Hypoventilation exacerbates the cardiovascular depression caused by a high volume of lumbosacral epidural bupivacaine in two isoflurane-anesthetized dogs.

This letter reports the cardiopulmonary changes resulting from lumbosacral epidural administration of bupivacaine (0.25%, 0.6 mL kg ) in isofluraneanesthetized dogs. Anesthesia was induced in two healthy, adult, mixed-breed male dogs (A and B) weighing 19.5 kg and 16.3 kg, respectively, with 5% isoflurane (Crist alia Produtos Qu ımicos Farmacêuticos Ltda, SP, Brazil) in oxygen (5 L minute ) administered by facemask. After endotracheal intubation, anesthesia was maintained at an end-tidal isoflurane concentration (FE0Iso) of 1.0 minimum alveolar concentration, previously determined for each dog (1.51% and 1.65%), and inspiratory fraction of oxygen (FIO2) of approximately 98% delivered from a circle breathing system with the dogs breathing spontaneously. With the dogs in lateral recumbency, a 20 gauge cephalic vein catheter and a 22 gauge dorsal pedal artery catheter (Nipro Medical Ltda, SP, Brazil) were placed to facilitate the infusion of lactated Ringer’s solution (3 mL kg 1 hour ) and to measure arterial blood pressure. An 8 Fr catheter introducer (Arrow International, Inc., PA, USA) was placed in the right jugular vein and a 7 Fr pulmonary artery (PA) catheter (Edwards Lifesciences LLC, CA, USA) was introduced. The placement of the tip of the catheter in the PA was confirmed according to the typical pressure waveforms displayed on the monitor (DX 2021; Philips Healthcare, Netherlands). All pressure transducers were calibrated using a mercury manometer and zeroed at the level of the manubrium. Cardiac output (CO) was measured by injection of 5 mL of cold saline solution (0–5 °C). An electrical heating pad was used to maintain body temperature, monitored by the PA thermistor, at 37.5–38.5 °C. The cardiac index (CI), systemic vascular resistance index (SVRI) and stroke volume index (SI) were calculated by normalizing CO and systemic vascular resistance to body surface area and stroke volume to body weight. Heart rate (HR), respiratory frequency (fR), end-tidal carbon dioxide tension (PE0CO2), FE0Iso and FIO2 were measured using a multi-channel monitor (Digicare Biomedical Technology, Inc., FL, USA). Tidal volume (VT) was measured by the ventilator’s pneumotachometer and minute ventilation (V̇E) was calculated as VT 9 fR. After instrumentation (197 and 182 minutes for dogs A and B, respectively), the dogs were placed in sternal recumbency and the lumbosacral area was aseptically prepared. An 18 gauge Tuohy needle (Becton Dickinson & Co., NJ, USA) was introduced in the lumbosacral space; placement in the epidural space was confirmed by the loss of resistance technique and the absence of blood and cerebrospinal fluid at the hub of the needle. Epidural administration of bupivacaine (Crist alia Produtos Qu ımicos Farmacêuticos Ltda) was performed over 2 minutes and cardiopulmonary variables were recorded at 0 (baseline), 5, 15, 30, 60 and 90 minutes. In comparison with baseline values, VT, fR and V̇E decreased, whereas PE0CO2 increased in both dogs at 15 minutes. HR decreased in dog A and MAP, CI and SVRI decreased in both dogs (Table 1). The recommended volume of local anesthetic solution to be injected into the lumbosacral epidural space in dogs is 0.2 mL kg 1 (Torske & Dyson 2000). Higher volumes up to 0.6 mL kg 1 have been investigated as alternatives to extend analgesia more cranially (Freire et al. 2010). In the present study, thoracic spread of epidural bupivacaine seemed to affect intercostal, abdominal and possibly diaphragmatic muscular function, resulting in hypoventilation, which, in the presence of a high sympathetic block, may lead to pronounced cardiovascular depression (Shibata