Doctors can use several techniques to measure stroke volume, which is the amount of blood expelled from a ventricle of the heart with each heart beat. The left ventricle is usually the topic of interest. These measurements can include direct measurements of cardiac output as well as indirect tests that involve the use of techniques like ultrasound to get a close estimation of the patient's stroke volume. When the volume is too low, it means the heart is not pumping enough blood with each beat and the patient could be at risk of complications.
Commonly, doctors express stroke volume in terms of milliliters per heartbeat. The number can vary depending on sex, age, and level of health. Usually it falls between 60 and 130 milliliters per beat. A doctor can determine the normal range for a patient, given medical history and other factors. This will be the baseline to determine if the patient's volume is too low.
One option is through cardiac catheterization, where a doctor threads a tube up to the heart. This may be done to perform a procedure on the heart, and it can also allow the doctor to measure the cardiac output. The doctor can divide this measurement by the heart rate to determine the stroke volume. Cardiac catheterization is invasive and may not be recommended for all patients.
Ultrasound and echocardiography provide another mechanism for measuring stroke volume. In this case, the doctor subtracts the measured end-systolic volume, the blood that remains in the heart after a beat, from the end-diastolic volume, the amount of blood present right before the heart beats. This provides a measurement of how much the heart pumps out with each beat. Ultrasound and similar techniques rely on the use of Doppler shifts to measure the movement of blood. The precision of the test depends on the accuracy of the machine and the doctor's skill.
A patient can also undergo an angiogram, an imaging study of blood vessels and the heart while they are in motion. The doctor can inject a tracer dye to follow blood as it moves through the circulatory system and may be able to determine cardiac output as the dye moves and dilutes. This will allow her to determine the patient's stroke volume through indirect measurement. The test also allows the doctor to spot vascular irregularities that may contribute to the patient's condition, such as weak or occluded blood vessels.