Overview
Direct continuous measurement with an intra-arterial catheter is the gold standard for determining arterial blood pressure (BP). The arterial waveform provides valuable information about cardiovascular function and hemodynamic status.
Arterial Waveform Fundamentals
The arterial waveform results from ejection of blood from the left ventricle into the aorta during systole, followed by peripheral runoff during diastole.
Key Timing
The peak of the wave occurs just after the QRS complex of the ECG. The peak correlates with the systolic blood pressure as measured by a normal non-invasive cuff. The trough (i.e. the lowest reading before the next pressure wave) is the diastolic pressure. The mean arterial pressure (MAP) is calculated from the area under the pressure curve during a single beat.
Waveform Components
- Systolic phase: Characterised by a rapid increase in pressure to a peak, followed by a rapid decline. This phase begins with the opening of the aortic valve and corresponds to the left ventricular ejection.
- Dicrotic notch: Represents the closure of the aortic valve.
- Diastolic phase: Represents the run-off of blood into the peripheral circulation.
Clinical Note
Unusual radial arterial pressure waveforms may occur due to pathology in the ascending aorta or aortic valve (e.g. aortic dissection, aortic valve replacement).
Information Derived from Arterial Waveform
Direct Measurements
- Heart rate
- Systolic pressure
- Diastolic pressure
- Mean arterial pressure
- Pulse pressure (low in hypovolaemia, decreases in stroke volume (SV) or increases in systemic vascular resistance (SVR); high in increases in SV and decreases in SVR)
- Changes in amplitude associated with respiration
Waveform Shape Analysis
- Slope of upstroke represents aortic valve and left ventricular outflow tract (LVOT) flow
- Slurred or collapsing wave in aortic stenosis (AS)
- Rapid systolic decline in LVOT obstruction
- Bisferiens wave in hypertrophic obstructive cardiomyopathy (HOCM)
- Low dicrotic notch in states with poor peripheral resistance
- Position and quality of dicrotic notch as a reflection of the damping coefficient
- Slope of diastolic decline varies with resistance in the arterial tree (if SV is constant, diastolic runoff decreases sharply if SVR is low but is more gradual if SVR is high)
Factors Affecting Blood Pressure Measurement
Site of Arterial Catheterization
Arterial pressure waveforms change as the pressure wave moves from the aorta to the periphery. Peripheral arterial waveforms have:
- Higher systolic BP
- Steeper slope of the systolic upstroke
- Lower diastolic BP
- A lower and later dicrotic notch
- Wider pulse pressure compared with measurements obtained at the aortic root
Transducer Level
The pressure transducer should be leveled to a point that corresponds with the level of the heart:
- Aim for 5 cm behind the sternum in a supine patient, or the mid-axillary line as an approximation
- Optimal transducer position is achieved by turning the stopcock off to the patient and open to air
- Adjust the height of the stopcock to align with the level of the heart
- "Zero" the system on the monitor
- The transducer should be zeroed before insertion, after disconnection from the cable, and when BP accuracy is in question
Damping of the Arterial Waveform
Rapid-Flush Test
Whether the degree of damping (i.e. dynamic responsiveness) in a monitoring system is appropriate can be assessed at the bedside by the rapid-flush test. This test is performed by briefly opening and closing the valve in the continuous flush device (rapid flushing), which produces a square wave on the monitor. The square wave is followed by ringing (oscillations), then a return to baseline.
Over-damping
- No ringing is observed after a rapid flush
- Causes underestimation of systolic pressure and overestimation of diastolic pressure
- Common causes:
- Air bubbles in the connecting tubing
- Clots in the catheter or tubing
- Loose connections
- Kinks in the tubing
- Arterial spasm
- Narrow tubing
Under-damping
- Excess ringing is observed after a rapid flush
- Causes overestimation of systolic pressure and underestimation of diastolic pressure
- Common causes:
- Excessive tubing length
- Tubing connected with multiple stopcocks
- Patient factors:
- Tachycardia
- High cardiac output
- Hypothermia
Clinical Applications
Hemodynamic Monitoring
- Continuous blood pressure monitoring in critically ill patients
- Assessment of volume status and response to fluid therapy
- Evaluation of vasopressor and inotropic therapy effects
- Detection of arrhythmias and their hemodynamic consequences
Waveform Analysis in Pathology
- Aortic stenosis: Slurred or delayed upstroke, narrowed pulse pressure
- Aortic regurgitation: Wide pulse pressure, rapid diastolic runoff
- Hypertrophic obstructive cardiomyopathy: Bifid pulse (bisferiens)
- Cardiac tamponade: Pulsus paradoxus (exaggerated respiratory variation)
- Hypovolemia: Narrowed pulse pressure, respiratory variation
Clinical Pearl
Always consider the clinical context when interpreting arterial waveforms. Correlation with other hemodynamic parameters and physical examination findings is essential for accurate diagnosis and management.