Invasive BP monitoring Flashcards
Normal arterial pressure waveform
Components
What may affect amplitude and contour (4)
- Systolic upstroke (anacrotic limb)
- Slightly rounded top
- Downstroke with dicrotic notch (represents closure of aortic valve)
SBP measured at peak of waveform
DBP measured just before beginning of next systolic upstroke
Amplitude and contour depends on
* Site of arterial cannulation
* Contractility of venticle
* Compliance of arterial wall
* Competence of aortic valve
Difference in waveform between cannula in aorta and one in peripheral arteries (e.g. radial or DP)
As arterial pulse pressure wave procedes distally from more central arteries to peripheral (e.g. DP, radial):
* High frequency components e.g. dicrotic notch, may dissapear
* Upstroke becomes steeper
* Systolic peak increases
* Diastolic trough decreases
Why:
* Peripheral arteries contain less elastic fibres -> stiffer and less compliant
* Narrowing and bifurcation of arteries -> impedence of forward blood flow -> backward reflection of the pressur wave
What clinical information is provided by an arterial pressure waveform (6)
Slope of upstroke -> inotropic component, relates to myocardial contractility
Slope of down stroke and position of dicrotic notch -> peripheral vascular resistance.
Areas under systolic compartment of waveform (from beginning of upstroke to dicrotic notch) -> stroke volume
Systolic time -> myocardial oxygen demand
Diastolic time -> myocardial oxygen supply
Increased ‘swing’ during respiratory cycle, particularly in ventilated patient -> hypovolaemic state (mainly due ot greater decrease in BP during expiratory phase of PPV)
Information about myocardial contractility from arterial waveform
Slope of upstroke indicates inotropic component which relates to myocardial contractility
Information about peripheral vascular resistance from arterial waveform
Slope of downstroke and position of dicrotic notch give indication of peripheral vascular resistance
Reduced peripheral vascular resistance -> Lowering of position of dicrotic notch
Information about stroke volume from arterial waveform
Area under systolic component (beginning of upstroke to dicrotic notch) is index of stroke volume.
SV * HR (derived from arterial waveform) = cardiac output
Information about myocardial oxygen demand and supply from arterial waveform
Systolic time indicates myocardial oxygen demand
Diastolic time indicates myocardial oxygen supply
Effects on arterial waveform
* Hypotension
* Hypertension
* Arrhythmia
* Hypovolaemia
* Positive pressure ventilation
Hypotension -> waveform appears flattened, more rounded, dicrotic notch may be absent
Hypertension -> upslope (ancrotic rise) is sharper with taller amplitude
Arrhythmia -> reduced SV associated with ectopic beats produces waveform with smaller amplitude
Hypovolaemia -> increased swing during respiratory cycle
Positive pressure ventilation -> reduces stroke volume
Components of invasive BP measurement system (5)
- Pressurized bag of NaCl 0.9% (plain or heparinised) connected to pressurized fluid-filled tubing system with flushing device. Pressure usually maintained 300mmHg, at which pressure, flushing device is activated, allowing 2-4ml/h flush (to prevent occlusion of cannula). Fluid filled tubing system provides a means of transmitting the pressure generated in the artery to the transducer
- Arterial cannula (wide, short, stiff, parallel sided, made of teflon or polyurethane
- Pressure transducer (changes the mechanical energy of the pressure wave into electrical energy). Has a low compliance silicone diaphragm that stretches in response to pressure changes within the column of fluid
- Microprocessor and display unit
Arterial cannula size
- Small catheters (22 or 20) have lower incidence of thrombus formation, larger catheter (18 or 16G) are less likely to kink or become blocked
- Wide, short, stiff and parallel sided to minimize effects on the resonant frequency of the system
Pressurized fluid filled tubing system: appropriate specifications, impact on resonance frequency
Column of fluid generates a system capable of oscillation and reduces the frequency response of the system.
(Note: resonance is the tendency of the system to oscillate, damping is the tendency to resist oscillation)
To minimize resonance and damping, tubing should be
* Short (max length 120cm)
* Stiff
* Filled with saline and free of air bubbles
Tubing should be under pressure of approx 300mmgHg to reduce risk of thrombus formation.
