Editing Electrocardiography (section)

===Rate and rhythm===
In a normal heart, the heart rate is the rate at which the [[sinoatrial node]] depolarizes since it is the source of depolarization of the heart.
Heart rate, like other [[vital signs]] such as blood pressure and respiratory rate, change with age.
In adults, a normal heart rate is between 60 and 100 bpm (normocardic), whereas it is higher in children.<ref>{{cite journal |last1=Fleming |first1=Susannah |last2=Thompson |first2=Matthew |last3=Stevens |first3=Richard |last4=Heneghan |first4=Carl |last5=Plüddemann |first5=Annette |last6=Maconochie |first6=Ian |last7=Tarassenko |first7=Lionel |last8=Mant |first8=David |title=Normal ranges of heart rate and respiratory rate in children from birth to 18 years of age: a systematic review of observational studies |journal=The Lancet |date=March 2011 |volume=377 |issue=9770 |pages=1011–1018 |doi=10.1016/S0140-6736(10)62226-X |pmc=3789232 |pmid=21411136 }}</ref>
A heart rate below normal is called "[[bradycardia]]" (<60 in adults) and above normal is called "[[tachycardia]]" (>100 in adults).
A complication of this is when the atria and ventricles are not in synchrony and the "heart rate" must be specified as atrial or ventricular (e.g., the ventricular rate in [[ventricular fibrillation]] is 300–600 bpm, whereas the atrial rate can be normal [60–100] or faster [100–150]).{{fact|date=November 2024}}

In normal resting hearts, the physiologic rhythm of the heart is [[normal sinus rhythm]] (NSR).
Normal sinus rhythm produces the prototypical pattern of P wave, QRS complex, and T wave.
Generally, deviation from normal sinus rhythm is considered a [[cardiac arrhythmia]].
Thus, the first question in interpreting an ECG is whether or not there is a sinus rhythm.
A criterion for sinus rhythm is that P waves and QRS complexes appear 1-to-1, thus implying that the P wave causes the QRS complex.<ref name="Conquering the ECG"/>

Once sinus rhythm is established, or not, the second question is the rate.
For a sinus rhythm, this is either the rate of P waves or QRS complexes since they are 1-to-1.
If the rate is too fast, then it is [[sinus tachycardia]], and if it is too slow, then it is [[sinus bradycardia]].

If it is not a sinus rhythm, then determining the rhythm is necessary before proceeding with further interpretation.
Some arrhythmias with characteristic findings:
* Absent P waves with "irregularly irregular" QRS complexes are the hallmark of [[atrial fibrillation]].
* A "saw tooth" pattern with QRS complexes is the hallmark of [[atrial flutter]].
* A [[sine wave]] pattern is the hallmark of [[ventricular flutter]].
* Absent P waves with wide QRS complexes and a fast heart rate are [[ventricular tachycardia]].

Determination of rate and rhythm is necessary in order to make sense of further interpretation.