Lymph Flow Pattern in the Intact Thoracic Duct in Sheep

1 To study the lymph flow dynamics in the intact thoracic duct, we applied an ultrasound transit‐time flow probe in seven anaesthetized and four unanaesthetized adult sheep (.60 kg). In unanaesthetized non‐fasting animals we found that lymph flow in the thoracic duct was always regular pulsatile (pulsation frequency, 5.2 ± 0.8 min−1) with no relation to heart or respiratory activity. At baseline the peak level of the thoracic duct pulse flow was 11.6–20.7 ml min−1 with a nadir of 0–3.6 ml min−1. Mean lymph flow was 5.4 ± 3.1 ml min−1. The flow pattern of lymph in the thoracic duct was essentially the same in the anaesthetized animals. 2 In both the anaesthetized and unanaesthetized animals, the lymph flow response to a stepwise increase in the outflow venous pressure showed interindividual variation. Some were sensitive to any increase in outflow venous pressure, but others were resistant in that lymph flow did not decrease until outflow venous pressure was increased to higher levels. This resistance was also observed in the high lymph flow condition produced by fluid infusion in the anaesthetized animal and mechanical constriction of the caudal vena cava in the unanaesthetized animals. Pulsation frequency of the thoracic duct flow initially increased and then decreased with a stepwise increase in the outflow venous pressure. This initial increase might be a compensatory response to maintain lymph flow against elevated outflow venous pressure. 3 To test the effect of long‐term outflow venous pressure elevation in unanaesthetized sheep, outflow venous pressure was increased by inflation of a cuff around the cranial vena cava for 1, 5 or 25 h. The cuff was inflated to a level where lymph flow was reduced. Lymph flow remained low or decreased further during the entire cuff‐inflation period. We calculated the lymph debt caused by the outflow venous pressure elevation and the amount ‘repaid’ when venous pressure returned to normal. Lymph debt for 25 h was 6400 ml but only 200 ml was repaid. Since we observed no visible oedema formation in the lower body of the sheep, the non‐colloidal components of the lymph must have been reabsorbed into the bloodstream, most likely in the lymph nodes.