Unlike other heart rate variability measures which focus on normal heart beats only, heart rate turbulence measures the heart’s response to an abnormal beat. The figure below shows normal heart beats with two abnormal beats called ventricular premature contractions (VPCs).
When a VPC occurs, it comes early and the heart does not contract normally. Therefore the amount of blood pumped out is less. At the same time, because the beat after a VPC is delayed, the heart has more time to fill, so that beat pumps out more blood than usual. Thus, the heart speeds up and blood pressure increases to deal with the loss of output after the VPCs then heart rate slows down and blood pressure decreased to deal with the increased output from the normal beats that follow the VPC. Finally, the system returns to normal, usually within 15 beats or so. HRT is a way to measure how well the heart deals with this situation.
Heart rate turbulence measures
Heart rate turbulence onset (TO)
- Measures initial compensatory acceleration in heart rate, if any, after a VPC.
- Calculated as the percent change in the average of the two normal beats after and the two normal beats before the VPCs (see below).
- Based in time between beats rather than heart rate.
- Shorter time between beats is faster heart rate, so if heart rate goes up, the difference between before and after is negative.
- TO positive is abnormal. In some studies TO=zero is considered normal, in others abnormal depending on the cutpoint.
- TO is a weak predictor of outcome.
Heart rate turbulence slope (TS)
- Turbulence slope measures the oscillation, if any, in heart rate after a VPC.
- TS is measured by fitting a line to each set of 5 beats after the VPC.
- The largest slope of any line is called TS (see below).
- Having TS less than 2.5 milliseconds/beat has identified cardiac patients at increased risk of dying and is an accepted cutpoint for this population.
- Having TS less than 3 milliseconds/beat has been the best cutpoint for identifying older adults at increased risk of cardiac death.
- In reality, the beats before and after VPCs are averaged so that the calculation is made from one event that is the average of all of the others.
- A minimum of 5 VPCs with two normal beats before and 15 normal beats after is needed to calculate HRT.
- Those with less than 5 VPCs in 24 hours can be considered to have normal HRT.
Combined HRT measures
- Although TO abnormal by itself is not a strong risk marker, having a combination of abnormal TO and abnormal TS is a strong marker for increased risk of cardiac death both in cardiac patients and in older adults.