The Effect of Changing the Mode of Ventilation on the Arterial‐to‐End‐Tidal CO 2 Difference and Physiological Dead Space in Laterally and Dorsally Recumbent Horses During Halothane Anesthesia

• Published in: Veterinary surgery Vol. 29; no. 2; pp. 200 - 205
• Main Authors: Teixeira Neto, Francisco J., Luna, Stelio P.L., Massone, Flavio, Thomassian, Armen, Vargas, José L.R., Junior, José R.S., D'Utra, Beatriz B., Crocci, Adalberto José
• Format: Journal Article
• Language: English
• Published: 01.03.2000
• ISSN: 0161-3499; 1532-950X
• DOI: 10.1111/j.1532-950X.2000.00200.x

Objective— To evaluate the effect of changing the mode of ventilation from spontaneous to controlled on the arterial‐to‐end‐tidal CO 2 difference [P(a‐et)CO 2 ] and physiological dead space (V D(phys) /V T ) in laterally and dorsally recumbent halothane‐anesthetized horses. Study Design— Prospective, experimental, nonrandomized trial. Animals Seven mixed breed adult horses (1 male and 6 female) weighing 320 ±11 kg. Methods— Horses were anesthetized in 2 positions—right lateral and dorsal recumbency—with a minimum interval of 1 month. Anesthesia was maintained with halothane in oxygen for 180 minutes. Spontaneous ventilation (SV) was used for 90 minutes followed by 90 minutes of controlled ventilation (CV). The same ventilator settings were used for both laterally and dorsally recumbent horses. Arterial blood gas analysis was performed every 30 minutes during anesthesia. End‐tidal CO 2 (PetCO 2 ) was measured continuously. P(a‐et)CO 2 and V D(phys) /V T were calculated. Statistical analysis included analysis of variance for repeated measures over time, followed by Student‐Newman‐Keuls test. Comparison between groups was performed using a paired t test; P < .05 was considered significant. Results— P(a‐et)CO 2 and V D(phys) /V T increased during SV, whereas CV reduced these variables. The variables did not change significantly throughout mechanical ventilation in either group. Dorsally recumbent horses showed greater P(a‐et)CO 2 and V D(phys) /V T values throughout. PaCO 2 was greater during CV in dorsally positioned horses. Conclusions and Clinical Relevance Changing the mode of ventilation from spontaneous to controlled was effective in reducing P(a‐et)CO 2 and physiological dead space in both laterally and dorsally recumbent halothane‐anesthetized horses. Dorsal recumbency resulted in greater impairment of effective ventilation. Capnometry has a limited value for accurate estimation of PaCO 2 in anesthetized horses, although it may be used to evaluate pulmonary function when paired with arterial blood gas analysis.