The Effect of a Right-to-Left Intracardiac Shunt on the Rate of Rise of Arterial and End-Tidal Halothane in Children

Ectothermy and cardiac shunts profoundly slow the equilibration of inhaled anaesthetics in a multi-compartment model

The use of inhalational anaesthesia is ubiquitous in terrestrial vertebrates. Given the dependence of these agents on delivery by the cardiorespiratory system, we developed a new computational model predicting equilibration of inhaled anaesthetics in mammalian and ectotherm conditions including the ability of reptiles to maintain vascular shunts. A multi-compartment model was constructed from simultaneously-solved equations, verified by comparison to the literature for endo and ectotherm physiology. The time to 90% equilibration of anaesthetic in arterial blood (t₉₀) is predicted and used to compare anaesthetics and physiologies. The five to tenfold lower cardiac output and minute ventilation of ectothermic vertebrates is predicted to slow equilibration times by five to ten times leading to 90% equilibration in ectotherm arterial blood of over 200 min, compounded by reduction in body temperature, and the extent of right-to-left vascular shunts. The impact of these findings is also influenced by the solubility coefficient of the anaesthetic, such that at net right-to-left shunt fractions of over 0.8, sevoflurane loses the advantage of faster equilibration, in comparison with isoflurane. We explore clinical strategies to regulate anaesthetic uptake in ectotherms by managing convectional flow especially by supportive ventilation and reduction of the right-to-left shunt.

Elimination of Intracardiac Shunting Provides Stable Gas Anesthesia in Tortoises

Inhalant anesthesia is challenging in chelonians due to a great capacity for breath-holding and an incomplete separation of the cardiac ventricle. Deoxygenated blood can recirculate back into systemic circulation by bypassing the lung in a process referred to as intracardiac right to left (R-L) shunting. Via electrocardiogram gated magnetic resonance imaging, a novel modality to investigate arterial flows in reptiles, intracardiac shunting and its elimination via atropine during gas anesthesia in tortoises (Chelonoidis carbonaria) was demonstrated. The great vessels of the heart were visualized confirming that after shunt-elimination, the flow (mean ± sd) in the pulmonary arteries increased significantly (54.6 ± 9.5 mL min⁻¹ kg⁻¹ vs 10.8 ± 3.4 mL min⁻¹ kg⁻¹; P < 0.008). Consequently, animals required significantly lower concentrations of inhaled anesthetics to maintain a stable anesthesia. To that end, the minimum anesthetic concentration (MAC) of isoflurane needed to maintain surgical anesthesia was measured. A significantly lower MAC was found after administration of atropine (mean MAC ± sd 2.2 ± 0.3% vs 3.2 ± 0.4%; P < 0.002). Previously, MAC has been indeterminable in chelonians likely due to intracardiac shunting, so this report constitutes the first MAC study performed in a tortoise.

A case of severe perioperative hypoxia in uncorrected tetralogy of fallot: Anesthetic management

Tetralogy of fallot (TOF) is one of the most common congenital heart disease (CHD) in children. With the development of pediatric surgery and intensive care units, increasing number of grown-up CHD patients are presenting for non-cardiac surgeries. Non-operated TOF patients suffer from chronic hypoxia and decreased pulmonary blood flow resulting in considerable alteration in the physiology. The optimal management of these patients, therefore, require a thorough understanding of the pathophysiology of the uncorrected TOF. We hereby report a case of successful management of a 10-year-old child with an uncorrected TOF posted for tibial external fixation device.

Sevoflurane induction in cyanotic and acyanotic children with congenital heart disease

INTRODUCTION

The aim of the study was to investigate the induction characteristics of sevoflurane in cyanotic and acyanotic children with congenital heart disease compared with healthy controls in a prospective, controlled, non-blinded study at a university hospital.

METHODS

After placement of standard noninvasive monitors, inhaled induction of anaesthesia was achieved via a facemask by using 7% sevoflurane in 100% oxygen, in all 3 groups (control group, n = 16; acyanotic group, n = 32; cyanotic group, n = 30). Three minutes after induction, tracheal intubation was performed without a muscle relaxant. In each case, the time from mask application to loss of eyelash reflex and to centralisation of the pupils, heart rate, blood pressure and oxygen saturation was recorded. After intubation, intubating conditions were graded and incidences of airway obstruction recorded. The demographic data was similar between the 3 groups except for the sex ratio in Group 1 compared with the other groups (P = 0.005).

RESULTS

The durations to loss of eyelash reflex were found to be significant between the patients with acyanotic and cyanotic heart disease (P = 0.044). Centralisation of the pupils, heart rate and blood pressure results, as well as the incidences of airway obstruction and intubation conditions, were similar between the groups (P > 0.05). In addition, oxygen saturation was significantly lower in the cyanotic group at all stages (P < 0.01).

CONCLUSION

Induction characteristics of sevoflurane in cyanotic and acyanotic children with congenital heart disease and in healthy children are similar. Sevoflurane induction is an effective and well-tolerated technique for cyanotic and acyanotic children with congenital heart disease.