When you look into your Apple Watch, you may have noticed that it reports a strange parameter called heart rate variability (HRV). Unlike heart rate which measures the number of time the heart beats per second, HRV is the measure of the variation in time between each heartbeat aggregated over time. The first step of measuring HRV is computing a time series of beat to beat distance (NN), from either an electrocardiogram (ECG) or an optical photoplethysmograph (PPG). Chest straps like Polar H10 use ECG sensors, whereas wristband based Apple Watch and Fitbit use a PPG sensor.
The human heart beats at regular intervals due to a periodic electrical impulse from a group of nerves located within the heart tissue, called the cardiac pacemaker or sinoatrial node. The sinoatrial node also receives additional secondary signals from the autonomic nervous system (ANS), located in the brain and the spinal cord. This ANS ensures that in the case of an emergency the heart can pump in large volumes of blood and trigger a fight-or-flight response (sympathetic response), and can thereafter return to its normal pace during rest (parasympathetic response). In individuals with a healthy heart, the switch between sympathetic and parasympathetic states should happen quickly and results in a high HRV, whereas patients with cardiovascular disorders show low HRV.
HRV can also be used to detect heart rhythm disorders such as Atrial Fibrillation (AF). AF happens when the top chambers of the heart beat faster and are out of sync with the bottom chambers. Although not immediately life-threatening, AF reduces the heart’s output by about 30% potentially leading to stroke.
Since HRV reported as a time series of RR signals is difficult to comprehend and compare, so HRV is often reported as an average RR. This approach is called a Time Domain Analysis and is most commonly used in the clinical interpretation of HRV. Other popular Time Domain Analysis include SDNN (Standard deviation of RR of normal-to-normal intervals), rMSSD(Square root of the mean squared difference of successive RR intervals), NNx (Number of valid adjacent NN values not separated by data breaks), pNNx (Proportion of valid adjacent RR values not separated by data breaks) and Cycles (Number of available NN values). The other approach is to compute a Frequency Domain Analysis which studies the relative distribution of NN signals in very low frequency, low frequency, and high-frequency domains.
Whatever might be the method of tracking HRV, the most exciting application of this physiological parameter is perhaps its ability to track stress. Tracking HRV can help individual incorporate healthy habits such as mindfulness, meditation, sleep, and physical activity into their life. Companies like Lief Therapeutics are building wearable HRV sensors that can help you calm down using breathing exercises. Wearable fitness trackers like the Garmin® Forerunner® 935 GPS Multisport Watch can provide an overall stress score by measuring your heart rate variability over a three minute period. Garmin uses Firstbeat’s recovery time metrics to achieve this, as does Huawei's Watch GT, Jabra's Sport Pulse Special Edition, and Huami's Amazfit Stratos.
One of the use cases of tracking your HRV is to ensure that you are not over-exercising. Polar allows you to do an Orthostatic Test to measure your balance between training and recovery. Trainers and athletes use this test to build a personalized strain baseline and then conducts weekly inspections to ensure that they are fully recovered. Polar measure HRV from the Polar H10 ECG chest strap, and displays the result on the Polar Vantage multisport watch, the Polar V800 GPS sport swatch or the Polar M460 GPS cycling computer. ithlete’s Heart Rate Variability App works in tandem with it’s ECG sensors and adjusts personal training regimens to maximize athletic performance. BioForce HRV’s Morpheus platform similarly uses a chest strap based ECG device, to prevent overtraining. Biostrap’s platform also uses HRV to monitor athlete fatigue and over-training.
HRV is also used physicians to track the overall health of patients. Kubios’ HRV analysis platform is suitable for doing a detailed examination of autonomic nervous system function and is used by clinical and public health researchers, professionals working on human or animal well-being and sports enthusiasts. Companies like Firstbeat and Kubios have simplified the process of understanding and working with HRV and will allow the next generation of health care providers to use this physiological parameter to it’s fullest capacity.
