Blood pressure is constantly monitored in patients admitted to the intensive care unit, often with an indwelling internal arterial catheter. This catheter sits inside the artery and gives a continuous reading of the pressure inside the artery.

An ICU patient has one of these arterial lines. The nurse takes blood pressure reading at the same time with a manual blood pressure cuff. The blood pressure cuff consistently reads 5 to 10 mmHg less than the arterial catheter.