What is stroke volume, and why should you care?

Dr. Gormican was an emergency physician for over 35 years and has over 45 years of medical experience. Retired from clinical practice as an Emergency Physician at Scripps Memorial Hospital – La Jolla, he is currently driving development of medical innovation as Chief Medical Officer of Rock West.

DISCLAIMER: The Heart Seat is currently under development and is NOT FDA cleared for any clinical measures at this time.

When I first heard about Casana, I knew that their Heart Seat would change treatment for heart failure patients. Not only can the Heart Seat collect vital signs effortlessly, but Casana found a way to collect a powerful vital parameter for measuring blood flow – stroke volume. Currently, measuring stroke volume is challenging. Depending on the setting, doctors often need to accept suboptimal surrogates for measuring flow.

Blood pressure (BP) is easy to obtain; Blood flow is not.

Since Pressure = Flow x Resistance (Ohm’s Law V=IR), pressure correlates to flow only if resistance is held constant. Stroke volume measures the amount of blood flowing out of the heart with each heartbeat. As an emergency physician for over 35 years, there were many times I wanted to measure blood flow to help inform the care pathway, but I would have to settle for the surrogate of blood pressure.

For example, whenever a patient would be admitted with sudden pulmonary edema – a critical form of heart failure that causes fluid to build up in the lungs because the heart isn’t properly pumping blood – my strategy was to lower resistance and thereby increase blood flow by administering IV nitroglycerin. When a patient already had low blood pressure, this strategy might lower the blood pressure further which might caused my nurses a lot of stress. The patient’s blood pressure worsened, however, their skin changed from bluish to pink and they began to appear more lucid. They were improving, even though a key vital sign, blood pressure, appeared to be worsening. The sooner I could improve the patient’s flow (stroke volume), the sooner their symptoms, which include shortness of breath or a feeling of suffocating, could be relieved.

The Heart Seat has the promise to deliver this blood flow measurement. This is a breakthrough that I believe could change the care of patients. It allows us to track this vital sign on a regular basis, while the patient sits comfortably at home, hopefully avoiding the ER visit altogether.

Stroke volume for assessing Heart Failure

Another benefit of stroke volume is that it may be superior to measuring the traditional ejection fraction (EF). Ejection fraction refers to the percentage of blood pumped out of the left ventricle with each beat. Many fractions (including EF) often hide important raw data. The denominator of EF is the peak size of the ventricle before the contraction, which is proportional to end diastolic volume or EDV. The numerator is the decrease of the ventricular size which is proportional to SV (stroke volume) so that EF=SV/EDV. If a numerator and denominator go down the same percent, the EF remains the same even though an important change has taken place. This is the so-called “heart failure with preserved ejection fraction” (HFpEF) which is present in nearly half of those with heart failure.

Ironically, the metric used to measure heart failure is normal in most people with heart failure. For monitoring changes in an individual, it seems preferable to use stroke volume (SV) or multiply SV by the heart rate to obtain cardiac output (CO) or divide CO by patient size to obtain cardiac index (CI), but all of these are based on SV so that measuring stroke volume and particularly changes in stroke volume is a more direct method of assessing heart failure. However, easily obtaining SV has traditionally been problematic. I believe The Heart Seat could be a game changer for treating heart failure and alter the approach to monitoring heart failure in the future.

Casana and the Heart Seat

Having led the Tech Coast Angel deal for an early investment in Casana, I had the privilege to briefly sit on the Board of Directors and get to know the Casana Founders. When new talent came in with the next investment, I was even more impressed. The team is incredible and the current momentum is inspiring. As you can tell, I am impatient to see the Heart Seat come to market. The Heart Seat’s potential to effortlessly collect heart rate, heart rhythm, PR Interval changes, QT interval changes, respiratory rate, O2 saturation, systolic blood pressure, diastolic blood pressure, stroke volume and cardiac output will create a massive database for patients and for medicine. Mining this database will advance home health care for heart failure patients in a way that previously was not possible. Thank you Casana.

DISCLAIMER: The Heart Seat is currently under development and is NOT FDA cleared for any clinical measures at this time.