Jugular venous pooling during lowering of the head affects blood pressure of the anesthetized giraffe

1 Institute of Physiology and Biophysics, 2 Institute of Clinical Medicine, Departments of CardioThoracic and Vascular Surgery, and 3 Zoophysiology, Department of Biological Sciences, Aarhus University, Aarhus, Denmark; 4 Department of Radiology and 5 Institute of Pathology, Aarhus University Hospital, Aarhus, Denmark; 6 Department of Anesthesiology, Rigshospitalet, 7 Department of Large Animal Science, University of Copenhagen, and 8 Center for Zoo and Wild Animal Health, Copenhagen Zoo, Copenhagen, Denmark; 9 Vendsyssel Hospital, Hjørring, Denmark; 10 Engineering College of Aarhus, Aarhus, Denmark; 11 Department of Companion Animal Clinical Studies, University of Pretoria, Pretoria, South Africa; and 12 EndoSomatic Technologies LLC, Sacramento, California Submitted September 26, 2008 ; accepted in final form July 14, 2009 How blood flow and pressure to the giraffe's brain are regulated when drinking remains debated. We measured simultaneous blood flow, pressure, and cross-sectional area in the carotid artery and jugular vein of five anesthetized and spontaneously breathing giraffes. The giraffes were suspended in the upright position so that we could lower the head. In the upright position, mean arterial pressure (MAP) was 193 ± 11 mmHg (mean ± SE), carotid flow was 0.7 ± 0.2 l/min, and carotid cross-sectional area was 0.85 ± 0.04 cm 2 . Central venous pressure (CVP) was 4 ± 2 mmHg, jugular flow was 0.7 ± 0.2 l/min, and jugular cross-sectional area was 0.14 ± 0.04 cm 2 ( n = 4). Carotid arterial and jugular venous pressures at head level were 118 ± 9 and –7 ± 4 mmHg, respectively. When the head was lowered, MAP decreased to 131 ± 13 mmHg, while carotid cross-sectional area and flow remained unchanged. Cardiac output was reduced by 30%, CVP decreased to –1 ± 2 mmHg ( P < 0.01), and jugular flow ceased as the jugular cross-sectional area increased to 3.2 ± 0.6 cm 2 ( P < 0.01), corresponding to accumulation of 1.2 l of blood in the veins. When the head was raised, the jugular veins collapsed and blood was returned to the central circulation, and CVP and cardiac output were restored. The results demonstrate that in the upright-positioned, anesthetized giraffe cerebral blood flow is governed by arterial pressure without support of a siphon mechanism and that when the head is lowered, blood accumulates in the vein, affecting MAP. Giraffa camelopardalis , venous pressure; heart rate; flow; Starling mechanism; siphon; waterfall Address for reprint requests and