Invasive BP Monitoring Flashcards
Arterial pressure waveform: what is
- Anacrotic limb?
- Dicrotic notch?
- Systolic upstroke
2. On the down stroke represents aortic valve closure
What clinical implication does position of the dicrotic notch indicate?
Indication of peripheral vascular resistance - the lower the notch position the lower the peripheral vascular resistance.
Which part of the arterial waveform indicates stroke volume?
Area under the systolic component (beginning of upstroke to dicrotic notch) is an index of stroke volume.
What clinical conditions affect arterial waveform?
What appearances do they have?
Hypotension - flattened, rounded, dicrotic notch maybe absent
Hypertension - upstroke/ ancrotic rise will be sharper with taller amplitude
Arrhythmia - ectopic beats
Hypovolaemia - increased swing in the waveform during the respiratory cycle, mainly due to decreased BP during the expiratory phase of positive pressure ventilation.
What are the components of the arterial blood pressure system?
- Pressurised bag of NaCl 0.9%
- Arterial catheter
- Pressurised fluid filled tubing system
- Pressure transducer
- Microprocessor and display unit
What is the purpose of the fluid filled tubing system?
Provide a means of transmitting the pressure generated in the artery to the transducer.
How is thrombus formation in the cannula prevented?
The fluid filled system contains normal saline or heparinised saline that has a constant flow of 2-4ml/hr via the flushing device that prevents this.
What pressure is fluid filled system maintained at?
300 mm Hg
What is the resonant frequency?
It is directly proportional the the diameter of the catheter and square root of compliance.
The resonance frequency is a guide to the frequency at which oscillation in the system is maximal.
What features are needed to minimise resonance and damping?
Short stiff tubing filled with saline and free of air bubbles.
What is a pressure transducer?
How does it work?
Device that changes the mechanical energy of pressure to electrical energy.
Consists of a low compliance silicone diaphragm that stretches in response to pressure changes within the column of fluid.
Uses a Wheatstone bridge circuit which converts the pressure into electrical signal.
Dependent changes in the resistance and the dependent changes in current are measured and displayed as systolic, diastolic and MAP.
What is under damping and how does it affect arterial BP measurement?
Seen as the presence of overshoot spike, gives falsely high systolic readings and low diastolic readings, however MAP not affects.
Due to increased resonance- stiff non compliant diaphragm and tubing.
How does over-damping affect accuracy of invasive BP monitoring?
results in a smoothed out trace without displaying sharp changes
- leads to under reading of systolic BP and over reading of diastolic BP.
Caused by loss of pressure in the fluid filled tubing system, soft complaint tubing, numerous connectors or kinks in the cannula, air bubbles and blood clots.
How can you ensure the transducer system is optimally damped?
A square wave test is used, flush the system by applying 300 mm Hg. In an optimally damped system there will be two or three oscillations before settling to zero.
Over damped settles to zero without any oscillations.
Under damped system oscillates for more than 3-4 cycles before settling to zero.
What is the the damping coefficient?
optimal damping coefficient?
The damping co-efficient indicates how fast the oscillating system will come to rest.
Optimal - 0.67
This is where if displaced from its equilibrium position, the mass would return to that position quickly with some over shoot.
