Final answer:
For a client with supraventricular tachycardia and vital signs indicating hemodynamic instability, synchronized cardioversion is the immediate action required by the nurse to restore a normal heart rhythm and stabilize the client.
Step-by-step explanation:
A client experiencing supraventricular tachycardia with a heart rate of 200 beats per minute and a blood pressure of 78 over 40 requires immediate intervention. Due to the critical state of the client characterized by the dangerously low blood pressure and high respiratory rate, the appropriate action to take is to perform synchronized cardioversion. This procedure, typically undertaken when a rapid heart rate is accompanied by hypotension, symptoms of shock, or is unresponsive to medication, involves a timed electrical shock to the heart to restore a normal rhythm.
In this scenario, increasing IV fluids could be considered as part of the overall management but is not the immediate priority given the client's rapid heart rate and hypotensive state. Instead, synchronized cardioversion directly addresses the immediate threat posed by the aberrant heart rate and potential instability of the client.