A high level of cardiorespiratory fitness is associated with a stiffer spleen in a population-based cohort study

Higher cardiorespiratory fitness (CRF) and muscle strength are independently associated with reduced risk of cardiovascular disease and morbidity. Higher CRF and muscle strength are also associated with an improved immune response. Since the spleen plays an integral role in the innate and adaptive immune system, we hypothesized that CRF and muscle strength would be associated with biophysical characteristics of the spleen. We related CRF and muscle strength with spleen stiffness as measured by shear wave elastography in a large population-based cohort from the Study of Health in Pomerania (SHIP).

We used cross-sectional data from 677 participants of the population-based, prospective Health Study in Pomerania (SHIP-START-4). A symptom-limited cardiopulmonary exercise test was performed according to a modified Jones protocol. Hand grip strength was determined using a JAMAR dynamometer. Time-harmonic elastography was used to measure splenic shear wave velocity as a surrogate for splenic stiffness. The shear waves in the spleen were generated by a loudspeaker integrated into an examination table, which emitted multifrequency vibrations in the range of 27 to 56 Hz. Tissue strain was encoded from radiofrequency data acquired by an ultrasound system. Shear wave velocity maps covering the entire visible spleen in B-mode were calculated using the k-MDEV method. Finally, mean values were calculated within a manually drawn region of interest. We used linear regression models adjusted for age, sex, and body weight to relate CRF and muscle strength to spleen shear wave velocity.

The median age of the study population was 61 (25th quartile: 52 and 75th quartile: 70) years and 54% were women. A one liter/min higher VO2peak was associated with a 0.05 (95% confidence interval [CI] 0.01 - 0.07; p < 0.01) m/s higher splenic shear wave velocity. A 100 watt higher peak workload was also associated with a 0.05 (95% CI 0.01 - 0.08, p < 0.01) m/s higher splenic shear wave velocity. When CRF was normalized to body weight, a 1 ml/kg/min higher CRF was associated with a 0.007 (95% CI 0.005 - 0.009, p < 0.01) m/s higher splenic shear wave velocity. We found no associations between hand grip strength and splenic shear wave velocity.

In our study, we found that higher VO2peak, VO2peak adjusted for body weight and maximal workload were associated with higher spleen stiffness. As a non-invasive and inexpensive test, time-harmonic shear wave elastography has potential for patient care. Interestingly, there seems to be no correlation between muscle strength and splenic shear wave velocity. It remains unclear why CRF, but not muscle strength, is related to splenic shear wave velocity.