Blood Flow In The Inferior Vena Cava At The Onset Of Cycling Exercise In Humans

Abstract

At the onset of exercise, the stroke volume (SV) is typically augmented through the Frank-Starling mechanism by increased venous return (V-Return) from blood storage regions such as the lower limbs and viscera. However, the influence of the respiratory pump in effecting this increased V-Return is not well understood. PURPOSE: We, therefore, aimed to measure blood flow (BF) in the inferior vena cava (IVC) at the onset of cycle ergometry, during modulation of respiratory and muscle pumps. METHODS: Seven healthy females (19-32 yrs) sat at rest in a semi-supine position before the onset of 10 seconds of low-intensity cycling exercise (40 W). BF in the IVC (2-3 cm below the renal vein bifurcation) and the portal vein (PV; as an index of BF of the visceral organs) was measured by echo-Dopplar ultrasonography for 30 s, across the rest (20 s) - exercise (10 s) transition. HR and blood pressure were measured continuously by Finapres. As a control, subjects breathed spontaneously throughout (C condition). To reduce the influence of respiratory pump, the subjects did a 20 s breath-hold (at end-expiration) from 10 s before exercise onset (non-RP condition). In addition, muscle pumped BF was reduced by inflating a cuff (80 mmHg) on the upper thighs from 10 s before exercise onset (non-MP condition). RESULTS: The time course of BF in the IVC was not different (P>0.05) between C and non-RP conditions: an abrupt increase at exercise onset (C; from rest 639 to 2035 at first 2 s after exercise onset, non-RP; 565 to 1848 ml/min, P<0.05) followed by a rapid reduction to a constant level (approx. 1500-1600 ml/min during 5-10 s). BF in the PV was also similar between C and non-RP, decreasing rapidly after exercise onset. The exercise-induced increase BF in IVC was completely ablated in the non-MP condition (averaged 440 ml/min throughout exercise). CONCLUSION: The offsetting of positive and negative effects on BF during inspiration and exhalation appeared to provide no net gain to V-Return, suggesting that the respiratory pump has little influence on the dynamics of cardiac output at exercise onset. In contrast V-Return was determined predominantly by muscle pump - even during low-intensity cycling exercise. (Supported by JSPS #20700526 to MYE, #19650158 to YF and #18207019 to SK)