## 3 Use Case: Watson Analytics as AaaS to Identify HF Patients Analyzing Only the ECG Signal
### Medical and ECG-Analysis Background
#### 3.2.3 ECG Signal Analysis through HRV  
The electrocardiogram (ECG) is the graphic reproduction of the heart activity during its cycle of operation, recorded via sensors (electrodes) placed on the skin. Specifically, the cardiac activity can be estimated by measuring the voltage differences in some defined point of the body. For decades the electrocardiogram has been the easiest, practical, less invasive and less expensive method to observe the electrical activity of the heart. The ECG outcome has a characteristic shape, whose variations can indicate problems. It contains several sections called waves, positive and negative, which are repeated for each cardiac cycle:  
* P wave: the first wave of the cycle, which corresponds to ventricular depolarization of the atria; the contraction is quite weak and the wave is small.
* QRS complex: set of three waves in rapid succession corresponding to the depolarization of the ventricles: the Q wave is negative and small, the R is a high positive peak, while S is again a small negative wave.
* T Wave: it refers to the ventricle repolarization.
* U Wave: due to the repolarization of the papillary muscles, which is also not always identifiable
* ST Section: period during which the ventricular cells are depolarized, therefore isoelectric, so electrical changes are not greater than 1 mm on the graph.
* QT interval: interval in which occurs ventricular depolarization and repolarization; its duration varies with the heart rate, but generally remains between 350 and 440 ms.  
Heart Rate Variability (HRV) nomenclature refers to the physiological phenomenon of time length variation between two heart beats; once defined the peak wave in the cardiac cycle as \"R\", we can also refer to HRV as \"RR variation\" or \"RR interval\", meaning the time frame between two R waves. HRV can be performed using two ways:  
* Long-term analysis: performed on a ECG signal acquired for 24 h in a row, using a device called Cardiac HolterShort-term analysis: performed on a ECG signal acquired for just 5 min or less  
HRV analysis can be carried on in both time and frequency domain. The values obtained from the ECG signal performing the time domain analysis are summarized in [28], and are:  
* SDANN: Standard deviation of the averages of NN intervals in all 5-min segments of a 24-h recording
* AVNN: Average of all NN intervals
* pNN50: Percentage of differences between adjacent NN intervals that are greater than 50 ms; a member of the larger pNNx family
* SDNNIDX: Mean of the standard deviations of NN intervals in all 5-min segments of a 24-h recording
* rMSSD: Square root of the mean of the squares of differences between adjacent NN intervals
* SDNN: Standard deviation of all NN intervals  
In the frequency domain parameters from the ECG signal are:  
* LF/HF: Ratio of low to high frequency power
* TOTPWR: Total spectral power of all NN intervals up to 0.04 Hz
* LF: Total spectral power of all NN intervals between 0.04 and 0.15 Hz.
* ULF: Total spectral power of all NN intervals up to 0.003 Hz
* HF: Total spectral power of all NN intervals between 0.15 and 0.4 Hz
* VLF: Total spectral power of all NN intervals between 0.003 and 0.04 Hz