Measuring Blood Pressure Automatically

Measuring blood pressure means that we'll be calculating the level of strength that your heart uses to pump blood around your body. This reading gives us a lot of valuable information about the patient's physiological state and is an excellent indicator of complications and deterioration. These notes will discuss the knowledge and nursing skills required when measuring blood pressure automatically. You can find the notes on measuring blood pressure automatically over here.


Measuring Blood Pressure Automatically: Introduction

Before we get into the how and why we measure blood pressure, we need to understand what we're actually measuring. Let's start off with some basic terminology; Millimetres of Mercury (mmHg) is the unit used to measure blood pressure. Systolic Blood Pressure is the pressure exerted on the arteries during ventricular systole, which means during heart contraction. The average systolic blood pressure in a healthy adult is 120mmHg, and it's written first whenever one documents blood pressure. Diastolic Blood Pressure is the pressure exerted on the arteries during ventricular diastole, which means during heart relaxation. The average diastolic blood pressure in a healthy adult is 80mmHg, and it's written last whenever one documents blood pressure.

Hypotension is the medical term for low blood pressure, and it refers to a systolic level of less than 90mmHg. Hypertension, on the other hand, is the medical term for high blood pressure, and it's divided into three stages:

  • Stage 1 Hypertension is when the systolic is between 130-139mmHg, while the diastolic is 80-89mmHg.

  • Stage 2 Hypertension has a systolic of 140 or higher and a diastolic of 90 or higher.

  • Stage 3 is called Hypertensive Crisis as it can be life-threatening. This is when the systolic is more than 180mmHg, and the diastolic is more than 120mmHg.

Measuring Blood Pressure Automatically: How it Works

If you've ever seen an automatic blood pressure machine, you'll notice that it has two important things:

  1. The Cuff that goes around a patient's arm, and

  2. The Device which has all the important features as you can see in the picture below.

When you press start on the device, the cuff starts to inflate automatically squeezing the patient’s arm. As the pressure increases, the cuff presses down on the brachial artery until the blood flow to the artery is completely occluded. Once it reaches that point, the cuff would typically inflate to an additional 20mmHg and then slowly start to deflate.

Once the cuff deflates slightly below the systolic pressure, blood starts to return into the artery and this return of blood causes a vibration on the arterial wall. The automatic device senses this vibration and registers the first vibration as the Systolic Blood Pressure. As the cuff continues to deflate, blood will eventually flow normally through the artery and the vibrations will stop. The automatic device senses the lack of vibration and registers the last vibration as the Diastolic Blood Pressure.

In most cases, automatic blood pressure devices are relatively accurate. Their design makes it simple and efficient to use, and in today’s world, most clinics and hospitals choose an automatic device vs a manual device. It has also made it possible for patients with blood pressure conditions to regularly monitor their blood pressure in the comfort of their own homes.

However, despite its many advantages, an automatic blood pressure device can give out inaccurate readings if the patient has an arrhythmia as the condition creates additional vibrations in the arteries which interfere with the natural vibration that the automatic device would be measuring. Moreover, if the patient has a very low pulse rate (extreme bradycardia) it would be too weak to be detected and the device would not be able to give out a reading.

Measuring Blood Pressure Automatically: The Procedure

Start by gathering all the equipment needed to check a blood pressure automatically, including:

  1. The proper sized cuff: Cuffs come in many sizes ranging from tiny paediatric cuffs to extra large adult cuffs, and choosing the right size is crucial to getting an accurate blood pressure reading. As a guide, the cuff is the proper size if it encircles more than 80% of the patient's arm.

  2. The blood pressure device: Check that it has batteries and that it functions well

Before going over to the patient, you should follow infection control guidelines and clean all the equipment with an alcohol swab. Get the patient's file and then go over to their bed.

Introduce yourself to the patient, apply an alcohol hand rub and explain why you'll be checking their blood pressure and what they can expect while you're doing it. Ask the patient about the risk factors that might affect their blood pressure measurement, including whether they've

  • been consuming alcohol, caffeinated drinks or tobacco in the last 15 minutes

  • having strenuous exercise before the procedure

  • been experiencing a high level of stress

  • been starved for a few hours (typically done before surgery or general anaesthesia)

Next, position your patient comfortably; preferably, they should be seated with their back supported and their legs uncrossed with their feet touching the ground. Unless contraindicated, place their left hand onto a table and use a pillow for support to keep the arm level with the heart. If the arm is placed higher or lower than the heart, the blood pressure reading will not be accurate.

Check your patient’s pulse, if it’s irregular or very faint you might need to switch to a manual blood pressure device. Before placing the cuff around their arm, you should ask them to remove tight and bulky clothing and assess the arm for contraindications; these might include:

  • lymphoedema

  • continuous IV infusions

  • trauma or previous surgery

  • fistula or venous problems

If there are no contraindications and the patient is seated comfortably, you can go ahead and start the procedure. All cuffs will have an Artery Index Marker, typically drawn on the cuff as a bold straight line. Align this marker with the brachial artery, and wrap the cuff around the patient's arm roughly 2cm above the brachial artery.

Most devices have the same button to switch on the device and start recording the blood pressure, if that’s not the case then you’ll need to first press the ON button and then the START button. The cuff will start to inflate automatically, and then deflate as it reads the blood pressure. The reading will then show on the monitor with the systolic blood pressure recorded at the top, and the diastolic blood pressure under it. (Some devices might show the systolic on the left and diastolic on the right).

Apply hand hygiene again, and document the reading on the patient's observation chart, noting any discrepancies from previous readings. You can use the hospital's protocol or the American Heart Association Blood Pressure Chart as a guide to understanding your patient's measurement. If the reading is not within a normal range, or if you note a large discrepancy from previous readings, you should immediately inform your senior nurse and the patient's medical team to implement a care plan.

However, you should keep in mind that sometimes an extreme value can result from an error during the measurement. Such errors can occur if the:

  • patient is agitated and restless during the measurement

  • equipment is defective

  • cuff is deflated too quickly, not level with the heart or the incorrect size

  • technique was performed poorly, or the observer was biased

  • patient has an irregular pulse


Great, that's all you need to know to pass your clinical nursing test on how to measure blood pressure automatically! If you'd like to read the rest of the notes on clinical nursing skills, you can follow the link here. If you have any questions, don't hesitate to send us a message or join our Facebook community

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  • Jevon P (2020) Blood pressure 2: procedures for measuring blood pressure. Nursing Times [online]; 116: 8, 26-28 ,t6 n

  • Rehman S, Nelson VL. Blood Pressure Measurement. [Updated 2021 Sep 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: