Measuring interstitial fluid pressure with fiberoptic pressure transducers

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.