1090-0233/01/010044 + 12 $35.00/0 © 2001 Harcourt Publishers Ltd INTRODUCTION The most common indications for blood collection in equine veterinary practice are for transfusion in life-threatening haemorrhage or haemolysis, and for plasma harvesting for storage or immediate infusion, mainly to hypoproteinaemic foals or adult horses. Current recommendations for volumes of blood that may be collected from horses without adverse clinical consequences range from 20 to 25% of blood volume (16–20 mL/kg or 8–10 L of blood) (Schmotzer et al., 1985; Williamson, 1992; Collatos, 1997). These recommendations are probably based on previous reports, clinical experience, welfare guidelines and the immediate requirements of the patient and hospital for blood or plasma. In addi- tion, several studies have been performed in horses where 20 to 31% (16 –25 mL/kg) of blood volume was acutely removed without adverse effects on clinical and some haematological variables (Torten & Schalm, 1964; Lumsden et al., 1975; Smith & Agar, 1976; Easley, 1985; Radin et al., 1986). However, physiological measurements such as arte- rial blood pressure, central venous pressure, heart rate and respiratory rate, as well as detailed hae- matological and biochemical data, have not been evaluated during or after blood collection of this magnitude in the horse. The purpose of the present study was to deter- mine the acute and long-term effects on some car- diorespiratory, haematological and biochemical variables, after removal of 20 mL/kg of blood (approximately 25% of blood volume: Jain, 1986; Williamson, 1992) from normal horses. We hypoth- esized that this volume of blood removal would have no adverse effects on horses and that there would be recovery to pre-blood removal values within one month. Correspondence to: Nicholas Malikides, University Veterinary Centre, Camden, University of Sydney, PMB4 Narellan Delivery Centre, Narellan, NSW, 2567, Australia. Tel.: +6102 9351 1735; Fax: +61 02 46 55 1212; E-mail: nickm@usyd.edu.au Cardiovascular, Haematological and Biochemical Responses After Large Volume Blood Collection in Horses N. MALIKIDES, J. L. HODGSON, R. J. ROSE and D. R. HODGSON University Veterinary Centre, Camden, The University of Sydney, PMB 4 Narellan Delivery Centre, Narellan, NSW, 2567, Australia SUMMARY To determine whether removal of 20 mL/kg of blood (approximately 25% of blood volume) resulted in adverse physiological effects in donor horses, we removed this volume of blood from five horses and selected cardiovascular, haematological and biochemical variables measured during collection and for 31 days thereafter. We found that alteration in most variables occurred, although the changes in values usually remained within published reference ranges. Also, recovery of these alterations to pre-collection values was rapid, occurring within 24–48 h in most instances. We concluded that volumes of blood less than or equal to 20 mL/kg when collected appropriately from healthy donor horses result in no adverse acute or chronic physiological changes. These results suggest that horses undergo adequate physiological com- pensation when ~25% of blood volume is removed for the purposes of blood donation or production of plasma. KEYWORDS: Horse; blood; arterial; venous; pressure; haematology; plasma. The Veterinary Journal 2001, 162, 44–55 doi: 10.1054/tvjl.2001.0583, available online at http://www.idealibrary.com on © 2001 Harcourt Publishers Ltd