Chp. 12: Blood Pressure Monitoring Flashcards
SAP
Pressure exerted by blood against arterial walls during systole of the cardiac cycle
DAP
Pressure exerted by blood on arterial walls during diastole
MAP
Determines perfusion pressure in tissues and directly influenced by CO and SVR
Area under pressure/time curve divided by cardiac cycle duration
Is digital pulse palpation accurate for assessing BP under GA?
No, because most sedative and anesthetic agents affect vasomotor tone.
Equations for MAP
MAP = CO x SVR
MAP = DAP + 1/3 (SAP-DAP) [rough estimate]
Cardiac Output Equation
CO = HR x SV
Chronotropy
Physiologic timing of HR
Lusitropy
Period of cardiac relaxation that occurs during diastole when cardiac myocytes have decrease level of systolic calcium
Inotropy
Contractility due to increased calcium uptake causing contractile effects
Dromotropy
Actual conduction of impulse
Systemic Vascular Resistance
Amount of force the vasculature system exerts on circulating blood, excluding the pulmonary circulation
How does SVR influence BP?
Through vessel length, vessel diameter, and viscosity of blood
Hagan-Poiseuille Equation
DeltaP = Q8Ln/(pi)r4
(reducing vessel diameter by half decreases flow to 1/16th of original)
Prehypertension
Hypertension
Severe hypertension
- Pre: SAP 140-150mmHg
- Hyper (may lead to target organ damage): SAP 160-179mmHg
- Severe: SAP >180mmHg
Autoregulation
Ability of organs to maintain a relatively constant blood flow despite changes in perfusion
Three main mechanisms of autoregulation
1) Metabolic regulation
2) Myogenic mechanisms
3) Shear stress-dependent (endothelial)
Metabolic autoregulation
Changes in metabolism results in the release of vasodilatory substances such as adenosine, CO2, and lactic acid
Myogenic mechanism of autoregulation (pressure-dependent)
Autoregulate flow via smooth muscle-lined vessels
Increase pressure leads to vasoconstriction; decreased pressure is followed by vasodilation in smaller arteries and arterioles
Shear-stress dependent (endothelial) autoregulation
Endothelial release of vasoactive factors such as NO and prostacyclin, causing vasodilation of smooth muscle-lined vessels, as well as endothelia, which modulates vasoconstriction
Main mechanism of autoregulation in the brain
Metabolic
Range of MAP for cerebral autoregulation (dog)
70-140mmHg
How are SAP, DAP, and MAP determined with invasive BP monitoring?
SAP and DAP are directly measured and MAP is calculated as the AUC over the cardiac cycle
Standard IBP components
1) Arterial catheter
2) Fluid-filled, non-compliant tubing
3) Pressure transducer
4) Signal conditioning and monitoring software
What is the role of saline-filled tubing in IBP measurement?
Produces “hydraulic coupling” between arterial circulation and transducer
Ohm’s Law
R = P/I
R is resistance, P is voltage, I is current
Describe how a BP transducer works.
Four strain gauges (resistors) are bonded to a movable diaphragm and arranged in a Wheatstone bridge circuit. Mechanical pressure variations associated with arterial pulsations physically deform the diaphragm, creating tension in two of the strain gauges and simultaneous compression of the other two. These changes lead to variations in electrical resistance and the bridge becomes unbalanced. The potential difference generated is proportional to the pressure applied.
Where is the IBP transducer leveled?
The level of the aortic root or base of RA.
What happens if the art line transducer is positioned above the RA? Below the RA?
If positioned above, artificially low pressures result. If below, artificially high pressures result.
For every 10cm above the RA that an arterial transducer is positioned, how many mmHg is added to displayed pressures?
7.4mmHg
True or False: Zeroing of the transducer must be performed at the level of the RA.
False, atmospheric pressure does not vary with modest positional changes.
Fourier analysis
The IBP waveform is created by Fourier analysis, which sums the various sine waves of differing amplitudes and frequencies into a single complex waveform
Should the IBP system have low or high natural frequency?
Highest possible, to minimize resonance distortion (esp. in patients with faster HRs and steeper systolic upstrokes).
What influences the natural frequency of the system?
Radius and length of tubing, elasticity of the system, density of the fluid.
How do you maximize the natural frequency of the IBP system?
1) Shorten the length of the pressure tubing
2) Increase the diameter of the pressure tubing
3) Use non-compliant pressure tubing
4) Use a low-density fluid (ie, saline) to fill the system
Will air bubbles increase or decrease the natural frequency of the IBP system?
Decrease
Damping
The frictional forces that oppose the oscillations and result in decrease wave amplitude
What is the optimal damping coefficient?
