Cardiovascular depression is commonly encountered in anesthetized neonatal foals and in foals with sepsis, perinatal asphyxia, or both.1 Cardiovascular monitoring in neonatal foals usually includes any or all of the following parameters: heart rate, ECG, pulse quality, arterial blood pressure (obtained by indirect or direct measurement), use of pulse oximetry and echocardiography, blood lactate concentration, and acid-base status. In hypotensive states, emphasis is placed on arterial blood pressure measurements; cardiovascular support with fluid therapy and vasoactive drugs is recommended when MAP readings of < 60 to 65 mm Hg are detected. In human patients, a minimum MAP of approximately 60 to 65 mm Hg is vital for adequate cerebral, renal, and coronary blood flow.2 Conversely, results of studies2,3 in human patients with sepsis indicate that increasing MAP from 65 to 85 mm Hg does not provide additional improvements in VO2, blood lactate concentrations, and renal function, compared with an MAP of 65 mm Hg.
The CO or CI is rarely measured under clinical situations, despite their ability to provide an assessment of overall cardiovascular function. Dobutamine and norepinephrine are used in critically ill neonatal foals to treat hypotension; however, to our knowledge, their effects on CI and derived variables have not been objectively evaluated and compared in a controlled study. A poor correlation between CI and MAP was recently reported for anesthetized neonatal foals.4 Similarly, in anesthetized dogs, CI is correlated with blood pressure measurements in various ways; high DAP has a negative effect on CI, whereas a high MAP has a positive effect on CI, and no correlation is found between SAP and CI.5 These findings underscore the importance of assessing the effect of vasopressors or inotropic drugs on the basis of multiple indicators of perfusion and cardiovascular function instead of simply relying on blood pressure data.
Arginine vasopressin, also called antidiuretic hormone, has gained popularity in human medicine as a drug for treatment of vasodilatory shock, including cardiac arrest.6–8 In some studies of critically ill human patients9 and of pigs with experimentally induced sepsis,10,11 administration of vasopressin has resulted in gastrointestinal hypoperfusion. Reports12,13 of the use of vasopressin in critically ill foals exist, but to our knowledge, controlled studies assessing its effects on the cardiovascular function and gastrointestinal perfusion in this population are lacking.
The use of indicators to assess tissue perfusion includes calculation of VO2 and DO2 and blood lactate concentrations. Their usefulness has been challenged over the years. More recently, evaluation of CO2 concentrations from gastric samples (gastric tonometry) has been used to demonstrate the high susceptibility of the splanchnic circulation to decreased tissue perfusion and oxygenation. Increased CO2 production from tissue metabolism under conditions of reduced perfusion is a good indicator of organ function, particularly in the gastrointestinal tract.14,15 Gastric tonometry is the only clinically available and FDA-approved method for detecting gastrointestinal hypoperfusion. The tonometer is a modified nasogastric tube with a balloon permeable to CO2 on the distal end. The partial pressure of CO2 measured from the balloon is a reflection of the PgCO2. The ΔCO2 reflects the mucosal blood supply–to–metabolic demand balance and is considered a marker of gastric mucosal perfusion. An increase in ΔCO2 can be attributed to decreased availability or diminished use of O2. The technique is now widely used to assess splanchnic perfusion in human intensive care units and during surgery in critically ill patients.15,16
The purpose of the study reported here was to determine and compare the effects of dobutamine, norepinephrine, and vasopressin on cardiovascular function and gastric mucosal perfusion in anesthetized foals during hypotension induced by isoflurane. By manipulating SVR, contractility, or both with the former drugs (vasoactive drugs and inhalational anesthetic), the most effective treatments for hypotension without negative effects on CO and splanchnic perfusion were determined.
Mean arterial pressure
Diastolic arterial pressure
Systolic arterial pressure
Gastric mucosal partial pressure of CO2
Systemic vascular resistance
Central venous pressure
Lithium dilution cardiac output
Low infusion rate
High infusion rate
Central venous hemoglobin saturation
Stroke volume index
Central venous O2 content
O2 extraction ratio
Mixed venous O2 content
DVM Stat, Corporation for Advanced Applications, Newburg, Wis
Mila International Inc, Florence, Ky
TONO-14 fr TRIP Tonometry catheter, Datex-Ohmeda Division, Helsinki, Finland
S/5 M-Tono, Tonometry Module, Datex-Ohmeda Division, Helsinki, Finland.
Abbott Laboratories, North Chicago, Ill
VetaKet, Phoenix Scientific Inc, St Joseph, Mo
IsoFlo, Abbott Laboratories, North Chicago, Ill
S/5, Datex-Ohmeda Division, Helsinki, Finland
DOT-34 NRC 300/375 M1014, Datex-Ohmeda Division, Helsinki, Finland
Baxter Healthcare Corp, Deerfield, Ill
LiDCO cardiac computer CM 31-01, LiDCO Ltd, London, UK
Flow through cell electrode assembly, LiDCO Ltd, London, UK
ABL System 605/600 and OSM3 Hemoximeter, Radiometer Medical A/S, Copenhagen, Denmark
LiDCO Ltd, London, UK
Bedford Laboratories, Bedford, Ohio
Levophed, Abbott Laboratories, North Chicago, Ill
American Regent Inc, Shirley, NY
Medfusion model 2010i syringe pump, Medex Inc, Duluth, Ga
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Formulas used for calculated variables.