The relationship between anaesthetic uptake and cardiac output

Response of hypotensive dogs to dopamine hydrochloride and dobutamine hydrochloride during deep isoflurane anesthesia

OBJECTIVE

To evaluate the dose-related cardiovascular and urine output (UrO) effects of dopamine hydrochloride and dobutamine hydrochloride, administered individually and in combination at various ratios, and identify individual doses that achieve target mean arterial blood pressure (MAP; 70 mm Hg) and cardiac index (CI; 150 mL/kg/min) in dogs during deep isoflurane anesthesia.

ANIMALS

10 young clinically normal dogs.

PROCEDURES

Following isoflurane equilibration at a baseline MAP of 50 mm Hg on 3 occasions, dogs randomly received IV administration of dopamine (3, 7, 10, 15, and 20 microg/kg/min), dobutamine (1, 2, 4, 6, and 8 microg/kg/min), and dopamine-dobutamine combinations (3.5:1, 3.5:4, 7:2, 14:1, and 14:4 microg/kg/min) in a crossover study. Selected cardiovascular and UrO effects were determined following 20-minute infusions at each dose.

RESULTS

Dopamine caused significant dose-dependent responses and achieved target MAP and CI at 7 microg/kg/min; dobutamine at 2 microg/kg/min significantly affected only CI values. At any dose, dopamine significantly affected UrO, whereas dobutamine did not. Target MAP and CI values were achieved with a dopamine-dobutamine combination at 7:2 microg/kg/min; a dopamine-related dose response for MAP and dopamine- and dobutamine-related dose responses for CI were identified. Changes in UrO were associated with dopamine only.

CONCLUSIONS AND CLINICAL RELEVANCE

In isoflurane-anesthetized dogs, a guideline dose for dopamine of 7 microg/kg/min is suggested; dobutamine alone did not improve MAP. Data regarding cardiovascular and UrO effects indicated that the combination of dopamine and dobutamine did not provide greater benefit than use of dopamine alone in dogs.

Predictive performance of a physiological model for enflurane closed-circuit anaesthesia: effects of continuous cardiac output measurements and age-related solubility data

BACKGROUND

The disposition of inhalation anaesthetics is governed by the factors described in the Fick principle.

METHODS

We have recalibrated a previously validated physiological model for enflurane closed-circuit inhalation anaesthesia, using individual continuous cardiac output measurements as well as age-related enflurane solubility coefficients as inputs to the model. Two model versions using 'calculated' (Brody's formula) or 'measured' (thoracic electrical bioimpedance) cardiac output values, and two versions with 'standard' (fixed) or 'age-related' solubility coefficients were formulated.

RESULTS

Data from 62 ophthalmic surgical patients were used to validate the predictive performance of the four model versions. The root mean squared errors (total error) and scatters (error variation) were similar with the extended model versions, but the group biases (systematic error component) were significantly less with the model versions that included age-related solubility compared with the versions using standard solubility coefficients (bias -0.76/-0.78% vs -3.44/-3.60%).

CONCLUSION

The inclusion of age-related solubility coefficients but not of continuous cardiac output measurements improves the predictive performance of the physiological model for closed-circuit inhalation anaesthetic conditions in routine clinical practice.

The effect of cardiac output changes on end-expired volatile anaesthetic concentrations--a theoretical study

Cardiac output is one of the major determinants of the rate of uptake, and therefore the end-expired concentration (F(E')) of volatile anaesthetic agents. The purpose of this theoretical study was to explore the effect of cardiac output changes on F(E') for a range of volatile anaesthetics. A multicompartment model of anaesthetic uptake and distribution which produces constant values of F(E') was used. The minimum detectable change in cardiac output was determined for a variety of anaesthetic agents for four patterns of cardiac output change. The effect of a step change in cardiac output from 5 to 10 l.min(-1) was also recorded. The smallest cardiac output changes (average 33%) were detected with isoflurane. As blood solubility increased or decreased, larger cardiac output changes were needed before they could be detected. With a large step change in cardiac output and with increasing solubility, the final change in F(E') increased but the initial rate of change of F(E') is decreased. A significant cardiac output change will produce a change in volatile anaesthetic uptake. An unexpected change in F(E') should be considered as a possible signal of a sudden cardiac output change. The difference between agents may represent a balance between the amount of agent taken up and the size of the tissue 'sink' for that agent.

The effect of cardiac output changes on end-tidal volatile anaesthetic concentrations

Cardiac output is a major determinant of the rate of uptake of volatile anaesthetic agents. Theoretical work suggests a 30% change in cardiac output will produce a measurable change in end-tidal volatile agent. We present three patients in whom significant changes in haemodynamic parameters, consistent with a large fall in cardiac output, were accompanied by an increase in end-tidal volatile agent concentrations. The changes in end-tidal volatile agent concentrations were comparable in magnitude but in the opposite direction to the changes in end-tidal CO2. Clinically, an unexplained change in end-tidal concentration of a volatile agent may signify a large change in cardiac output.

The uptake of sevoflurane during anaesthesia

The rate of uptake of sevoflurane during clinical anaesthesia (1.3 MAC) was measured by computer-controlled injection of liquid anaesthetic into a closed breathing system. The cumulative uptake of sevoflurane was 4.8 ml, 7.4 ml, 9.5 ml and 11.5 ml at 30, 60, 90 and 120 min, respectively. The ratio of inspired to end-expired sevoflurane was greater than similar measurements we have made for desflurane in the past, but the absolute rate of sevoflurane uptake was less than the rate of uptake of desflurane in these cases. The rate of uptake was equivalent to 059e-0.32t + 0.039e-0.036t + 0.105e-0.0034t ml.min-1 liquid sevoflurane. Plasma urea and creatinine measured on the first postoperative day were not significantly different from pre-operative values.