Blood pressure readings can be falsely high or falsely low due to various causes. You'll need to assess the situation and respond accordingly.
REACTION AND CAUSES NURSING ACTIONS
FALSE-HIGH READING
  • Cuff too small
  • Cuff wrapped too loosely, reducing its effective width
  • Slow cuff deflation, causing venous congestion in the arm or leg
  • Tilted mercury column
  • Poorly timed measurement—after patient has eaten, ambulated, appeared anxious, or flexed arm muscles
  • Make sure that the cuff bladder is 20% wider than the circumference of the arm or leg being used for measurement.
  • Tighten the cuff.
  • Never deflate the cuff more slowly than 2 mm Hg/heartbeat.
  • Read pressures with the mercury column vertical.
  • Postpone blood pressure measurement or help the patient relax before taking pressures.
FALSE-LOW READING
  • Incorrect position of arm or leg
  • Mercury column below eye level
  • Failure to notice auscultatory gap (sound fades out for 10 to 15 mm Hg, then returns)
  • Inaudible low-volume sounds
  • Make sure the arm or leg is level with the patient's heart.
  • Read the mercury column at eye level.
  • Estimate systolic pressure by palpation before actually measuring it. Then check this pressure against the measured pressure.
  • Before reinflating the cuff, instruct the patient to raise the arm or leg to decrease venous pressure and amplify low-volume sounds. After inflating the cuff, tell the patient to lower the arm or leg. Then deflate the cuff and listen. If you still fail to detect low-volume sounds, chart the palpated systolic pressure.