Thiopental respiratory effects

Hildebrand S V, Arpin D, Cardinet G III 1990 Contracture test and histologic and histochemical analyses of muscle biopsy specimens from horses with exertional rhabdomyolysis. Journal of the American Veterinary Medical Association 196 1077-1083 Hubbell J A, Bednarski R M, Muir W W 1989 Xylazine and tiletamine-zolazepam anesthesia in horses. American Journal of Veterinary Research 50 737-742 llkiw J E, Haskins S C, Patz J D 1991 Cardiovascular and respiratory effects of thiopental administration in hypovolemic dogs. American Journal of Veterinary Research 52 576-580... 

Various barbiturates such as the short acting agent pentobarbital and the ultra-short acting agents thiopental and methohexital are used for anesthesia induction. They produce loss of consciousness without analgesia and with little effects on the cardiovascular system. Unconsciousness is combined with respiratory depression as the barbiturates produce non-selective CNS depression. 

With large doses, thiopental causes dose-dependent decreases in arterial blood pressure, stroke volume, and cardiac output. This is due primarily to its myocardial depressant effect and increased venous capacitance there is little change in total peripheral resistance. Thiopental is also a potent respiratory depressant, lowering the sensitivity of the medullary respiratory center to carbon dioxide. 

In the horse, induction with thiopental usually results in a brief increase in heart rate and a decrease in cardiac output but no significant change in arterial blood pressure. Arterial blood pressure is typically lower after thiopental induction than in ketamine-based anesthetic techniques (Bennett et al 1998, Muir et al 2000). However, when horses are placed on inhalation anesthetics for maintenance of anesthesia, the hemodynamic effects of the induction agents is short lived and the hypotension and reduced cardiac output typical of inhalation anesthesia predominates (Bennett et al 1998, Wagner et al 1996). Respiratory depression is significant with an accompanying increase in Paco2 and decrease in pH. 

Respiratory Barbiturates are respiratory depressants. Induction doses of thiopental decrease minute ventilation and tidal volume with a smaller and inconsistent decrease in respiratory rate reflex responses to hypercarbia and hypoxia are diminished by anesthetic barbiturates at higher doses or in the presence of other respiratory depressants such as opiates, apnea can result. With the exception of uncommon anaphylactoid reactions, these drugs have little effect on bronchomotor tone and can be used safely in asthmatics. 

Opioid analgesics would be expected to potentiate the respiratory depressant effects of barbiturate anaesthetics. A study has found that the dose of thiopental required to induce anaesthesia was reduced by pretreatment with fentanyl. The manufacturer recommends reduced doses of thiopental in patients premedicated with opioids. ... 

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