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Evaluation of ECG and Respiration Specific Functions
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6.2.2 Lead Derivation
The demonstration kit is configured to generate two ECG leads (Lead I and Lead II) from the four
electrodes using two channels of the ADS1192 and ADS1292 data. The other four ECG leads are
computed from Lead I and Lead II using the following equations:
Lead III = Lead II - Lead I
Lead aVR = - Lead II + 0.5 * Lead III
Lead aVL = Lead I - 0.5 * Lead II
Lead aVF = Lead III + 0.5 * Lead
The ADS1292R demonstration kit is configured to generate one ECG lead (Lead I) and one respiration
channel from the four electrodes using the two channels.
6.2.3 QRS and Respiration Rate Detection
QRS detection is based on first derivative of the Lead I ECG signal and threshold. Once five consecutive
QRS are detected, the heart rate is calculated by taking average of the five RR intervals.
The following steps are involved for calculating heart rate:
1. Calculate first derivative of the Lead I ECG signal samples. The first derivative for any sample is
calculated as
y0(n) = |x(n+1) – x(–1)|
Where,
y0(n) is the first derivative.
x (n+1) is the sample value for (n + 1) th sample.
x (n – 1) is the sample value for (n – 1) th sample.
2. The initial two seconds of the first derivative is stored in a buffer and the maximum value (P) in this
buffer is obtained.
3. Calculate the threshold as 0.7 x P.
4. Whenever a derivative crosses the threshold, the ECG sample index (S1) of that particular sample is
marked.
5. The QRS peak is detected by scanning the next 40 derivatives (MAXIMA_SEARCH_WINDOW = 40)
and obtaining the maxima (M1) .This maxima (M1) value is stored in another buffer.
6. After detecting a QRS peak, the next 50 samples (SKIP_WINDOW = 50) are skipped to take care of
the minimum RR interval that can occur in the case of maximum detectable heart rate (i.e. 240 BPM).
7. Next five QRS peaks are detected by repeating steps 4 to 7.
8. RR interval is calculated as the number of samples between two consecutive QRS peaks.
9. Heart rate is calculated using the below formula HR per Minute = (60 * Sampling Rate)/ (Average RR
interval for 5 consecutive RR intervals )
10. Threshold also gets recalculated from the QRS peak values detected.
36 ADS1x9xECG-FE Demonstration Kit SLAU384A – December 2011 – Revised April 2012
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