Overview
This lecture covers the components of the arterial pressure tracing seen in ICU arterial lines, factors affecting waveforms, and how to assess waveform accuracy using the square wave test.
Arterial Pressure Tracing Basics
- The arterial line displays a pressure waveform called the arterial pressure tracing.
- The rapid upward slope is the systolic upstroke, indicating when the aortic valve opens and systole begins.
- The highest point is peak systolic pressure (SBP), representing maximum ventricular contraction.
- The waveform's downward movement after the peak is the systolic decline; heart still contracting but less forcefully.
- The dicrotic notch marks aortic valve closure and the end of systole.
- The area after the dicrotic notch is diastolic runoff, leading down to minimum diastolic pressure (DBP).
- Pulse pressure is the difference between systolic and diastolic pressures.
- The area under the waveform represents mean arterial pressure (MAP).
Clinical Significance & Advanced Analysis
- Slope of the systolic upstroke correlates with cardiac contractility.
- Area under the systolic curve reflects stroke volume; stroke volume times heart rate gives cardiac output.
- Slope of diastolic runoff correlates with vascular resistance.
Relationship to ECG
- QRS complex occurs before the systolic upstroke on the arterial tracing.
- There is a ~180 ms delay from the ECG R wave to the start of the arterial upstroke.
Distal Pulse Amplification
- The further from the heart the arterial line is placed, the higher the peak systolic pressure appears.
- Two waveforms add together: forward flow and rebound pressure wave, more pronounced distally.
Damping in Arterial Lines
- Damping refers to loss of waveform energy, affecting accuracy.
- Overdamped lines show flattened waveforms and falsely low SBP/narrow pulse pressure, often due to excess energy loss.
- Underdamped lines have exaggerated waves, falsely high SBP, and low DBP from too much energy.
- MAP readings are generally reliable despite damping issues.
Square Wave Test (Dynamic Response Test)
- Used to assess damping in an arterial line system.
- A quick flush produces a square wave with 1–2 oscillations if optimally damped.
- Overdamped: slow up/downstroke, no oscillation, and flattened tracing.
- Underdamped: fast up/downstroke, many oscillations, and exaggerated waveform.
Key Terms & Definitions
- Arterial Pressure Tracing — Visual representation of blood pressure changes in an artery over time.
- Systolic Upstroke — Rapid rise in pressure as the heart contracts.
- Dicrotic Notch — Small upward deflection marking aortic valve closure.
- Pulse Pressure — Difference between systolic and diastolic blood pressures.
- Mean Arterial Pressure (MAP) — Average pressure in arteries during one cardiac cycle.
- Damping — Reduction of waveform energy leading to altered pressure readings.
- Square Wave Test — Test to evaluate the accuracy (damping) of an arterial line system.
Action Items / Next Steps
- Review arterial line waveform components and damping concepts.
- Practice interpreting waveforms and performing the square wave test.
- Watch for details on troubleshooting damping problems in future lessons.