This is the third in a series of blog posts on the pitfalls of ACLS algorithms for adults with a pulse, and how a systematic approach to 12-lead ECG can help with ECG acquisition, interpretation, and application. The first looked at unstable bradycardia, the second stable bradycardia, and this one will look at unstable tachycardia. The ACLS algorithm for unstable tachyarrhythmia is simple: immediate cardioversion. This works well if it is a primary tachyarrythmia that is causing the instability (eg SVT or VT), but there are a number of pitfalls in this assumption. ECG acquisition: is it actually a tachy-arrhythmias? Artifact can mimic a tachy-arrhythmia. This can be identified by unaffected leads recorded at the same time, and narrow QRS complexes marching through the noise ECG interpretation: is the tachy-arrhythmia a primary electrical problem? There are other tachycardias in unstable patients that may fail to respond to cardioversion: AF (irregularly irregular rhythm), where the patient’s instability may be related to a secondary cause Sinus tachycardia (in response to a secondary cause): can best be identified by upright P waves in II and biphasic P waves in V1. ECG application: Is the rhythm causing the instability, or is there a secondary cause – including one that might be revealed by the 12-lead, like hyperkalemia or occlusion MI? For live and highly interactive ECG courses to elevate your skills - including online courses, or an in-person pre-conference course at CAEP, go to www.heartsECGcourse.com