Both of these rhythms originate from the heart's normal pacemaker, travel the normal conduction pathway, and produce a normal-looking complex. The only abnormality is speed: sinus bradycardia is a sinus rhythm below 60 beats per minute, sinus tachycardia is a sinus rhythm above 100. That is why they are the two rhythms most often mismanaged. A rhythm that is structurally normal but fast or slow is almost always the heart responding correctly to something else. The sinus node speeds up because the body is demanding more output — from fever, pain, hypovolaemia, hypoxaemia, anxiety, or infection. It slows because vagal tone is high, because a medication is acting on it, or because the node itself is failing. The clinical error that follows is treating the number. Slowing a compensatory tachycardia removes the compensation and can drop cardiac output sharply in a client who was depending on it. The correct move in nearly every case is to find and treat what the sinus node is responding to. The nurse's contribution is therefore diagnostic rather than reactive: establish whether the rate is...