Beat-to-beat evaluation of cardiac function in low-dose endotoxemia using a conscious sheep model

The effects of very low doses of endotoxin (20 ng/kg/h for 8 h) were evaluated in a conscious sheep model in which introducers for catheters for monitoring pressure and ventricular dimensions had been previously inserted so that the studies could be performed without anesthesia and without a previous thoracotomy. Analog data was obtained at hourly intervals for 10 h and again at 24 h and was used to construct a beat-to-beat analysis of left ventricular performance. Only minimal effects were observed on heart rate, end diastolic pressure, arterial pressure, cardiac output, or cardiac work, although there was a significant rise in pulmonary artery pressure at 1 h of infusion. Despite the absence of changes in heart rate, preload, or afterload, maximal dp/dt decreased significantly by 4 h and remained decreased for the 10-h observation period; it returned to normal at 24 h. End systolic elastance decreased at 6 h and Pmax/EDV, a new indicator of performance, also decreased at 6 and 9 h. Thus, systolic performance decreased. Negative dp/dt did not change, but time for relaxation from 80% to 20% of peak ventricular pressure increased significantly at 5, 6, and 8 h. Plasma TNF-alpha was also measured and showed a significant rise at 2 h, but rapidly decreased thereafter. These results indicate an early depression of myocardial contractility and distensibility at doses of endotoxin insufficient to produce measurable effects on arterial pressure or cardiac output.