Myocardial preservation during cardiopulmonary bypass: comparison of retrograde with antegrade cardioplegia on right ventricular protection

INFLUENCE

OF INTRACARDIAC LEFT-TO-RIGHT SHUNTS ON THERMODILUTION MEASUREMENTS OF CARDIAC OUTPUT

A. Weyland*,

G. Wietaschs,

Depts. of Anaesthesiology*

A. Hoeft*,

W. Buhre*,

and Pediatric Cardiology§,

Introduction:Thermodilution (TD) measurements of cardiac output (CO) by use of Swan-Ganz catheters represent pulmonary arterial blood flow. In printiiple, this is also true in the presence of intracardiac left-toright shunts due to atrial or ventricular septal defects. However, early recirculation of indicator may give rise to serious methodological problems in these cases. We sought to determine the influence of intracardiac left-toright shunts on different devices for TD measurements of CO using an extracorporeal flow model. Methods: Blood flow was regulated by use of a centrifugal pump that at the same time enabled complete mixing of the indicator after injection. Pulmonary and systemic circulation were simulated by use of two membrane oxygenators and a systemicvenous reservoir to delay systemic recirculation of indicator. Control measurements of pulmonary (Cl,) and systemic (Qs) blood flow were performed by calibrated electromagnetic flow meters (EMF). Blood temperature was kept constant using a heat exchanger without altering the indicator mass balance in the pulmonary circulation. Left-to-right shunt was varied at different systemic flow levels (3 and 4 I/min) applying a Qp:Qs ratio ranging from 1:l to 2.5:1. Triple TD measurements of pulmonary flow were performed by use of two different TD catheters that differed considerably with respect to the time constants of thermistors (1080 msec and 440 msec, respectively). Catheters were connected to commercially available CO computers (Oximetrix III, Abbott, and IVC 3, Pulsion). Additionally, TD curves were recorded on a microcomputer and analysed with custom made software that enabled iterative regression analyses of the initial decay to determine that part of the downslope that fitted best to a monoexponentially declinig function. Extrapolation of the TD curve then was based on the respective curve segment in order to eliminate indicator recirculation due to shunt flow. Results: At moderate left-to-right shunts (Qp:Qs < 2:l) all TD measurements showed a close agreement with control measurements. At higher shunt flows (Qp:Qs t 2:1), however, conventional extrapolation procedures of CO computers considerably underestimated pulmonary blood flow (Fig.). This was particularly true when a slow-response thermistor (SRT) catheter was used. The reason for this underestimation of Qp was an overestimation of the area under curve because of inadequate mathematical elimination of indicator recirculation by standard truncation methods. However, curve-alert messages of the commercially implemented

40

B. Allgeiefi,

University

W. Weyland*,

of Giittingen,

H. Sonntag*

37075 Gtittingen,

FRG

software did not occur. A high level of agreement could be consistently obtained by use of a fast-response thermistor (FRT) together with individual definition of extrapolation limits according to logarithmic regression analyses. Discussion: Under varying levels of a left-to-right shunt, both, the reponse time of TD catheters and the algorithms for calculation of flow, considerably influenced the validity of TD measurements of pulmonary blood flow in an extracorporeal flow model. The use of computer-based regression analysis to define the optimal segment for monoexponential extrapolation could effectively eliminate indicator recirculation from the initial portion of the declining TD curve and showed the closest agreement with EMF measurements of Qp. Limitations of this method may occur if very early recirculation of indicator critically truncates the curve segment for extrapolation or completely contaminates the downslope of the TD curve. Insufficient elimination of indicator recirculation results in flow values that closely resemble systemic rather than pulmonary blood flow. This finding is in accordance with clinical observations1 and could be verified by a mathematical analysis of the underlying Steward-Hamilton equation if an indefinite number of recirculations would be included in the area under curve.

Q,

(TD)

[Vmin]

0

SRT (standard truncation method. 82-30%)

V

FRT (standard truncation method, 82-30%)

A

FRT (“adjusted extrapolation’)

,

Q,(EMF)

;

6

8

10

QJQS

1.0

1.5

2.0

2.5

I

I

I

I

[Vmin]

Ref.: 1. Pearl RG, Siegel LC: Thermodilution CO measurement with a large left-to-right shunt. J Clin Monit 106: 146-153; 1991

Journal of Cardiothoracic and Vascular Anesthesia, Vol8. No 5, Suppl 3 (October), EACTA 94 Abstracts: Cardiovascular Pharmacology

1994