Haemodynamic monitoring: Non-invasive and invasive blood pressure measurement in modern cardiology
Precise monitoring of non-invasive and invasive blood pressure forms the foundation of modern haemodynamic diagnostics. These two complementary measurement techniques enable cardiologists and intensive care physicians to detect life-threatening changes in blood pressure at an early stage and initiate appropriate therapeutic measures.
Basic principles of blood pressure monitoring
Physiological basics
Arterial pressure monitoring is a cornerstone of haemodynamic monitoring in acutely or critically ill patients. Continuous monitoring of blood pressure is of utmost importance for the early detection and treatment of hypotension and hypertension. Both hypotension and hypertension can impair the function of vital organs such as the brain, heart and kidneys.
Haemodynamic stability: Blood pressure monitoring must be tailored to the individual patient. In stable, low-risk patients, intermittent oscillometric blood pressure measurements are usually sufficient. Patients at risk of haemodynamic instability should be monitored by continuous blood pressure measurement.
Non-invasive blood pressure monitoring
Oscillometric measurement technique
Oscillometric method: In automatic blood pressure measurement, a cuff is inflated to a pressure that blocks arterial flow to the extremity. The pressure is then slowly released while small pressure fluctuations in the cuff are closely monitored by the device.
The oscillometric technique detects characteristic pressure oscillations:
- Systolic detection: When the cuff pressure drops below the systolic pressure, arterial flow is resumed.
- Diastolic assessment: The frequency of oscillations increases and then decreases as the flow through the vessel becomes smooth.
- Mean arterial pressure: This is measured directly and serves as the basis for calculating systolic and diastolic values.
Technical specifications of modern NIBP systems
Automated pressure control: Modern electronic blood pressure monitors have internal pumps for inflating the cuff. Inflation is monitored electronically based on pulse and pressure, with the device providing a display of blood pressure.
Calibration requirements: To maintain accuracy, calibration must be checked regularly, unlike the inherently accurate mercury manometer. Devices vary greatly in accuracy and should be checked at specified intervals and recalibrated as necessary.
Oscillometric measurement requires less skill than the auscultatory technique and may be suitable for use by untrained personnel and for automated patient monitoring at home.
Continuous non-invasive systems
Volume clamping method: Continuous non-invasive arterial blood pressure measurement (CNAP) combines the advantages of two clinical “gold standards”: it measures blood pressure continuously in real time like the invasive arterial catheter system and is non-invasive like the standard upper arm sphygmomanometer.
Applanation tonometry: This technique has been refined and is now able to assess mean arterial pressure in the radial artery, enabling the calculation of diastolic and systolic arterial pressure using population-based algorithms.
Invasive blood pressure monitoring
Arterial catheterisation
Arterial catheter systems: Invasive blood pressure measurement via arterial catheterisation is considered the clinical reference method (criterion standard). In routine clinical practice, it is often performed during high-risk surgery and in intensive care.
The usual method of invasive blood pressure monitoring involves the percutaneous insertion of a small (18- to 22-gauge) plastic catheter into a peripheral artery. The catheter is physically connected to an electronic pressure transducer and a display unit via high-pressure plastic tubing.
Technical components of invasive systems
Pressure transducer technology: The transducer is a sterile, miniaturised, self-contained assembly that contains the electromechanical components in a transparent plastic housing. Most transducers have an integrated mechanism for continuous, slow flushing of the sterile solution through tubes and catheters to prevent clotting.
Signal transmission: Each heart contraction exerts pressure, which causes mechanical movement of the flow within the catheter. The mechanical movement is transmitted via rigid, fluid-filled tubes to a transducer, which converts this information into electrical signals.
The following components are required for invasive blood pressure systems:
- High-precision catheters: Different gauge sizes for different vessels
- Fluid-filled tube systems: Rigid transmission of mechanical signals
- Electronic pressure transducers: Conversion into digital signals
- Monitoring monitors: Real-time display of waveforms and numerical values
Anatomical access routes
Radial artery access: The radial artery is the most commonly catheterised site for blood pressure monitoring. Due to its low complication rate, the radial artery is the preferred site for invasive blood pressure monitoring.
