The RP interval is often so short that the inverted P waves may not be seen on the surface electrocardiogram (ECG) as they are buried within or immediately after the QRS complexes, appearing as a "pseudo R prime" wave in lead V1 or a "pseudo S" wave in the inferior leads. Atypical AVNRT In atypical AVNRT, the anterograde conduction is via the fast pathway and the retrograde conduction is via the slow pathway ("fast-slow" AVNRT). Multiple slow pathways can exist so that both anterograde and retrograde conduction are over slow pathways. ("slow-slow" AVNRT).Because the retrograde conduction is via the slow pathway, stimulation of the atria will be delayed by the slow conduction tissue and will typically produce an inverted P wave that falls after the QRS complex on the surface ECG. Diagnosis If the symptoms are present while the person is receiving medical care (e.g., in an emergency department), an ECG may show typical changes that confirm the diagnosis i.e., QRS duration <120 ms, unless a heart block is suspected. If the palpitations are recurrent, a doctor may request a Holter monitor (portable, wearable ECG recorder). Again, this will show the diagnosis if the recorder is attached at the time of the symptoms. In rare cases, disabling but infrequent episodes of palpitations may require the insertion of a small device under the skin that continuously record heart activity (an implantable loop recorder). All these ECG-based technologies also enable the distinction between AVNRT and other abnormal fast heart rhythms such as atrial fibrillation, atrial flutter, sinus tachycardia, ventricular tachycardia and tachyarrhythmias related to Wolff-Parkinson-White syndrome, all of which may have symptoms that are similar to AVNRT. Blood tests commonly performed in people with palpitations are: thyroid function tests (TFTs) – an overactive thyroid increases the risk of AVNRT electrolytes – disturbances in potassium, calcium and magnesium may predispose to AVNRT cardiac markers – if there is a concern that myocardial infarction (heart attack) has occurred either as a cause or as a result of the AVNRT; this is usually only the case if the patient has experienced chest pain Treatment Treatments for AVNRT aim to terminate episodes of tachycardia, and to prevent further episodes from occurring in the future. These treatments include physical manoeuvres, medication, and invasive procedures such as ablation. Arrhythmia termination An episode of supraventricular tachycardia due to AVNRT can be terminated by any action that transiently blocks the