A classic revisited: Speed of Inhalational Induction of Anesthesia: Does Intracardiac Shunting Play A Role?
Viviane Nasr MD, Jim DiNardo MD, Susan Nicolson MD, Lindsey Loveland Baptist, MD
Original article
Tanner GE, Angers DG, Barash PG, Mulla A, Miller PL, Rothstein P. Effect of left-to-right, mixed left-to-right, and right-to-left shunts on inhalational anesthetic induction in children: a computer model. Anesth Analg. 1985 Feb;64(2):101-7. PMID: 3970355.
A question frequently asked during anesthesia training and in Board examinations: Does a patient with right to left (R-L) intracardiac shunting have a slower or faster induction with an inhalational anesthetic? The speed of inhalational anesthetic induction is determined by how rapidly adequate partial pressures of anesthetic reaches the brain. The three determining factors are: 1) rate of anesthetic delivery to the lungs, 2) rate of transfer from lungs to the arterial blood, and 3) rate of transfer from the blood to the brain.
This article is considered a classic in pediatric and pediatric cardiac anesthesiology literature. It describes the impact of left-to-right (L-R) shunt, R-L shunt and mixed shunts on the rate of inhalation induction. The authors use a seven-compartmental physiologic model: Vessel rich group, vessel poor group, right heart, left heart, lung gas, lung tissue, and lung blood. Three anesthetics with low, medium and high solubility coefficients were chosen: nitrous oxide, halothane and diethyl ether. The authors defined 7 rate equations and 18 constants including a body weight of 25 kg and modelled the intracardiac shunts using the Dynamo programming environment. Rather than typing code, in Dynamo, programs are created by manipulating graphic elements called “nodes”. It is a visually oriented programming approach best suited for architects, designers, and engineers. In Dynamo, each node performs a specific task. Nodes have inputs and outputs. The outputs from one node are connected to inputs on another using “wires”. The program or “graph” flows from node to node through the network of wires. Dynamo is a program that solves equations, created in 1958. (https://ia801906.us.archive.org/25/items/dynamousersmanua00pugh/dynamousersmanua00pugh.pdf)
Today's equivalent would be a program like Desmos or Wolfram Alpha which can exactly solve these equations rather than numeric approximations.
To understand the cardiac compartment and its impact on effective pulmonary blood flow which will determine the amount of anesthetic reaching the brain, we share these figures about normal systemic blood flow (Qs) and pulmonary blood flow (Qp) and pulmonary blood flow in the setting of shunting.
In this model, patients with L-R shunt did not have a significant change in the speed of inhalation induction. Hence, patients with an atrial septal defect with L-R shunting will have inhalation induction times comparable to patients without shunting. In patients with R-L shunting (tetralogy of Fallot), induction was slower for all anesthetics with the slowest induction time for anesthetics with lower solubility (eg.nitrous oxide). Less anesthetic is taken up by the lungs and thus less anesthetic reaches the brain. Patients with mixed shunts such as unrestrictive ventricular septal defect or tetralogy of Fallot with a BTT shunt essentially have a R-L shunt in conjunction with L-R shunt; the presence of the L-R shunt was seen to attenuate the slowing of induction seen with a pure R-L shunt.
Recognizing the slower induction with R-L shunts, it is advisable in patients with cardiac disease to be patient when doing an induction with sevoflurane and not be tempted to increase the concentration to 6-8% potentially causing depression of myocardial function.
References:
Rivenes SM, Lewin MB, Stayer SA, Bent ST, Schoenig HM, McKenzie ED, Fraser CD, Andropoulos DB. Cardiovascular effects of sevoflurane, isoflurane, halothane, and fentanyl-midazolam in children with congenital heart disease: an echocardiographic study of myocardial contractility and hemodynamics. Anesthesiology. 2001 Feb;94(2):223-9PMID: 11176085.
Laird TH, Stayer SA, Rivenes SM, Lewin MB, McKenzie ED, Fraser CD, Andropoulos DB. Pulmonary-to-systemic blood flow ratio effects of sevoflurane, isoflurane, halothane, and fentanyl/midazolam with 100% oxygen in children with congenital heart disease. Anesth Analg. 2002 Nov;95(5):1200-6, table of contents. PMID: 12401594.