Wheatstone bridge circuit in pressure transducer
Converts the pressure signal (from stretch in low compliance silicone diaphragm that stretches in response to pressure changes in the column of fluid) into electrical signal.
Wheatstone bridge circuit consists of four resistors:
* R1 and R2 have known resistance, and form one limb of the circuit
* R3 and R4 form other limb of circuit. R3 has a variable resistance, and resistance of R4 is unknown
* Resistance in R4 varies with the stretch of the transducer’s diaphragm
* By adjusting the variable resistance (R3), constant null deflection is maintained through the galvanometer and the two limbs of the bridge circuit are balanced (‘zeroed’).
* Resistance in R4 can be calculated, as R1/R2 = R3/R4
Resonance
Definitions, relevanvce to IBP measurement
Resonance:
* The pressure transducer system can be described as a second-order dynamic system, a harmonic oscillator
* Natural frequency of the system is the frequency at which it will oscillate freely (in the absence of sustained stimulus)
* All systems tend to resonante at a natural frequency - this frequency is known as the resonant or resonance frequency.
* Resonance = the amplification of signal when its frequency is close to the natural frequency of a system
An arterial waveform is a composite of many waveforms of increasing frequencies (harmonics), the amplitude of which decreases as frequency increases. At least 5 harmonics must be analysed to accurately represent the pulse pressure, and at least 8 harmonics must be analysed to represent the waveform with sufficient resolution to see the dicrotic notch
-> transducer system must have a natural frequency well above the 8th harmonic frequency of a rapid pulse, i.e. higher than 24Hz
At the resonant/natural frequency, oscillation in the system is maximal (resonance occurs). Resonant frequency is:
* Directly proportional to the diameter of the catheter (cannula) and to the square root of the system compliance
* Inversely proportional to the square root of the length of the tubing and density of the fluid
*Therefore shorter tubing -> higher natural frequency -> lower chance of resonance. *
Narrow tubing -> lower natural frequency -> increased chance of resonance
Damping
Definition, relevance to IABP measurement
= the process of the system absorbing the energy (amplitude) of oscillations
Damping coefficient = index of the tendency of the system to resist oscillations. Indicates how fast oscillating system will come to resit
* Damping coefficient = 0: no damping, oscillation will continue indefinitely
* Critically damped: damping coefficient =1. Just enough damping to prevent oscillations. If the mass in a critically damped system is displaced, it will return to its equilibrium position in an exponential fashion
* Over-damped (damping coefficient >1): long time to return to its original position.
In an adequately damped transducer system:
* Amplitude change due to resonance should not occur even when it is close to the system’s natural frequency
* Frequency response of the system (the range of frequencies over which there is minimal amplitude change from resonance) should encompass the clinically relevant range of frequencies
* Optimal damping coefficient = 0.7 (actually 0.67)
* If dispersed from equilibrium position, mass returns to that position quickly, with some overshoot.
Accuracy of invasive BP measurement system compared to NIBP
Invasive BP measurement system generally over-reads the systolic pressure, particularly in systolic hypertension and arteriopathic patients, compared to NIBP
How to ensure that transducer system is optimally damped
Square wave test (fast flush test)
* Flush system by applying a pressure of 300mmHg
* Results in square waveform, followed by oscilations
* Optimally damped: 2-3 oscillations before settling to zero
* Over-dampled: settles to zero without any oscillations
* Under-damped: oscillates for >3-4 cycles before settling to zero
Note also: in an under-damped system, without fast flush test, will see an overshoot spike (‘ringing’)
Calibrating the transducer system
Zero calibration, offset drift, gradient drift
Zero calibration elimiates the effect of atmospheric pressure on the monitored pressure. Elimitates the offset drift (zero drift). Transducer system is ‘zeroed’ to a reference point.
* For patient in supine position, mid-axillary line is appropriate reference point
* However if patient is in sitting position and interested in MAP at level of brain, different reference point should be chosen
Calibration at a higher pressure is required to eliminate gradient drift. Transducer is connected to an aneroid manometer using sterile tubing, through a three-way stopcock, and manometer pressure raised to 100 and 200mmHg. Monitor display should read same pressure as is applied to transducer
Which trace is normal
B.
A: over-damped
C: under-damped, demonstrates overshoot or ringing of arterial waveform
Based on this square wave test, which waveform is over-damped
A is over-damped
B is a normal trace
