Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)
Sinus rhythms are those that arise from the pacemaker in the sinus node (also called the SA node). The sinus pacemaker is our heart's normal pacemaker. We have three basic rhythms that originate in the sinus node. If the heart's rate is below 60 beats/min, we call it sinus bradycardia. If the rate is above 100 beats/min, the rhythm is called sinus tachycardia. If everything is just right, the rhythm is called normal sinus rhythm (NSR). (These rates apply to human adults. Pediatrics is a whole 'nother world.)
Of course, if all rhythms started in the sinus node, we probably wouldn't have a section devoted specifically to sinus rhythms. While the normal heart rhythm is of sinus origin, there are many arrhythmias that do not start in the sinus. Why is the sinus the best place to start?
One reason is that an impulse that originates in the sinus node follows a certain path that allows the atria to contract before the ventricles. If the two sets of chambers contracted at the same time, the atria would push against closed valves. Because there are no valves that separate the atria from their veins (vena cava and pulmonary vein), blood would flow backwards. This often causes the jugular veins in the neck to pulse.
Between the atria and the ventricles is the A.V. node. On the diagram of the firecracker (figure x-x), this node is represented by the yellow tunnel. Conduction of the electrical impulse slows down in the A.V. node, allowing the atria to completely depolarize before the ventricles so that atria may contract first. On the ECG, the isoelectric part between the P wave and the QRS complex demonstrates this pause. If the conduction through the A.V. node were slowed too much, the ECG would show this as a PR interval that is longer than 0.20 seconds.
How can you tell if an ECG rhythm originates in the sinus node? One thing to do is to look at the P wave. A rounded, upright P wave is often indicative of the sinus pacemaker. (Occasionally, the amplitude of an ECG is so great that the P wave will end up looking pointed even though it is sinus. Be very careful with this.) In non-sinus (i.e. ectopic) pacemakers, the P waves are often either notched, very pointed, inverted, or absent. To know what a normal rhythm looks like, it is best to be familiar with abnormal rhythms.
Don't be caught off guard by a weird looking QRS complex. Just because it doesn't look like the "textbook example" of the QRS does not mean that it is abnormal. A normal QRS is less than 0.12 seconds, but it is not limited to a single shape. Although the QRS complexes can differ from ECG to ECG, it should not be considered normal if they were to differ in the same ECG.
Normal sinus rhythm
A normal sinus rhythm (NSR) is the common, everyday rhythm. It must be, of course, sinus in origin. It must be regular and have a rate between 60 - 100 per minute. It must have a normal PRI and QRS duration.
Often times, an arrhythmia is described by saying the "underlying rhythm" and adding to that anything abnormal (e.g. sinus rhythm with first degree heart block). If any abnormalities exist, do not include the word normal when designating the underlying rhythm.
This is just like a normal sinus rhythm except that the rate is slower than 60 per minute.
Remember that the limits of 60 and 100 are arbitrary. A person who has a rate of 59 beats/min would not feel much different than he would at a rate of 60 beats/min. Some people (especially athletes) have a normal resting heart rate below 60. When President (George W.) Bush passed out after choking on a pretzel, it was revealed that his normal heart rate was around 45 beats/min. Many pundits became alarmed and criticized the president for not revealing his "disease" prior to the incident. The talking cardiologist heads were quick to point out that "disease IS as disease DOES"- that a disease is based on the patient and not always on standard one-size-fits-all guidelines. Some people may become symptomatic when there heart rate falls even though it is still above 60. We would call this relative bradycardia.
This would be a good time to reiterate : treat the patient, not the machine.
This is just like a normal sinus rhythm except that the rate is faster than 100 per minute.
Sinus tachycardia is common in everyone. If you are a paramedic or EMT, you will probably find plenty of patients in sinus tachycardia simply because they are nervous or excited. In more serious cases, a person in the early stages of shock may have a fast heart rate to compensate for the would-be-fall in blood pressure. Sinus tachycardia in itself is not always a bad thing; treatment should be aimed at the underlying cause.
Sinus arrhythmia is similar to normal sinus rhythm except that it the rate is irregular. It often matches the patient's breathing pattern, speeding up when the patient inhales and slowing down when the patient exhales.
Sinus arrhythmia can be relatively common in young and is often asymptomatic.
How irregular is irregular? The criterion used by many is that the longest R-R interval should differ from the shortest by at least 0.16 seconds.