Ventilation and Cardiovascular Studies during Mechanical Control of Ventilation in Horses

Abstract

SUMMARY Eleven out of 12 horses were underventilating while breathing spontaneously during halothane anaesthesia with high arterial carbon dioxide tensions. In addition, large alveolar to arterial oxygen tension gradients were found to be present. Mechanically, controlled ventilation with an intermittent positive pressure of 20–30 cm H2O reduced arterial carbon dioxide levels to normal. The alveolar to arterial oxygen gradients did not increase and in some cases decreased. These (A − a) Po2 gradients were due mainly to true shunt of the order of 30 per cent and not to ventilation perfusion inequality. RÉSUMÉ Anesthèsiés sous halothane, onze chevaux sur douze ont montré une hypoventilation accompagnée d'un taux artériel de CO2élevé. En outre une large variation des taux d'oxygène alvéolaire et artériel fut constatée. La ventilation assistée, réduisit le taux de CO2 artériel à la normale. Les variations du taux d'oxygène alvéolaire et du taux d'oxygène artériel ne furent pas accrues et furent quelquefois amoindries. On attribue ces variations plus à un shunt vasculaire qu'à une insuffisance de la ventilation. ZUSAMMENFASSUNG Elf von zwölf Pferde zeigten während der Spontanat‐mung unter Halothannarkose eine Hypoventilation mit hohen, arteriellen CO2‐Spannungen. Zusätzlich wurden ausgeprägte alveolär‐arterielle O2‐Spannungsgradienten festgestellt. Die mechanisch kontrollierte Beatmung mit inter‐mittierendem, positivem Druck von 20–30 cm H2O verringerte die CO2‐Werte auf die Norm. Die alveolär‐arteriellen O2‐Gradienten blieben unverändert oder nahmen in einigen Fällen ab. Diese (A‐a) PO2‐Gradienten waren vor allem die Folge eines echten Shunts von 30% und nicht eines Beatmung – Durchblutung – Ungleichgewichts.

Intravenous fluid therapy in the horse

“…This is in line with reports (Weaver and Walley 1975;Whitehair and Willits 1999) which suggest that the use of intermittent positive pressure ventilation does not always improves PaO 2 but may even decrease it. It is thought that over-inflating the compliant upper alveoli (with high V/Q) which are less subject to abdominal counter pressure not only increases vascular resistance in this zone but also leads to additional compression of dependent alveoli.…”

Effects of an alveolar recruitment maneuver on cardiovascular and respiratory parameters during total intravenous anesthesia in ponies

“…In horses, as in other species, IPPV strongly interferes with cardiovascular function. 36,40,41 There are marked additional depressing effects on cardiovascular parameters when IPPV is combined with PEEP, 37,38,42 decreasing CO by as much as 50%. 39 The purpose of the study reported here was to evaluate pulmonary and cardiovascular effects of an RM combined with PEEP during TIVA in ponies.…”

Effects of inspired oxygen concentration on ventilation, ventilatory rhythm, and gas exchange in isoflurane-anesthetized horses

“…Pulmonary shunting in anesthetized horses reportedly increases by 21% to 51% (mean, 34%), compared with 4% to 7% in standing horses prior to anesthesia. 11,a Furthermore, administration of a high concentration of inspired oxygen (> 95%) during anesthesia in horses has been associated with an increase in intrapulmonary shunting and a decrease in gas exchange. 6 Airway closure and atelectasis formation in humans can be responsible for as much as 74% of gas exchange impairment.…”