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Rates >100 represent tachycardia. Whether they are sinus is dependent on other factors.

Sinus tachycardia

Sinus rhythm with a rate of  >100 per minute. RR interval less than 3 big boxes (0.6 secs).

Sinus Tachycardia can be seen in:

• Exercise
• Anemia
• Fever
• Anxiety
• Thyrotoxicosis
• Hemorrhage

and many other conditions.

Sinus Bradycardia

Sinus rhythm with a rate of < 60 per minute. RR interval more than 5 big boxes (1 sec)

Sinus Bradycardia can be seen in:

• Hypothyroidism
• Raised Intracranial tension
• Obstructive jaundice
• Sinus node dysfunction
• Athletes
• Medications
  • Digoxin
  • Beta blockers
  • Morphine

Premature atrial beats (P')

• Single irritable focus in atria discharging an impulse before the next regular SA node impulse
• P' wave is premature
• Contour of the premature P' wave is different
• P'-R is often longer - always different from PR of sinus beats
• QRS following P' is usually normal but may be non-conducted or conducted with ventricular aberrancy
• Pause following P' is not usually fully compensatory
• Encountered in
  • Normal
  • Anxiety
  • Tobacco
  • Alcohol
  • Amphetamine
  • Cocaine
  • Coronary artery disease

Paroxysmal Atrial Tachycardia.

• Single irritable focus in atria taking over pacemaker function of Heart
• Starts with a premature P wave
• Rate is between 140-250 per minute
• P waves are frequently not discernible
• Rhythm is perfectly regular, RR interval constant.
• QRS is usually unchanged

Paroxysmal Atrial Tachycardia. with block.

• P waves are abnormal
• Rate is usually under 200 / minute
• 2:1 or variable block is present

Multifocal atrial tachycardia.

• Frequent premature P waves of varying contour
• Rate is more than a 100/minute
• P'-P' intervals are irregular
• Most commonly seen in severe pulmonary disease

Atrial Flutter

• Irritable focus in atria contracting at a rate that exceeds the rate at which Ventricles are capable of contracting.
• Ventricles cannot exceed rate of 250/min.
• AV node filters and prevents too many impulses reaching Ventricles.
• Atrial rate is 250-350/minute and regular
• P waves are replaced by "F" waves (saw tooth pattern in II, III, AVF)
• Ventricular rate is lower. Frequently 2-1 or 4-1 A-V conduction is present
• QRS contour is unchanged
• Encountered in
  • Post MI
  • CAD
  • Hypertension
  • Rheumatic Heart disease
  • Pericarditis

Ventricular Tachycardia

• Single irritable focus in Ventricles
• Ventricular rate 150-250 bpm
• Wide QRS complexes
• No P wave
• RR intervals approximately equal
• A run of three or more VPB's in a row @ rate greater than 100/min
• AV dissociation may be present
• Atrial capture beats may be present
• Ventricular fusion beats may be present
• Retrograde concealed conduction
• VPB's with the same shape QRS

Idioventricular rhythm

• Looks like Ventricular tachycardia
• Except the rate is below 100/minute
• Functions as escape rhythm when higher pacemaker fail

  Atrial fibrillation 

• Multiple foci in atria begin initiating impulses.
• No unified atrial contraction, atria primarily quiver.
• AV node permits only few impulses to pass to ventricles at random.
• Atrial rate >400 bpm-- is fibrillating
• Absent P waves
• Ventricular rate variable
• If there is tachycardia then called rapid A-fib
• Irregularly irregular
• Encountered in
  • Post MI
  • CAD
  • ASD
  • COPD

Ventricular Fibrillation

• Multiple irritable foci in Ventricles, uncoordinated quivering of Ventricles.
• QRS complexes indistinct chaotic undulations.
• Boundary between the QRS and the ST-T wave of a ventricular tachycardia becomes ill-defined
• Isoelectric baseline not clear
• Low amplitude chaotic rapid ventricular rate >250 bpm
• P waves not conducted

Premature supraventricular complexes 

• P' wave contour different from P wave
• Temporary shortening of PR interval

Premature ventricular complexes (PVC)

• Single irritable focus in Ventricle discharging impulse before the next regular impulse delivered from sinus node.
• Basic rhythm interrupted by single premature ectopic beat.
• Absent P wave for the beat
• Wide QRS complex

• Occasional
• Bigeminy
• Trigeminy
• Quadrigeminy
• Runs of PVC