Editing Electrocardiography (section)

===Axis===
[[File:Rapid Axis Vector.svg|thumb|Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.]]
The heart has several axes, but the most common by far is the axis of the QRS complex (references to "the axis" imply the QRS axis).
Each axis can be computationally determined to result in a number representing degrees of deviation from zero, or it can be categorized into a few types.<ref>{{Cite web |title=Sample records for qrs complex relationship |url=https://www.science.gov/topicpages/q/qrs+complex+relationship}}</ref>

The QRS axis is the general direction of the ventricular depolarization wavefront (or mean electrical vector) in the frontal plane.
It is often sufficient to classify the axis as one of three types: normal, left deviated, or right deviated.
Population data shows that a normal QRS axis is from −30° to 105°, with 0° being along lead I and positive being inferior and negative being superior (best understood graphically as the [[hexaxial reference system]]).<ref>{{Cite book |title=Chou's electrocardiography in clinical practice : adult and pediatric |url=https://archive.org/details/chouselectrocard00macc |url-access=limited |last1=Surawicz |first1=Borys |last2=Knillans |first2=Timothy |date=2008 |publisher=Saunders/Elsevier |isbn=978-1416037743 |edition=6th |location=Philadelphia, PA |page=[https://archive.org/details/chouselectrocard00macc/page/n277 12]}}</ref>
Beyond +105° is [[right axis deviation]] and beyond −30° is [[left axis deviation]] (the third quadrant of −90° to −180° is very rare and is an indeterminate axis).
A shortcut for determining if the QRS axis is normal is if the QRS complex is mostly positive in lead I and lead II (or lead I and aVF if +90° is the upper limit of normal).<ref>{{Citation |last1=Kashou |first1=Anthony H. |title=Electrical Right and Left Axis Deviation |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK470532/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29262101 |access-date=2022-10-28 |last2=Basit |first2=Hajira |last3=Chhabra |first3=Lovely}}</ref>

The normal QRS axis is generally ''down and to the left'', following the anatomical orientation of the heart within the chest. An abnormal axis suggests a change in the physical shape and orientation of the heart or a defect in its conduction system that causes the ventricles to depolarize in an abnormal way.<ref name="Conquering the ECG"/>

{| class="wikitable"
|-
! Classification
! Angle
! Notes
|-
| Normal
| −30° to 105°
| Normal
|-
| [[Left axis deviation]]
| −30° to −90°
| May indicate [[left ventricular hypertrophy]], [[left anterior fascicular block]], or an old inferior STEMI
|-
| [[Right axis deviation]]
| +105° to +180°
| May indicate [[right ventricular hypertrophy]], [[left posterior fascicular block]], or an old lateral STEMI
|-
| ''Indeterminate axis''
| +180° to −90°
| Rarely seen; considered an 'electrical no-man's land'
|}

The extent of a normal axis can be +90° or 105° depending on the source.