Noncardiac surgery in the patient with congenital heart disease

The patient with congenital heart disease who presents for noncardiac surgery requires careful evaluation and planning to avoid adverse perioperative events. This chapter presents a physiological approach to the management of anesthesia for the most common congenital heart lesions. The various congenital heart defects are categorized into lesions resulting in: (1) left-to-right shunting; (2) right-to-left shunting; (3) complete mixing of pulmonary and systemic circulation; (4) complete separation of the pulmonary and systemic circulations; (5) increased myocardial work; and (6) mechanical obstruction of the airway.

Effect of ventilation with soluble and diffusible gases on the size of air emboli

Pulmonary hypertension resulting from venous air embolism is known to increase after ventilation with highly soluble and diffusible gases. Exacerbation of the hypertension could be due to further blockage of the circulation if the bubbles enlarge as a result of ingress of gas by diffusion. This mechanism has been frequently cited but lacks direct proof. To determine directly whether intravascular air bubbles actually enlarge when highly soluble and diffusible gases are inspired, we used microscopy to measure the size of gas emboli in vivo. When air bubbles were injected into the right atrium, the bubbles that appeared in pulmonary arterioles were larger during ventilation with helium or nitrous oxide than with air. Air bubbles injected into the pulmonary artery enlarged when the inspired gas was changed to helium or nitrous oxide. The direction, magnitude, and timing of changes in bubble size were consistent with a net diffusion of gas into the bubbles. These data support the idea that venous air emboli enlarge during ventilation with soluble and diffusible gases and thereby cause further vascular obstruction.

Evaluation of a new high-efficiency blood warmer for children

Because currently available blood warmers are inadequate for infants and children requiring massive transfusion, the performance of a new high-efficiency pediatric blood warmer (System 250TM, LEVEL 1 Technologies Inc., Marshfield, Massachusetts) was evaluated and compared with a commonly used conventional blood warmer (Model DW1000A, American Pharmaseal, Valencia, California). Cold (5-6 degrees C), diluted red blood cells (RBC) (Hct = 30%) were infused through the warmers over a series of flow rates, and the resulting temperatures of the infusate were measured. The flow rates of diluted packed RBC were also measured over a series of infusion pressures. At a flow rate of 225 ml/min, the output temperature of the System 250TM was 33.6 degrees C compared with 24.6 degrees C (P less than 0.05) for the conventional warmer. Above a flow rate of 250 ml/min, however, the water bath of the System 250TM cooled significantly, resulting in a deterioration of performance and an output temperature of only 24.2 degrees C at a flow rate of 400 ml/min. With a 16-G catheter attached, the flow rate at a pressure of 300 mmHg was 223 ml/min through the System 250TM compared with 160 ml/min (P less than 0.05) for the conventional warmer. The System 250TM produced higher output temperatures and a lower resistance to flow compared with the conventional warmer, but flow rates of cold blood through the System 250TM should be restricted to 250 ml/min or less to ensure adequate warming.

Fate of air emboli in the pulmonary circulation

The lung serves an important nonrespiratory function by trapping and excreting venous air emboli. The site of trapping and the mechanism of excretion, however, are uncertain. To observe the behavior of bubbles in the pulmonary circulation, we injected venous air emboli into anesthetized dogs and videotaped their elimination from the pulmonary microcirculation by using in vivo microscopy. Small intravenous bubbles lodged exclusively in pulmonary arterioles and were eliminated from that site. To determine whether the gas was dissolving into nearby blood and then was carried to the capillaries for excretion, the rate of bubble radius change was measured during nonperfused conditions produced by balloon occlusion of lobar blood flow and compared with perfused conditions. Bubble volume decreased at the same rate during perfused and nonperfused conditions and thus was independent of regional blood flow. Molecular diffusion of gas directly across the arteriolar wall into alveolar spaces was the most likely mechanism of elimination because calculations based on the Fick equation for molecular diffusion predict an elimination rate nearly identical with those observed experimentally.

