invasive and non-invasive BP monitoring Flashcards
why is BP monitoring significant?
- key indicator of perfusion
- most important dereminant of LV afterload
- reflects the workload of the heart
describe manual indirect BP measurement
rapid systolic estimation (return of flow technique)
-palpation: during deflation, palpable pulse retruns
-visual: during deflation, finger pulse oximetry or arterial catheter waveforms reappear
systolic and diastolic measurement
-auscultatory: (Korotkoff) turbulent flow sounds
-systolic: point at which the first turbulent arterial flow sound returns
-diastolic: point at which the sound becomes muffled/diminished or no longer heard
-no mean BP estimation available
what are errors with cuff measurement?
- shock or pressors obliterate sound generation causing false low reading
- low compliance (distendability) of tissues causing false high reading (shivering, non-compressible arteries d/t arteriosclerosis)
- cuff size: width should be 20% greater than arm diameter; cuff too large causes false low reading, too small false high
- too rapid a cuff deflation rate causes false low (3mmHg a second or 2 mmHg per heart beat)
describe automated non-invasive BP (NIBP)
- also known as the dynamap or oscillator
- data interpretation algorithms are utilized to determine BP
- measures systolic, diastolic, and mean BP
- electronic storage of data occurs
- automated which allows the clinician to devote their time to other patient care needs
- based on oscillometry (measuring vibrations): arterial pulsations cause varying amplitudes which are measured along with the rate of change of amplitudes
how do NIBP monitors determine BP?
- systolic pressure: amplitude of pulsations are increasing and are at 25-50% of maximum
- mean arterial pressure (MAP): peak amplitude of pulsations
- diastolic pressure: amplitude of pulsations has declined from the peak value approx. 80%
- diastolic reading is considered the most inaccurate
what are clinical indications for invasive BP monitoring?
need for real-time continuous pressure monitoring
-hemodynamic instability
-vasoactive agent use
-deliberate hyper or hypotensive state required
-precision BP control needed
cuff measurement is unreliable
-poor circulation/perfusion- low BP
-erratic pulse (a fib, PVCs, tachycardia)
-burns, AV shunts
waveform diagnostics desired
repeated blood sampling needed
what are sites for art lines?
- radial artery (most utilized)
- ulnar (rarely used d/t principle source of blood flow to hand)
- brachial (no collateral circulation, near median nerve)
- axillary (left more than right)
- femoral (resembles aortic pressure; accessible in a low perfusion state
- dorsalis pedis/posterior tibial: requires long tubing, increased reflectance d/t peripheral vascular tree
what are complications associated with invasive BP monitoring?
- ischemia distal to site
- hematoma-compartmental syndrome
- arterial trauma
- infection
- thrombus formation
- vasospasm
- bleeding
- fistula
- air embolus
- heparin overdose
what is dynamic response of invasive BP monitoring?
- physical behavior of the system
- based on three physical properties: elasticity, mass, friction
- characterized and assessed by: natural frequency and damping coefficient
define Hertz (Hz)
-unit for measuring frequency, number of cycles per second; 1 cycle per second = 1 Hz
define oscillation
- back and forth repeated motion
- a quantity that repeatedly and regularly fluctuates above and below some mean value, as the pressure of a sound wave
- normal occurrence
define harmonics
- stretch and recoil of spring (bouncing vibrations/oscillations)
- a series of oscillations in which each oscillation has a frequency that is an integral multiple of the same basic frequency
- abnormal occurrence
define resonance
- exaggerated wave amplitudes occurring when the monitored frequency matches the systems natural frequency resulting in overshoot or overestimated wave reading
- when harmonics occur the system is resonate
- resonance is useful for the ear b/c the amplification can help distinguish sounds but resonance is not good for direct arterial BP measurement
describe electrical transducers
-a fluid filled catheter and tubing sends a pressure wave to fluid filled transducer where diaphragm displacement/movement creates an electrical signal
what happens when the system is undamped?
diaphragm moves too easily it may oscillate too long and if subsequent wave arrives while it’s still oscillating, stacking occurs
what happens when the system is overdamped?
the diaphragm is too stiff and fails to oscillate in response to a pressure wave
describe natural frequency
- how easily or rapidly the sytem oscillates
- all objects have a natural frequency at which they optimally vibrate when disturbed or struck (based on object’s properties; slower through denser objects)
- measured in Hz unit (cycles per second)
- higher the natural frequency, the more precise and accurate the signal quality with less distortion
what should natural frequency be?
at least 5 times the frequency of the waveforms to be monitored
*if HR 180 (3Hz or 180/60) then 5 x 3 = 15Hz NF required
describe the damping coefficient
- numerical indicator of the degree of damping
- damping defines an objects tendency to cease vibrating/oscillating (how rapidly an object will return to resting baseline)
- frictional forces contribute to damping
what is considered completely undamped?
coefficient of 0.0
*object will continue to oscillate indefinitely
what is considered completely damped?
coefficient of 1.0
*object will instantly return to baseline resting state as soon as the stimulus is withdrawn
what is the critical damping?
when one displacement causes one vibration
*damping coefficient 0.4
what is the desired damping coefficient for invasive BP monitoring?
