Stroke and minute distance in pregnancy: a longitudinal study using Doppler ultrasound.

Using Doppler ultrasound directed at the aortic arch, stroke and minute distance were measured in 45 normal singleton pregnancies and compared with 42 controls. In the non-pregnant controls, stroke distance was 5% more in the supine than in the left lateral position (P less than 0.01) and during pregnancy this relativity was maintained until 36 weeks gestation. In the supine and left lateral positions respectively, stroke distance was increased by 8% (both) at 4 weeks (P less than 0.05 and NS), by 14 and 11% at 8 weeks (P less than 0.001 and P less than 0.01) and showed a maximum increase of 21 and 20% at 16 weeks gestation (both P less than 0.001). At 40 weeks, supine stroke distance was 14% less than the non-pregnant control value (P less than 0.001), and 6% less than the value in the left lateral position (P less than 0.05), which was itself not significantly different from that in the non-pregnant controls. The maximum increase of heart rate was at 32 weeks, by 24% in the supine and by 19% in the left lateral position (both P less than 0.001). Minute distance was maximally increased by 37% in the supine at 16 weeks (P less than 0.001) and by 30% in the left lateral position at 24 weeks (P less than 0.001). Thereafter, minute distance, measured in both positions, declined until at 40 weeks it was not significantly different from the non-pregnant control value.

How far is the cardiac output?

Measurement of volumetric cardiac output is technically difficult and poorly reproducible, and the technique is unsuitable for use in the majority of patients in whom a knowledge of overall cardiovascular function would be desirable. Interpretation of a single measurement in a patient is difficult because little is known about volumetric cardiac output in normal subjects and the available techniques of measurement are very complex or invasive. Linear cardiac output, in contrast, can be simply, non-invasively, and reproducibly measured at the bedside by doppler ultrasound. The systolic-velocity integral of a single heart beat (stroke distance) multiplied by the heart rate gives minute distance; this may be envisaged as the distance travelled by mid-stream blood in the aorta in a minute. Providing there is no aortic disease, minute distance may be used as an absolute indication of cardiac output and for following serial changes.

Assessment of cardiac output by the Doppler ultrasound technique alone.

The normal range of aortic blood velocity was established in 140 healthy adults, using a non-invasive Doppler ultrasound technique. Integration of the area under the velocity-time curve for each heart beat gave stroke distance, which, when multiplied by heart rate, gave minute distance. Stroke distance and minute distance are an indication of stroke volume and cardiac output respectively and both show a progressive decline with age of about 1% per annum of adult life. Stroke distance gave the greatest discrimination between patient groups and, compared with age corrected normal values, was increased by 43% in 12 pregnant women, normal in 16 patients convalescing after myocardial infarction, and decreased by 14% in 15 patients with hypertension, by 31% in 12 patients with atrial fibrillation, and by 43% in 13 patients with cardiac failure. Measurement of aortic blood velocity and its derivatives alone, without determination of aortic size, is a safe, simple, and physiologically valid way of assessing cardiac output.

Reproducibility of linear cardiac output measurement by Doppler ultrasound alone.

Inclusion of a pig aorta in an artificial circulation with pulsed blood flow allowed correlation of minute distance, measured in the aorta by Doppler ultrasound, and absolute blood flow, measured by timed blood-volume collection. The correlation coefficient was 0.99 with a standard error of prediction that was 5.4% of the minute distance predicted at a standard flow rate of 5 litres per minute. The horizontal distance between 95% confidence limits for a single prediction expressed as a percentage of 5 litres per minute was 33%, and this corresponded to the range of flow rates of 1.65 litres per minute that could give rise to the same measurement. In 142 patients duplicate measurements of minute distance were made with repositioning of the ultrasound transducer between recordings. The mean difference between paired readings, expressed as a percentage of the average (SD) of each pair was 5.4 (4.7)%. Thus, the non-invasive measurement of linear cardiac output by Doppler ultrasound is similarly reproducible in vitro and in vivo and compares favourably with the measurement of volumetric cardiac output by thermodilution.