Dopamine reverses cardiovascular depression of toxic doses of pentobarbitone in dogs

Pentobarbitone, 20 mg X kg-1 IV followed by infusion of 25 mg X kg-1 X hr-1, produced a progressive decrease in mean arterial pressure in dogs from 113 +/- 17 mmHg (SD) after one hour of infusion to 82 +/- 21 mmHg after 3.5 hours and to 49 +/- 22 mmHg after 5.5 hours. EEG silence occurred at 3.6 +/- 0.6 hours. In dogs similarly treated with pentobarbitone, a two hour infusion of dopamine 5 micrograms X kg-1 X min-1 beginning at the time of EEG silence prevented the further decrease in pressure and restored pressure to 87 +/- 18 mmHg. The mechanism for this effect of dopamine was an increase in cardiac output as systemic vascular resistance was unaffected by dopamine. The cardiac output increase was mainly the result of an increase in stroke volume as heart rate increased only slightly. Since reduced stroke volume was the main reason why pentobarbitone lowered blood pressure, the effect of dopamine on stroke volume and thus on blood pressure makes it an appropriate antagonist to the cardiovascular effects of toxic doses of pentobarbitone.

Measurement of cardiac output in adult and newborn animals by ascorbic acid dilution

We have developed an ascorbic acid-dilution method for measuring cardiac output which requires minimal blood withdrawal. Ascorbate is injected into a central venous catheter. The indicator-dilution curve is obtained by drawing blood from an arterial catheter through an amperometric cell at 0.96 ml/min for 35 s. The current is measured by a picoammeter . A calibration curve is obtained in 15 s prior to each indicator-dilution curve. An on-line digital computer measures the curve areas and calculates the cardiac output. Cardiac outputs of heparinized dogs anesthetized with pentobarbital and halothane measured by this method (AA) compared closely to cardiac outputs measured by the dye-dilution method (CG) (AA = 0.96 CG + 20 ml/min, r = 0.98). Both the cardiac output and the arterial blood pressure remained stable during replicate measurements of the cardiac output of 1-day-old piglets. This system allows cardiac output determinations of neonatal subjects without excessive blood removal and, with further development, should be practical in human neonates.

Effect of age on epinephrine-induced arrhythmias during halothane anaesthesia in pigs

The effect of age on the arrhythmogenicity of epinephrine during halothane anaesthesia was studied in pigs of two different age groups. At a stable alveolar concentration of 0.84 volumes per cent halothane, ventricular arrhythmias could not be elicited in one- to three-day-old pigs by a 100 micrograms X kg-1 infusion of epinephrine. PVCs were produced in 50- to 55-day-old pigs at a mean epinephrine dose of 9.55 micrograms X kg-1. Heart rate, systolic blood pressure, and rate-pressure product were significantly higher before and during the epinephrine infusion in the 50- to 55-day-old pigs. It is concluded that there is an age dependent effect upon epinephrine induced arrhythmias during halothane anaesthesia in pigs.

Comparison of cardiovascular effects of thiopental and pentobarbital at equivalent levels of CNS depression

Thiopental and pentobarbital have been used in high doses to protect the brain from injury following hypoxia or to reduce intracranial pressure. This study was performed to determine whether these barbiturates differ in cardiovascular effects when present in plasma concentrations that produce equivalent CNS effects. The effects of thiopental and pentobarbital on heart rate, stroke volume/kg, cardiac output/kg, systemic vascular resistance, mean arterial pressure, and central venous pressure were statistically indistinguishable at plasma concentrations of each barbiturate ranging from 50% to 100% of their concentration producing EEG silence. Three of the seven dogs given thiopental developed ventricular bigeminy at plasma concentrations ranging from 45% to 65% of their concentration producing EEG silence. Lidocaine (1.4-2.0 mg/kg intravenously) reversed the bigeminy to sinus rhythm. When given more than the amount needed to produce a flat EEG, five of the seven dogs given thiopental died, but all dogs given pentobarbital survived. Pentobarbital may be a better choice than thiopental when large doses are indicated.

Anaesthetic implications of neonatal necrotizing enterocolitis

A retrospective analysis of infants with necrotizing enterocolitis was done to evaluate the effects of preoperative abnormalities upon anaesthesia and mortality. Mortality was significantly increased in infants weighing less than 1500 grams (p less than .001). Sixty-nine per cent of the infants had hyaline membrane disease and 35 per cent had platelet counts less than 50 X 10(9) cells/litre (50,000/mm3). Perioperative problems include peritonitis, sepsis, hypovolaemia, acidosis, and prematurity. Other ramifications of prematurity and anaesthesia are discussed.

Anesthetic considerations for ophthalmic surgery in the pediatric patient

In conclusion our review of 2,187 ophthalmic procedures revealed that even the very young pediatric patient can be anesthetized safely by adhering to basic principles and meticulous attention to detail.