0.6-0.7
what happens with an underdamped system?
- transducer diaphragm vibrates too long after being subjected to mechanical displacement
- system rings/oscillates abnormally with each pressure signal transmitted, harmonics occurs
- characterized by SBP and DBP overshoot
- falsely widened pulse pressure
- artifact and other external interference invade the pressure waveform
- if system is still oscillating when a second pressure wave arrives there may be summation and stacking of waves
- mean still accurate
what is seen with overdamping?
smooth waveform
no dicrotic notch
what are some causes of overdamping?
- components absorb energy and “dampen” the pressure reflectance (tennis ball hitting foam wall)
- vasodilation
- aortic stenosis
- low CO
- clots
- air bubbles
- stopcocks
- kinks
- blood in transducer
- empty or lack of pressure on flush bag
what is seen with underdamping?
sharp, exaggerated waveform
what are some causes of underdamping?
- HTN
- hyperdynamic flow states
- catheter whip (excessive movement)
- atherosclerosis
- vasoconstriction
- aortic regurgitation
- components have no compliance to absorb energy or dampen reflectance (tennis ball hitting hard wall)
described an overdamped system
- if transducer diaphragm has little to no vibration/oscillation
- characterized by slurred upstroke, absent dicrotic notch, loss of fine detail
- can demonstrate a erroneous narrowed pulse pressure
- systolic BP will be falsely low and diastolic BP will be falsely high
- mean BP mostly unaffected
- small clots on end of catheter, tubing kinks, bubbles, stopcocks most common causes
what is the affect of NF on damping?
increased NF slows a lower damping coefficient so it has minimal impact on waveform
which can be corrected most b/w underdamping and overdamping?
underdamping
how can dynamic response be assessed?
- fast-flush test: accurate assessment of DR
- calculating natural frequency of system
- calculating damping of system
- square wave test
describe the square wave test
counting oscillations
- optimally damped 1.5-2 oscillations before returning to baseline
- underdamped: > 2 oscillations before returning to baseline
- overdamped:
what factors affect dynamic response?
- increasing damping will decrease NF b/c damping causes slower vibration
- the longer the tubing length, the less NF: 6 in. = 33 Hz, 6 ft = 8 Hz
- optimal length is 4 ft
- stiffer noncompliant (nondistensible) tubing is optimal so the walls of the tubing wont absorb the wave
- bigger diameter tubing is optimal
- decreased damping and NF augments the wave and introduces resonance into system
- DR issues affect SBP the most and MAP the least; MAP most accurate
- properties of the system control NF; clinician control damping
how can you optimize DR?
- most systems are underdamped (0.2)
- NF as high as possible (> 7.5 Hz); shortest length tubing; fewest stopcocks; non compliant tubing, no T connectors
- remove air bubbles from system and from pressure bag before spiking (air is compressible and has own NF causing decreased NF and increased damping)
- withdraw blood and remove clot from end of catheter
- commercial available devices are available (filters out high frequencies and only allows low frequencies to pass through
how do you establish zero reference?
- open stopcock and expose transducer to ambient atmospheric pressure
- press the zero button, the screen pressure tracing should overlie with zero pressure line
how do you level the art line?
- align transducer with cardiac chambers (more accurate than midchest alignment
- 5 cm below sternal border and 4th ICS corresponds to aortic root
what are the effects of transducer placement on measurement?
- 1 cm of height = 0.75 mmHg (10 cm = 7.5 mmHg)
- raising the pt. above the transducer will produce higher pressures
- lowering the pt. below the transducer will produce lower pressures
- no re zeroing required b/c atmospheric pressure will not change significantly with a few cm
describe neurosurgery placement of the transducer
- level of the external auditory meatus or tragus of ear = Circle of Willis in the brain and estimates CPP
- lower pressure than level of heart d/t the vertical column and hydrostatic pressure difference
- 20 cm difference in placement from heart alignment to Circle of Willis alignment so pressure drops at circle of willis by 15 mmHg
what waveform changes are seen with distal pulse amplification?
- upstroke steeper, systolic peak higher
- dicrotic notch appears later, end diastolic lower
- wider pulse pressure; delayed arrival of wave
what waveform changes are seen with aortic stenosis?
- delayed upstroke
- narrowed pulse pressure
what waveform changes are seen with aortic regurgitation?
- sharp rise
- double peak
what waveform changes are seen with cardiomyopathy?
-spike and dome d/t midsystolic obstruction
what causes distal wave amplification
d/t more resistance as go further from the heart
- widening pulse pressure farther form the heart
- older people have more resistance, decreasing perfusion
- stiff arteries, more pressure on systole b/c less compliance, then pressure drops with diastole
describe placement technique of an art line
- radial artery most common d/t good collateral circulation
- Allen’s test (unreliable assessment)
- mild wrist dorsiflexion, immobilize with towel roll
- locate pulse, prep area; use local anesthetic
- enter at 45 degree angle until flash of blood
- dip to 30 degree angle and advance to hub
- occlude artery and connect pressure tubing
- most common cause of failure to cannulate is needle is in artery but cannula no
- smaller catheters reduce incidence of thrombus formation and increase damping
