Introduction

Blood pressure is pulsatile in nature. With each heartbeat, the heart produces a pressure wave that travels through the circulatory system. This pressure wave generally consists of an increase in pressure up to systolic (SYS), followed by a decrease in pressure down to diastolic (DIA). The systolic time interval (STI) is the time interval to reach SYS from the start of a pressure pulse. The diastolic time interval (DTI) is the time interval from SYS to the start of the next pressure pulse. These phases of the cardiac cycle are measured in milliseconds (ms) and are displayed as part of the Pulsewave results for each reading.

The timing of these cardiac events is used as measure of left ventricular function and thus has diagnostic and prognostic value. Traditionally, STI has been used as an index for myocardial energy expenditure and stroke volume of the left ventricle, and DTI has been used as an index of measuring subendocardial blood flow (during blood flow to the muscles of the heart) and ventricular filling.

Measurement of Systolic Time Interval and Diastolic Time Interval

The phases of the cardiac cycle can be measured using invasive techniques; however, these techniques carry physical risks and are limited to special circumstances. Non-invasive techniques can also be used to measure STI and DTI. Such techniques may be based on the use of echocardiography, stethoscopes, electrocardiography, or, as in the case of a Pulsewave reading, the pulse waveform.

Relationship to Pulse Rate

Pulse time is inherently related to pulse rate. The sum of STI and DTI provide an indication of the duration of a pulse, while the pulse rate provides an indication of the rate at which the pulse occurs. This can be described as an inverse relationship, e.g., an increase in pulse rate corresponds to a decrease in pulse time. However, while measurement of pulse rate provides an indication of pulse time, it does not provide an indication of the components of pulse time, i.e., STI and DTI.

Additional Resources

Wikipedia. Cardiac Cycle [Online]. Available: http://en.wikipedia.org/wiki/Cardiac_cycle. Accessed: 2011-02-22.