Which of the following is the most critical vital sign to assess in a patient with suspected cardiac arrest?

In a patient with suspected cardiac arrest, the most critical vital sign to assess is the pulse. This is because the presence or absence of a pulse is a direct indicator of cardiac function and circulation. In cardiac arrest, the heart is unable to pump effectively, which results in the absence of a palpable pulse rather quickly.

Assessing for a pulse helps responders determine whether the patient is in a state of cardiac arrest and requires immediate resuscitation efforts. If no pulse is detected, it signifies that the patient is not experiencing effective circulation, complicating the situation further as the chances of survival decrease with each passing minute without intervention.

While respiratory rate, heart rate, and blood pressure can provide useful information about a patient's overall condition, they are not as immediately indicative of cardiac arrest. For instance, during cardiac arrest, the heart rate will typically be absent, and blood pressure will fall, making an initial pulse check paramount for life-saving decisions.