Alternative access sites: Other proposed catheterisation sites for blood pressure monitoring include the axillary artery, dorsalis pedis artery, ulnar artery or posterior tibial artery. However, cannulation of an artery can be time-consuming, must be performed by a trained operator and is associated with potentially major complications.
Technical challenges and quality assurance
System attenuation and artefacts
Damping phenomena: System damping affects the accuracy of the data. This is the amount of resonance in the system and affects systolic and diastolic pressure while maintaining the correct mean arterial pressure.
Under-damping effects: If the system is insufficiently damped, there is an excess of resonance, which leads to an overestimation of systolic pressure and an underestimation of diastolic pressure.
Overdamping correction: If the system is overdamped, the systolic pressure will be falsely low, but the diastolic pressure will normally be accurate. Overdamping can be caused by a clot or fibrin build-up at the tip of the catheter.
Calibration and maintenance
Transducer positioning: Correct positioning of the transducer is critical for accurate blood pressure monitoring and patient care. The position of the transducer should be checked each time the operating table or patient bed is repositioned.
Zero calibration: The arterial pressure transducer was levelled and zeroed at the intersection of the anterior axillary line and the fifth intercostal space. The investigators then flushed the system of any air bubbles using a special inflated flushing system at 300 mmHg.
Clinical applications and indications
Perioperative monitoring
High-risk surgery: Indications for the insertion of an arterial catheter include the need for continuous blood pressure monitoring, the impracticality of non-invasive blood pressure measurements, or the need for repeated arterial blood sampling.
Intensive care: Continuous invasive blood pressure monitoring with an arterial catheter is still recommended for critically ill patients. Various standards, such as national anaesthesia societies, recommend blood pressure monitoring at least every 5 minutes in anaesthetised surgical patients.
Special patient populations
Haemodynamic instability: Patients at risk of haemodynamic instability should be monitored by continuous blood pressure measurement. Whether continuous non-invasive blood pressure monitoring can improve patient outcomes in specific patient populations or clinical settings is the subject of ongoing clinical research.
Cardiac surgery considerations: In cardiac surgery patients, cannulation of the brachial artery is more reliable than cannulation of the radial artery, especially after cardiopulmonary bypass.
Comparative evaluation of measurement techniques
Accuracy and reliability
Gold standard comparison: Direct measurement of blood pressure via arterial cannulation is considered the clinical reference method. Invasive blood pressure measurement with an arterial catheter, which allows continuous blood pressure measurements, detected almost twice as many episodes of hypotension.
Measurement deviations: Non-invasive oscillometric blood pressure measurement with an upper arm cuff tends on average to overestimate blood pressure during hypotension and underestimate it during hypertension, with significant bias and considerable variation.
Complication profiles
Advantages of the non-invasive method: Non-invasive auscultatory and oscillometric measurements are simpler and faster than invasive measurements, require less expertise in fitting, have virtually no complications and are less uncomfortable and painful for the patient.
Invasive risks: Potential problems include symptomatic or asymptomatic arterial thrombosis, infection, accidental injection of IV medications, nerve damage due to trauma or haematoma during placement, and exsanguination due to accidental separation.
Future prospects in blood pressure monitoring
The development of wearable sensors for continuous blood pressure measurement and derived cardiovascular variables has the potential to revolutionise patient monitoring. Innovative technologies such as continuous non-invasive haemodynamic monitoring (CNAP2GO) are showing promising results through direct blood pressure measurement using volume control technology.
Modern systems integrate advanced algorithms for real-time analysis of blood pressure waveforms, improved artefact detection and automatic quality control. These developments are helping to improve the accuracy of both measurement techniques while optimising clinical applicability.
The future of haemodynamic monitoring lies in the intelligent combination of non-invasive and invasive blood pressure measurement technologies, enabling clinicians to select the optimal monitoring strategy for each patient while maximising both safety and diagnostic accuracy.
Note: This article is for informational purposes only and is not a substitute for professional medical training. The selection of the appropriate blood pressure monitoring method should always be based on the individual patient’s situation and clinical requirements.