A heart rate of 37 beats/min is dangerously slow and needs urgent medical assessment now, especially in someone with a mitral-valve condition or valve replacement.

Call emergency services / go to the emergency department immediately if she has any of these: fainting or near-fainting, dizziness, confusion, extreme weakness, chest pain, shortness of breath, blue/gray lips, sweating, very low blood pressure, or reduced responsiveness. Do not have her drive.

Even if she feels reasonably well, contact her cardiologist urgently today or seek emergency evaluation. A rate this low can arise from a conduction block or from medicines such as beta-blockers, digoxin, diltiazem/verapamil, amiodarone, etc., and needs an ECG, blood pressure/oxygen check, medication review, electrolytes, and sometimes pacing. The AHA adult life-support guidance treats symptomatic bradycardia, generally below 50/min, with signs of poor circulation as an emergency.

Regarding INR: 3.09 may be within the prescribed target for some mechanical mitral valves, often target INR 2.5-3.5, but the correct range depends on her exact valve and her cardiologist's plan. Do not change, skip, or double warfarin on your own. INR 3.09 does not explain a pulse of 37.

While arranging help:

A sudden fall to 37/min can happen when the heart’s normal electrical signal is slowed or blocked. Possible causes include:

For someone with valve disease/surgery, this needs an ECG urgently to tell whether it is a medication-related slow sinus rhythm or a heart block. The current AHA guidance lists ischemia, hypoxia, electrolyte disturbance, acidosis, and hypothermia among causes clinicians must urgently evaluate in acute bradycardia, while supporting circulation (AHA adult life-support guidance).

INR 3.09 is unlikely to be the direct reason her heart rate is 37. Do not change warfarin yourself. But please treat this as urgent: call emergency services now if the 37 rate is current or repeat-confirmed, especially if she has dizziness, fainting, confusion, breathlessness, chest discomfort, weakness, or low blood pressure. Do not wait to see whether it settles or give/withhold heart medicines without an urgent clinician's direction.

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