0.7
Dynamic Pressure Response Test (a.k.a. “Fast Flush Test)
High pressure is rapidly introduced to the catheter for 1-2 seconds and then abruptly stopped. A square waveform is produced. The waveform should return to baseline within one to two oscillations, neither greater than 1/3 the height of the previous. Absence indicates overdamping; many oscillations indicate underdamping.
Is a smaller gauge catheter more likely to be over or underdamped?
Overdamped
What factors cause overdamping of the IBP system?
Air bubbles, catheter obstruction, overly compliant tubing, long tubing, too many stopcocks
What is the effect of overdamping on SAP, DAP, and MAP? Of underdamping?
Overdamping underestimates SAP and DAP is overestimated or accurate. Underdamping overestimates SAP and underestimates DAP. MAP is least affected.
How does Doppler BP measurement work?
Movement of RBCs results in a change in pitch of reflected sound waves (generated from contact of piezoelectric crystals with pulsatile RBCs). The cuff is inflated until audible signal is lost and then 20-30 additional mmHg. Pressure is released slowly and the sphygmomanometer pressure correlating to return of the first Korotkoff sound is SAP.
Does Doppler BP over or underestimate BP in cats?
Underestimate
How does oscillometric BP measurement work?
It is a counter pressure measurement technique whereby the arterial pulse produces changes in the volume of a limb, which can be transmitted to and detected as changes in pressure within a cuff encircling the limb. The cuff is inflated to 20-30mmHg past obstruction of blood flow and then slowly deflated via linear or step-deflation methods. Arterial flow gradually returns and corresponding changes in cuff pressure amplitudes persist until inflation pressure is released and flow returns to normal.
What is the most accurate oscillometric NIBP value?
MAP
When is the Oscillometric BP method inaccurate?
Reduced blood flow, poor cuff sizing, patient movement/shivering, extremes of patient size, severe hypo/hypertension, cardiac arrhythmias
Central Venous Pressure
Measures the hydrostatic pressure of the intrathoracic vena can, as a close reflection of RA pressure (an intravascular rather than transmural pressure)
Why has CVP fallen out of favor?
Relationship between CVP, CO, and vascular system is complex and complicates interpretation. The non-linear nature of the Frank-Starling curve and factors causing changes in cardiac and pulmonary compliance results in circumstances where pressure does not correlate well with volume status.
Reference range for CVP
0-10 cmH20, more commonly 0-5 cmH2O
Hypovolemic, euvolemic, and hypervolemic CVP ranges
Hypovolemia: Negative values
Euvolemia: 0-5 cmH2O
Hypervolemia: 7-10 cmH2O
Kussmaul’s Sign
Decrease in CVP observed during inspiration in fluid responsive patients
Risks of jugular catheter placement
Cardiac arrhythmias (APCs or VPCs), hemorrhage, hematoma, air embolus, thrombus, pneumomediastinum, pneumothorax, hemothorax, tracheal trauma, carotid artery puncture
Long-term jugular catheter complications
Infection, recurrent laryngeal nerve damage, jugular vein stenosis or occlusion
Contraindications to jugular catheter placement
Increased ICP, coagulopathies, those at increased risk of thromboembolic events
Pulse Pressure Variation
Measure of difference between systolic and diastolic pressures during IPPV.
IPPV increases pleural pressure > compresses vena cava > increases RAP > decreases venous return > decreases RA preload > decreases RV output and PA blood flow > decreases LV filling and output
During inspiration, RV preload is decreased and RV afterload is increased while LV preload is increased and LV afterload is decreased
What are the four main reasons for hypovolemic patients demonstrating greater respiratory variations in SV and arterial pressure?
1) Vena cava is more collapsible
2) Underfilled RA is more sensitive to transmission of pleural pressure during inspiration
3) Effect of inspiration on RV afterload is more significant where West’s zone I and II conditions are met
4) Left and right ventricles are functioning on the steep portion of the Frank-Starling curve where there is greater sensitivity to preload changes
What PPV values predict fluid responsiveness?
PPV values > 7-16%
What factors influence PPV?
Volume status, fluid type and volume, ventilator settings, chest wall compliance, intra-abdominal pressure, venous vasomotor tone, presence of cardiac abnormalities
Plethysmography Variability Index
Built into specific pulse oximeters.
During IPPV, RV SV is minimal at end-inspiration due to reduction in venous return and LV preload and SV decrease and are minimal at end-expiration, causing changes in pulse wave amplitude. Magnitude of variation is proportional to likelihood that SV will increase with augmented preload.
What PVI values predict fluid responsiveness?
Greater than or equal to 13%
