8), -3.25 mm Hg (SEM-/+0.478, n=4), -3.34 mm Hg (SEM-/+0.312, n=6), and -2.85 (SEM-/+0.57, n=6) for the FOBPS fiberoptic transducer, the SAMBA fiberoptic transducer, the transducer-tipped catheter, and the wick-in-needle technique, respectively. There was no difference between these techniques to measure IFP (Friedman test, p=0.7997). The subcutaneous IFP measurements showed strong linear correlation between fiberoptic transducer and transducer-tipped catheter (R(2)=0.9950) and fiberoptic transducer and wick-in-needle technique (R(2)=0.9966). Fiberoptic pressure transducers measure the interstitial fluid pressure accurately, comparable to conventional techniques. The simplified IFP measurement procedures described in this report will allow investigators to easily measure IFP, and elucidate the unit pressure change per unit volume change (dP/dV) in normal or cancer tissues in the presence of strong electromagnetic fields encountered in MRI.

Figure

Interstitial fluid pressure measurement with fiberoptic pressure transducers. A. The distal end of the WPI-FOBPS-18 fiberoptic transducer tip is not sensitive to pressure while the side of the transducer (arrow) is sensitive to the same. This feature of WPI-FOBPS-18 prevents piston effect and consequent pressure artifact. Millar SPC 320, a non-fiberoptic transducer tipped catheter, is shown with an asterisk for comparison Scale bar indicates 590 μm. B. The distal end of the SAMBA 420 MR fiberoptic transducer (arrow) is sensitive to pressure while the side of the transducer is not. This feature of SAMBA 420MR causes piston effects and temporary pressure artifacts when the transducer is placed (introduced) into the tissue interstitium for IFP measurement. Scale bar indicates 250 μm. C. The WPI-FOBPS-18 fiberoptic transducer is easily introduced through the interstitial space of tissues in a protective metal guide (18 gauge needle) (asterisk). The needle guide is withdrawn slowly while the sensor is introduced into the interstitial space in subcutaneous tissue. D. Implantation of a perforated polyurethane tube is necessary to prevent the SAMBA-420 MR fiberoptic transducer from breaking. E. Through a small (0.5 mm) skin incision the perforated tube can be inserted in its entirety to the subcutaneous tissue easily (arrow). F. The mean IFP values obtained in subcutaneous tissues were −3 mm Hg (SEM −/+−0.462, n=8), −3.25 mmHg (SEM −/+ 0.478, n=4), −3.34mm Hg (SEM−/+ 0.312, n=6), and −2.85 (SEM −/+ 0.57, n= 6) for FOBPS fiberoptic transducer, SAMBA fiberoptic transducer, Millar catheter, and wick-in–needle technique, respectively. There was no statistically significant difference between these four techniques to measure IFP (Friedman test, p=0.7997) G. The subcutaneous IFP measurements showed strong linear correlation between FOBPS and Millar catheter (R²= 0.9950). H. The subcutaneous IFP measurements showed strong linear correlation between FOBPS and wick–in-needle technique (R²= 0.9966 ) I. Pressure measurement with fiberoptic pressure transducers is based on Fabry-Perot optical interferometry principle. Two parallel, partially reflecting surfaces are spaced less than a coherence length apart, thereby forming an optical reflecting cavity. If one of the partial reflecting surfaces is a pressure-sensitive diaphragm, changes in external pressure will alter optical cavity depth. A change in optical cavity depth will cause a change in optical cavity reflectance. The transducer comprises a sensor element mounted on the tip of an optical fiber. When the pressure surrounding the sensor element is changing, the reflected light signal will change.