How to Calculate Expiratory to Inspiratory (E:I) Ratio from ECG Signal Recordings

Calculating the Expiratory to Inspiratory (E:I) ratio from continuous Electrocardiogram (ECG) data is a fundamental skill in clinical electrophysiology, autonomic research, and medical thesis studies. This guide breaks down the step-by-step computational methodology.

Step 1: Record Signal & Detect R-Peaks

In a standard Lead II ECG recording (sampled at 1000 Hz or higher), the sharp QRS complexes contain R-peaks that define consecutive heartbeat timestamps \(t_1, t_2, \dots, t_N\).

The inter-beat RR interval \(\Delta t_i\) in milliseconds is calculated as:

\(\text{RR}_i = (t_{i+1} - t_i) \times 1000\)

Step 2: Identify Expiratory and Inspiratory Peaks

During a 1-minute 6-breath deep breathing protocol (5 seconds inhalation, 5 seconds exhalation per cycle):

• Expiratory Peak (High RR): Exhalation causes cardiac slowing, resulting in maximal RR interval duration (e.g. 1150 ms to 1300 ms).
• Inspiratory Trough (Low RR): Inhalation causes cardiac acceleration, resulting in minimal RR interval duration (e.g. 750 ms to 900 ms).

Step 3: Mean E:I Ratio vs. Median E:I Ratio

In clinical practice, a single cycle can be corrupted by an ectopic beat or movement artifact. It is recommended to calculate the E:I ratio for all 6 breathing cycles and compute both:

1. Mean E:I Ratio: The arithmetic average across all valid respiratory cycles.
2. Median E:I Ratio: The median value, robust against single-cycle outliers or ectopic spikes.
3. Ratio of Means: The mean of all maximum expiratory RR intervals divided by the mean of all minimum inspiratory RR intervals.

Step 4: Handling Signal Artifacts & BIOPAC Files

When working with native BIOPAC .acq files, manual verification of R-peaks is vital. Ectopic beats (such as Premature Ventricular Contractions) create artificially low or high RR intervals that distort the E:I ratio.