Vasoconstrictor responses in the upper and lower limbs to increases in transmural pressure

Abstract

The purpose of this study was to examine upper and lower limb vasoconstrictor responses to changes in transmural pressure in humans. Brachial and femoral blood mean blood velocity (MBV) and vessel diameter (Doppler ultrasound) were measured in 20 supine healthy subjects (10 men and 10 women; 27 +/- 1 yr; mean +/- SE) during four levels of limb suction at -25, -50, -75, and -100 mmHg, respectively. Limb suction led to an initial rise in MBV followed by a rapid fall in flow velocity to a level below MBV baseline, indicating a vasoconstriction effect. Femoral compared with brachial vessels exhibited a greater fall in flow velocity at all levels of suction (-89 +/- 17 vs. -10 +/- 2, -142 +/- 11 vs. -14 +/- 2, -156 +/- 22 vs. -13 +/- 2, and -162 +/- 29 vs. -12 +/- 2 ml/min for -25, -50, -75, and -100 mmHg, respectively; interaction effect, P < 0.05). Even at low tank suction levels (i.e., -10 and -20 mmHg), significant brachial flow velocity vasoconstriction from baseline values was demonstrated, reflecting downstream resistance vessel changes (n = 14). Brachial and femoral diameters did not change during changes in negative tank pressure. During suction, changes in limb volumes were significantly greater in the forearm (1.4 +/- 0.5%, 2.4 +/- 0.8%, 3.5 +/- 1.0%, and 4.3 +/- 1.1%) compared with the calf (0.9 +/- 0.5%, 1.4 +/- 0.7%, 2.0 +/- 0.8%, and 2.8 +/- 1.1%) at all levels of negative tank pressures (-25, -50, -75, and -100 mmHg, respectively). Simultaneous measurements of both upper limbs and both lower limbs suggested that the majority of the reduction in flow was due to myogenic influences except when -100 mmHg of suction was applied to the lower limb. The greater vasoconstriction responses in the leg compared with the arm with suction appear to be influenced by both myogenic and sympathetic mechanisms.

Fig. 1.

Flow and hemodynamic responses in the brachial (left panels) and femoral artery (right panels) to changes in suction (−25, −50, −75, and −100 mmHg) in 1 subject. Dashed line, change to suction. Negative pressures used: ▴, −25 mmHg; ▵, −50 mmHg; ▪, −75 mmHg; □, −100 mmHg. MBV, mean blood velocity (cm/s); MAP, mean arterial pressure (mmHg). Heart rate is in beats/min; flow is in ml/min; conductance is in ml·min⁻¹·mmHg⁻¹.

Fig. 2.

Absolute peak MBV and diameter changes (Δ), timing of peak MBV, and dynamic response to abrupt changes in suction (−25, −50, −75, and −100 mmHg) were examined. Arms compared with legs had a higher peak MBV at the higher tank pressures (−50, −75, and −100 mmHg; A). There were no changes in limb diameters at peak velocity across all limb tank pressures (B). Legs had a delayed response to all applications of tank pressure compared with arms (C). Arms had a greater dynamic response after the peak velocity (D). Baseline, baseline levels at ambient pressure where Δ = 0; • and solid line, arm; ○ and dashed line, leg; *Significant difference between arm and leg at a specific negative tank pressure. **Significant main effect difference between arm and leg.

Fig. 3.

Absolute MBV, diameter, flow, and conductance changes (Δ) under sustained increases in tank suction (−25, −50, −75, and −100 mmHg) in the brachial and femoral arteries. Legs compared with arms had greater velocity reductions in response to changes in suction (A) with no change in diameters in either limb (B). Leg flow and conductance changes were greater at every tank suction level compared with the arm (C and D). Baseline, baseline levels at ambient pressure where Δ = 0; • and solid line, arm; ○ and dashed line, leg; *Significant difference between arm and leg at a specific negative tank pressure. **Significant main effect difference between arm and leg.

Fig. 4.

Absolute flow changes (Δ) normalized by limb volumes under sustained increases in tank suction (−25, −50, −75, and −100 mmHg) in the brachial and femoral arteries. Leg flow and conductance when normalized by limb volume was lower at every tank suction level compared with the arm (A and B). Baseline, baseline levels at ambient pressure where Δ = 0; • and solid line, arm; ○ and dashed line, leg. **Significant main effect difference between arm and leg.

Fig. 5.

Sympathetic nervous system (SNS) and myogenic influences on vasoconstriction during changes in limb tank pressures (n = 17). There was little change in contralateral flow in the arm during changes in tank suction (A), whereas opposing leg limb blood exhibited vasoconstrictor responses (B). Myogenic contribution to the vasoconstriction was calculated from limb MBV within the tank minus limb MBV in opposing limb for the arm (C) and leg (D). The myogenic response compared with SNS influence appeared to have a greater contribution to vasoconstriction in the arm with changes in tank suction. Although the myogenic response still had a strong influence in leg vasoconstriction, sympathetic activation also appears to contribute to vasoconstriction in the lower limbs, especially at the highest tank pressure (−100 mmHg). Baseline, baseline levels at ambient pressure where Δ = 0; • and solid line, tank limb; ○ and dashed line, nontank limb; solid bars, myogenic influence; hatched bars, SNS influence. **Significant main effect difference between arm and leg.

Fig. 6.

In a separate group of subjects (n = 14), forearm MBV responses were examined 60 s after application of −10, −20, −30, −40, and −50 mmHg tank pressure. At 60 s under negative pressure, MBV was below baseline levels for all levels of suction (P < 0.05). A greater vasoconstriction response (ΔMBV) was observed with higher negative pressures (−30 and −40 mmHg) compared with lowest negative pressure (−10 mmHg). Baseline, baseline levels at ambient pressure where Δ = 0. *Significantly different from −10 mmHg.