Normal Oxygen Recovery Kinetics in Pelvic Floor Muscle Using Near Infrared Spectroscopy

Objectives: Near infrared spectroscopy (NIRS) uses light to monitor changes non-invasively in the concentration of oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) in skeletal muscle in real time. NIRS provides in vivo assessment of the workload capacity of muscle (oxygen extraction) and fatigue resistance (oxygen recovery) to serve as an objective assessment of muscle fitness. As objective parameters for muscle fitness are lacking for the assessment of patients with pelvic floor muscle (PFM) dysfunction, this study sought to establish normal values of PFM fitness in healthy subjects using NIRS.

Methods: Females without lower urinary tract symptoms were recruited, and confirmed to be asymptomatic using the UDI-6 questionnaire. Subjects underwent PFM assessment with a discriminate pelvic floor exam and transvaginal NIRS monitoring during serial work/rest cycles. A vaginal probe incorporating a dual-channel NIRS emitter/detector array was linked wirelessly to a four-wavelength, continuous wave NIRS instrument. Concentration changes in O2Hb and HHb were recorded, and the difference between the two (HbDiff) was calculated for the right and left pelvic floor musculature over the course of five maximum voluntary contractions (MVCs) and one sustained maximum voluntary contraction (SMVC) of the PFM. Half-recovery time of hemoglobin difference, HbDiff(½RT), is a validated, NIRS-derived, oxygen kinetic parameter indicating muscle metabolic function; this parameter was calculated for each subject following sustained maximal voluntary contraction (SMVC).

Results: 24 asymptomatic women with median age of 33 years (IQR 23-46), with an average BMI of 25 (IQR 23-27), gravity of 1 (IQR 0-2), and parity of 0 (IQR 0-1.25) were enrolled. NIRS data sets were obtained for all subjects. UDI-6 median score was 0 (IQR 0-8.3). Oxford Pelvic floor strength was 5. 62% of participants had dominance of their left PFM, while 25% had equal responses between right and left PFM. The typical pattern during MVC showed decreased O2Hb and HHb, followed by recovery to baseline with relaxation allowing for the calculation of HbDiff(½RT) time (Figure 1). HbDiff(½RT) did not vary with age and was 1.82 seconds. The mean maximum depth of response was 10.1 micromoles. Heatmaps showing real time imaging in video format were derived to demonstrate detail in regional differences in O2BHb and HHb during PFM short contractions and during SMVC (Figure 2).

Conclusions: Oxygen kinetic parameters did not vary with age in healthy volunteers, although a natural dominance of one side of the PFM was frequently seen. With normative values now established, NIRS systems are suitable for further objective assessment of the strength and endurance of pelvic floor muscles in women with dysfunction, allowing for better understanding of the pathophysiology of PFM conditions and objective monitoring in response to treatment.