Studying the Basic ECG

INTRODUCTION

Normal sinus rhythm (NSR) is the characteristic rhythm of the healthy human heart. NSR is considered to be present if the heart rate is in the normal range, the P waves are normal on the ECG, and the rate does not vary significantly. If, however, the R-R interval is variable, the rhythm is called sinus arrhythmia. A sinus rhythm faster than the normal range is called a sinus tachycardia (see “Sinus tachycardia”), while a slower rate is called a sinus bradycardia (see “Sinus bradycardia”).
The sinoatrial (SA) node, due to its small mass, does not have a visible manifestation on the ECG. The behavior of the SA node, therefore, must be inferred from the atrial response. The upper right atrium is depolarized first, followed by the simultaneous depolarization of the remainder of the right and some of the left atrium, and finally by depolarization of the left atrial appendage. The characteristics of NSR are discussed here. The anatomy and electrophysiology of the SA node are considered elsewhere. (See “Anatomy and electrophysiology of the sinoatrial node”.)

NORMAL HEART RATE

The normal heart rate has been considered to be between 60 and 100 beats/min. However, the range (defined by 2 standard deviations from the mean) is between 43 and 93 beats/min in men and 52 and 94 beats/min for women (table 1) [1,2]. There is also important variability in age in young children. The normal heart rate is 110 to 150 beats/min in infants, with gradual slowing over the next 6 years.

NORMAL SINUS RHYTHM

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – (60-100 bpm)
• QRS Duration – Normal
• P Wave – Visible before each QRS complex
• P-R Interval – Normal (<5 small Squares. Anything above and this would be 1st degree block)
• Indicates that the electrical signal is generated by the sinus node and travelling in a normal fashion in the heart.

SINUS BRADYCARDIA

A heart rate less than 60 beats per minute (BPM). This in a healthy athletic person may be ‘normal’, but other causes may be due to increased vagal tone from drug abuse, hypoglycaemia and brain injury with increase intracranial pressure (ICP) as examples.

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – less than 60 beats per minute
• QRS Duration – Normal
• P Wave – Visible before each QRS complex
• P-R Interval – Normal
• Usually benign and often caused by patients on beta blockers

SINUS TACHYCARDIA

An excessive heart rate above 100 beats per minute (BPM) which originates from the SA node. Causes include stress, fright, illness and exercise. Not usually a surprise if it is triggered in response to regulatory changes e.g. shock. But if their is no apparent trigger then medications may be required to suppress the rhythm.

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – More than 100 beats per minute
• QRS Duration – Normal
• P Wave – Visible before each QRS complex
• P-R Interval – Normal
• The impulse generating the heart beats are normal, but they are occurring at a faster pace than normal. Seen during exercise

SUPRAVENTRICULAR TACHYCARDIA (SVT)

A narrow complex tachycardia or atrial tachycardia which originates in the ‘atria’ but is not under direct control from the SA node. SVT can occur in all age groups.

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – 140-220 beats per minute
• QRS Duration – Usually normal
• P Wave – Often buried in preceding T wave
• P-R Interval – Depends on site of supraventricular pacemaker
• Impulses stimulating the heart are not being generated by the sinus node, but instead are coming from a collection of tissue around and involving the atrioventricular (AV) node

ATRIAL FIBRILLATION

Many sites within the atria are generating their own electrical impulses, leading to irregular conduction of impulses to the ventricles that generate the heartbeat. This irregular rhythm can be felt when palpating a pulse

Looking at the ECG you’ll see that:

• Rhythm – Irregularly irregular
• Rate – usually 100-160 beats per minute but slower if on medication
• QRS Duration – Usually normal
• P Wave – Not distinguishable as the atria are firing off all over
• P-R Interval – Not measurable
• The atria fire electrical impulses in an irregular fashion causing irregular heart rhythm

ATRIAL FLUTTER

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – Around 110 beats per minute
• QRS Duration – Usually normal
• P Wave – Replaced with multiple F (flutter) waves, usually at a ratio of 2:1 (2F – 1QRS) but sometimes 3:1
• P Wave rate – 300 beats per minute
• P-R Interval – Not measurable
• As with SVT the abnormal tissue generating the rapid heart rate is also in the atria, however, the atrioventricular node is not involved in this case.

VENTRICULAR TACHYCARDIA

Looking at the ECG you’ll see that:

• Rhythm – Regular
• Rate – 180-190 Beats per minute
• QRS Duration – Prolonged
• P Wave – Not seen
• Results from abnormal tissues in the ventricles generating a rapid and irregular heart rhythm. Poor cardiac output is usually associated with this rhythm thus causing the pt to go into cardiac arrest. Shock this rhythm if the patient is unconscious and without a pulse.

VENTRICULAR FIBRILLATION (VF)

Disorganised electrical signals cause the ventricles to quiver instead of contract in a rhythmic fashion. A patient will be unconscious as blood is not pumped to the brain. Immediate treatment by defibrillation is indicated. This condition may occur during or after a myocardial infarct.

Looking at the ECG you’ll see that:

• Rhythm – Irregular
• Rate – 300+, disorganised
• QRS Duration – Not recognisable
• P Wave – Not seen
• This patient needs to be defibrillated!! QUICKLY

ASYSTOLE / FLATLINE

Looking at the ECG you’ll see that:

• Rhythm – Flat
• Rate – 0 Beats per minute
• QRS Duration – None
• P Wave – None
• Carry out CPR!!

 

About these ads