What Should You Know about ECG Monitoring?
An electrocardiogram (ECG) is a recording of waveforms that reflects the electrical activity of the heart. Each heart beat involves 5 major stages, reflected in P, QRS and T waves, the size and shape of which help in assessing normality of heart functions.
What Does the ECG Wave Represent?
The sinoatrial (SA) node, the natural pacemaker of the heart, creates electrical impulses which trigger each heartbeat. As electrical impulses flow through the heart, the atria and the ventricles contract and pump blood out to lungs and body. Meanwhile, the process of depolarization (action state) and repolarization (resting state) occurs.
- The P wave represents atrial depolarization.
- The QRS complex represents ventricular depolarization.
- The T wave represents ventricular repolarization.
Throughout the cardiac cycle, blood pressure increases and decreases.
Why is ECG Monitoring Necessary?
- Problems with Heart Beat Frequency
The frequency of the cardiac cycle is described by the heart rate, which is expressed as beats per minute (bpm). The cases below reflect the abnormality of heartbeat frequency:
- Bradycardia: if a heart rate is too slow.
In this case, the heart does not supply sufficient blood to the body.
- Tachycardia: if a heart rate is too fast.
In this case, the heart works too hard without any particular reason.
- Ventricular fibrillation: if the heart cannot pump.
In this case heart muscles just quiver rather than contract properly. Ventricular fibrillation is a medical emergency which requires prompt interventions.
- Problems with Heart Beat Sequence
Each heart beat has a particular course: the SA (sinoatrial) node indicates the impulse which spreads across both atria, then it gets to the AV (atrioventricular) node after which the AV bundle carries it to the ventricles.
The heart contracts: pumps blood to the lungs and the body then it rests for a short time (diastole) to start the whole procedure again. Wrong heart beat sequence may result in premature contractions (early beat).
- The Spread of Impulse is Distracted (tissue damage)
The spread of blood flow to the heart muscle may be distracted due to ischemia, which is a lack of oxygen to the heart in most cases due to clogged arteries.
- The Heart cannot Contract (tissue death)
The heart may not contract due to damage to the heart that involves the death of a certain amount of heart muscle (infarction).
What is Considered during an ECG Examination?
- QRS Height and Width
High and broad QRS may signal an old myocardial infarction.
- Shape and Duration of Consecutive Heart Cycles
The heart might beat too quickly (tachycardia), too slowly (bradycardia) or in an unpredictable way (atrial fibrillation).
- ST Segment
ST segment should be flat or slightly upsloping in shape, and it should be on the baseline level. ST elevation may predict myocardial infarction while ST depression may signal ischemia (restriction of blood supply to tissues, causing lack of oxygen).
- Heart Rate Variability (HRV)
HRV describes the cyclic variations in heart rate (beat-to-beat RR intervals). Reduced heart rate variability indicates a higher risk for adverse cardiac events.
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