Objective: To evaluate the use of midazolam, ketamine, and xylazine for total IV anesthesia (TIVA) in horses. Animals: 6 healthy Thoroughbred mares. Procedures: Horses were sedated with xylazine (1.0 mg/kg, IV). Anesthesia was induced with midazolam (0.1 mg/kg, IV) followed by ketamine (2.2 mg/kg, IV) and was maintained with an IV infusion of midazolam (0.002 mg/kg/min), ketamine (0.03 mg/kg/min), and xylazine (0.016 mg/kg/min). Horses underwent surgical manipulation and injection of the palmar digital nerves; duration of the infusion was 60 minutes. Additional ketamine (0.2 to 0.4 mg/kg, IV) was administered if a horse moved its head or limbs during procedures. Cardiopulmonary and arterial blood variables were measured prior to anesthesia; at 10, 20, 30, 45, and 60 minutes during infusion; and 10 minutes after horses stood during recovery. Recovery quality was assessed by use of a numeric (1 to 10) scale with 1 as an optimal score. Results: Anesthesia was produced for 70 minutes after induction; supplemental ketamine administration was required in 4 horses. Heart rate, respiratory rate, arterial blood pressures, and cardiac output remained similar to preanesthetic values throughout TIVA. Arterial partial pressure of oxygen and oxygen saturation of arterial hemoglobin were significantly decreased from preanesthetic values throughout anesthesia; oxygen delivery was significantly decreased at 10- to 30-minute time points. Each horse stood on its first attempt, and median recovery score was 2. Conclusions and Clinical Relevance: Midazolam, ketamine, and xylazine in combination produced TIVA in horses. Further studies to investigate various dosages for midazolam and ketamine or the substitution of other α2-adrenoceptor for xylazine are warranted.

The objective of this study was to determine the effects of the administration of a high volume of isotonic crystalloid at a rapid rate on cardiovascular function in normovolemic, isoflurane-anesthetized dogs during induced hypotension. Using a prospective study, 6 adult dogs were induced to general anesthesia and cardiovascular and hematological values were measured while the dogs were maintained at 3 hemodynamic states: first during light anesthesia with 1.3% end-tidal isoflurane (ETI); then during a hypotensive state induced by deep anesthesia with 3% ETI for 45 min while administered 1 mL/kg body weight (BW) per minute of isotonic fluids; and then decreased to 1.6% ETI while receiving 1 mL/kg BW per minute of fluids for 15 min. End-tidal isoflurane (ETI) at 3.0 ± 0.2% decreased arterial blood pressure (ABP), cardiac index (CI), and stroke volume index (SVI), and increased stroke volume variation (SVV) and central venous pressure (CVP). Fluid administration during 3% ETI decreased only SVV and systemic vascular resistance index (SVRI), while CVP increased progressively. Decreasing ETI to 1.6 ± 0.1% returned ABP and SVI to baseline (ETI 1.3 ± 0.1%), while CI and heart rate increased and SVV decreased. There was significant progressive clinical hemodilution of hemoglobin (Hb), packed cell volume (PCV), total protein (TP), colloid osmotic pressure (COP), arterial oxygen content (CaO2), and central-venous oxygen content (CcvO2). High-volume, rapid-rate administration of an isotonic crystalloid was ineffective in counteracting isoflurane-induced hypotension in normovolemic dogs at a deep plane of anesthesia. Cardiovascular function improved only when anesthetic depth was reduced. Excessive hemodilution and its adverse consequences should be considered when a high volume of crystalloid is administered at a rapid rate.

Objective-To measure the effects of tidal volume, ventilatory frequency, and oxygen insufflation flow on the fraction of inspired oxygen in cadaveric horse heads attached to a lung model. Sample-8 heads of equine cadavers. Procedures-Each cadaveric horse head was intubated with a nasotracheal tube that extended into the proximal portion of the trachea. Oxygen was delivered through an oxygen catheter contained within and extending to the tip of the nasotracheal tube. The trachea was connected to the lung model by use of a spiral-wound hose with a sampling adaptor. Eight treatment combinations involving 2 tidal volumes (5 and 8 L), 2 ventilatory frequencies (6 and 12 mechanical breathes/min), and 2 insufflation rates (10 and 15 L/min) were applied to each head. Hand-drawn inspired gas samples were collected and analyzed for oxygen concentrations. Results-The fraction of inspired oxygen (measured at mid trachea) ranged from 26.8% to 39.4%. Fraction of inspired oxygen was significantly higher with a smaller tidal volume, lower ventilatory frequency, and higher insufflation rate. Conclusions and Clinical Relevance-In the study model, measured fraction of inspired oxygen varied with ventilatory pattern as well as oxygen insufflation rate. Clinically, this information could be beneficial for interpretation of data regarding arterial blood gases and hemoglobin saturation and in making appropriate oxygen insufflation decisions for anesthetized horses that are breathing room air.

Objective: To evaluate the use of the oxygen content–based index, Fshunt, as an indicator of venous admixture (Qs/Qt) at various fractions of inspired oxygen (Fio2s) in anesthetized sheep undergoing Flung or 2-lung ventilation. Animals: 6 healthy adult female sheep. Procedures: Sheep were anesthetized and administered 5 different Fio2s (0.21, 0.40, 0.60, 0.80, and 1.00) in random order during 2-lung mechanical ventilation. Arterial and mixed venous blood samples were obtained at each Fio2 after a 15-minute stabilization period. Vital capacity alveolar recruitment maneuvers were performed after blood collection. The previously used Fio2 sequence was reversed for sample collection during Flung ventilation. Blood samples were analyzed for arterial, pulmonary end-capillary, and mixed venous oxygen content and partial pressure and for hemoglobin concentration. Oxygen hemoglobin saturation, Qs/Qt, Fshunt, and oxygen tension–based indices (OTIs; including Pao2:Fio2, alveolar-arterial difference in partial pressure of oxygen [Pao2 – Pao2], [Pao2 – Pao2]:Fio2, [Pao2 – Pao2]:Pao2, and Pao2:Pao2) were calculated at each Fio2; associations were evaluated with linear regression analysis, concordance, and correlation tests. Intermethod agreement between Qs/Qt and Fshunt was tested via Bland-Altman analysis. Results: Strong and significant associations and substantial agreement were detected between Fshunt and Qs/Qt. Relationships between OTIs and Qs/Qt varied, but overall correlations were weak. Conclusions and Clinical Relevance: Whereas OTIs were generally poor indicators of Qs/Qt, Fshunt was a good indicator of Qs/Qt at various Fio2s, regardless of the magnitude of Qs/Qt and could be potentially used as a surrogate for Qs/Qt measurements in healthy sheep.