ECG Continued

The standard ECG cycle consists of a sequence of electrical events represented by specific deflections: the P wave, the QRS complex, and the T wave, which repeat periodically.
P Wave Deflection:
- Represents atrial excitation or depolarization.
- The duration of the P wave averages approximately $0.08\,s$ .
QRS Complex Deflection:
- Represents ventricular depolarization.
- The duration of the QRS complex averages approximately $0.08\,s$ .
T Wave Deflection:
- Represents ventricular repolarization.
- The duration of the T wave is significantly longer than the P wave, averaging approximately $0.16\,s$ (roughly twice the length of the P wave).

PR Interval:
- This interval spans from the beginning of the P wave to the beginning of the QRS complex.
- It represents the time from the start of atrial excitation to the start of ventricular excitation.
- The signal is typically paused at the AV node for approximately $100\,ms$ ( $0.1\,s$ ).
- The transcript notes the PR interval itself lasts approximately $0.1\,s$ .
PR Segment:
- This is the specific time period between the P wave and the QRS complex where no electrical activity is recorded on the baseline.
- The duration is approximately $0.1\,s$ .
ST Segment:
- This segment occurs from the end of the QRS complex to the beginning of the T wave.
- It represents the period during which the entire ventricle is depolarized.
- Physiologically, this corresponds to the plateau phase of the cardiac action potential, characterized by the entry of Calcium ( $Ca^{2+}$ ) ions into the cardiac muscle cells.
Q-T Duration (End of QRS to End of T):
- The interval from the start of the QRS complex to the end of the T wave averages approximately $0.38\,s$ .

ECGs are recorded by placing electrodes (leads) across the heart to capture electrical activity, which is then printed onto moving graph paper.
Grid Dimensions:
- The paper consists of large boxes and small boxes.
- Large Boxes: Measure $5\,mm$ in length.
- Small Boxes: Each large box contains five smaller boxes, each measuring $1\,mm$ .
- Consequently, two large boxes ( $10\,mm$ ) equal $1\,cm$ .
Feed Rates (Paper Speed):
- The speed at which paper moves through the machine can be adjusted.
- Common settings include $10\,mm/s$ , $15\,mm/s$ , and $20\,mm/s$ .
- The most common standard feed rate in clinical practice is $25\,mm/s$ .

To determine the heart rate (beats per minute or BPM), one must measure the distance between specific peaks (Peak-to-Peak distance).
Measurement Methods:
- The measurement is typically taken from the R peak of one QRS complex to the R peak of the next (R-R interval).
- It can also be measured from P-peak to P-peak or T-peak to T-peak.
Calculation Formula:
- The formula involves dividing one beat by the measured distance in millimeters and converting units using the paper feed rate and seconds per minute.
- $\text{BPM} = \frac{1\,\text{beat}}{\text{distance in mm}} \times \text{feed rate in mm/s} \times 60\,\text{s/min}$
Worked Example from Transcript:
- Measured Peak-to-Peak distance: $32\,mm$ .
- Feed rate: $20\,mm/s$ .
- Step 1: $\frac{1\,\text{beat}}{32\,mm}$
- Step 2: $\frac{1}{32} \times 20\,mm/s = 0.625\,\text{beats/s}$
- Step 3: $0.625 \times 60\,s/min = 37.5\,\text{BPM}$ .
Double-Checking Values (Heuristics):
- If a heart rate were exactly $60\,\text{BPM}$ , there would be one beat every second.
- At a feed rate of $20\,mm/s$ , a $60\,\text{BPM}$ rate would show a beat exactly every $20\,mm$ .
- If the measure is $40\,mm$ (beat every $2\,s$ ), the rate is $30\,\text{BPM}$ .
- Because $32\,mm$ is between $20\,mm$ and $40\,mm$ , a calculated rate of $37.5\,\text{BPM}$ is mathematically consistent.

Tachycardia:
- Defined as a resting heart rate greater than $100\,\text{BPM}$ .
- This occurs when the SA node's basic rhythm (typically around $100\,\text{BPM}$ ) is not dampened by the parasympathetic nervous system, or when the sympathetic nervous system is activated (e.g., exercise or excitement).
Bradycardia:
- Defined as a resting heart rate less than $60\,\text{BPM}$ .
- A rate this slow often suggests the SA node is not setting the rhythm, and the heart may be following a junctional rhythm (regulated by the AV node, usually $40-60\,\text{BPM}$ ).
- Athletic bradycardia: Highly conditioned individuals may have heart rates significantly lower than $60\,\text{BPM}$ due to efficient heart pumping and healthy tissue requiring less oxygen at rest.
- Anecdotal example: A professional tennis player was recorded with a resting heart rate of approximately $35\,\text{BPM}$ .
- Limits of life: Heart rates below $30\,\text{BPM}$ are generally considered incompatible with life.

Junctional Rhythm:
- Occurs when the SA node is inactive or "knocked out."
- Characteristic ECG feature: Missing P waves.
- The AV node takes over pacing the ventricles, followed by a T wave.
Heart Block:
- Occurs when the electrical signal from the SA node fails to reach the ventricles via the AV node, Bundle of His, or bundle branches.
- Manifests as missing P waves or "double P waves" where the signal is delayed or blocked.
- Example pattern: A QRS-T sequence followed by a P wave, then a long delay, then another P wave before a QRS complex finally triggers.
Ventricular Fibrillation (V-fib):
- Characterized by a sporadic, chaotic ECG tracing with "no rhyme or reason."
- The myocardium does not contract in unison; instead, various parts of the ventricles fire dyssynchronously.
Normal Sinus Rhythm:
- The healthy, standard repeating pattern of P wave, QRS complex, and T wave.
Left Ventricular Hypertrophy:
- Diagnosed by measuring the height (voltage) of the waves on the graph paper.
- Indicated by an extremely large QRS complex, specifically with a height of $22\,mm$ or greater.
- The increased wave height is caused by a larger muscle mass undergoing depolarization.