Pmsf, SV, CO Flashcards
Pmsf
Mean Systemic Filling Pressure
Definition of Pmsf
System‐wide equilibrated pressure after cardiac arrest (usually about 10–15mmHg)
o Very similar to postcapillary venous pressure in an animal with a beating heart
o Represented by P1 in Flow = (P1-P2)/R, R = resistance to flow (Ohm’s Law)
o Rewritten: as venous return = (Pmsf – CVP)/venous resistance
Three methods for determining Pmsf
- Inspiratory Hold Maneuver
- Mathematical Modeling
- Tourniquet Technique
Inspiratory Hold Maneuver
series of inspiratory hold maneuvers at Paw 5, 15, 25, 35 cmH2O + simultaneous CVP, CO measurements
Tourniquet Technique
rapidly inflating tourniquet (to provide stop-flow event) applied to appendage with preplaced AC or VC attached to pressure-measuring device
20-30s: ABP, VBP equilibrated – pressure ~ Pmsf
Use of Pmsf
Characterizes functional status of circulating blood volume, identify hypovolemic patients who would benefit from fluid therapy
Measurements Obtained Using Pmsf
Venous return (assumed = CO), Pmsf, CVP measurements used to calculate venous resistance
total systemic compliance calculated from a known volume load, pre/post‐Pmsf measurements.
Functional estimate of Pmsf, circulating BV derived from fact that PPV impedes intrathoracic venous return, diastolic heart filling, SV
Magnitude of SV decrease by PPV used as index of central blood volume
–Magnitude of thoracic BF impairment depends on peak Paw, inspiratory time, cycle rate (essentially pulse pressure
Magnitude of decrement in SV assessed by:
Systolic blood pressure
Mean blood pressure
Pulse pressure (systolic – diastolic pressure)
Digital evaluation of pulse quality (area under pulse pressure waveform)
Plethysmographic monitoring of area under PP waveform, caused by inflating lung
Limitations of Pmsf
Only intended for use in patients with normal lungs, closed chest
Diffuse disease decreases compliance (change in V/change in P) – diminishes transfer of pressure from airways to pleural space – diminishes magnitude of thoracic blood flow impairment to given ventilator pressure setting
Area under pulse pressure waveform decrements of >10–13% were reported to predict hypovolemia and fluid bolus responsiveness
Stroke Volume Measurements
- Estimation by Doppler
- Area under PP waveform
SV: Doppler measurements
o Ventricular end‐diastolic diameter (EDD), ventricular end‐systolic diameter (ESD) measured; end‐diastolic volume (EDV), end‐systolic volume (ESV) calculated
o SV calculated as difference btw EDV, ESV
o Calculated by measuring flow velocity through structure (often aortic valve) of known diameter
o CT, MRI - primarily research tools in anesthetized patients
SV: Area under PP waveform - partial correlation: qualitative characterization
Tall wide (bounding) pulse likely associated with large SV
Short, narrow or thready pulse likely associated with small SV
Arterial Compliance
At given arterial compliance, assoc btw change in area under PP wave form, SV
Basis for most cardiac output measuring devices
When compliance or impedance changes, qualitative relationship btw PP waveform, SV also changes
Commercial measurement devices usually require intermittent resetting of computation constant to account for changes in compliance, flow impedance DT retrograde reflected pressure waves over time
Cardiac Output
vol of blood ejected from each ventricle per minute, L/min, product of HR*SV
o CI = cardiac index, CO/BSA or BW – L/min/m2 or L/min/kg
o Summarizes in single value contribution of CV system to global DO2
Advantages of CO Monitoring
monitoring hemodynamic changes, assessing effectiveness of fluid responsiveness
o Trends > actual values, ‘functional CO monitoring’ – positive response = acute increase 20-25%
5 Primary Variables of CO
o HR
o Rhythm
o Preload
o Contractility
o Afterload
Basic Principles of CO Measurement
o Results obtained must be of clinical relevance to patient
o Data obtained must be sufficiently accurate
o Therapeutic intervention must improve outcome
o Patient’s BP: important, complementary info
Low CO in hypotensive patient: hypovolemia, decreased cardiac function
High CO in hypotensive patient: decreased SVR
Which is the only technique that allows for direct CO measurement?
electromagnetic flowmetry
Requires sx implantation of flow probe circumferentially to main PA
Fick’s Principle for CO
1870 – first technique to measure CO
o Measurement of CaO2, CvO2, O2 consumption
Measurement of O2 consumption = limitation of technique, requires accurate collection/analysis of exhaled gases
What is the reference standard for CO monitoring?
PAC thermodilution
Law of Conservation of Mass and the Fick Principle
Law of Conservation of Mass: quantity of O2, CO2 leaving lungs = quantity of gas taken up or expelled by blood flowing in pulmonary circulation
Limitation: absence of any CP shunting
Requires PA cath for MvB sampling
Modified Fick Technique
estimates for VO2
CO = VO2/(CaO2-CvO2)
Indirect Fick Method of Measuring CO: NICO
–MOA: Elimination of CO2 rather than uptake of O2
Intermittent periods of partial rebreathing – estimates PaCO2, PvCO2 from ETCO2 partial pressure during normal breathing and rebreathing
* VCO2 calculated from minute ventilation, CO2 content
* CaCO2 estimated from ETCO2
*PvCO2 ~ CvCO2 (blood draw)
Essentially CO = (VCO2)/(CVCO2-CaCO2)
Equilibrium Point Assoc with Indirect Fick Method
CO2 elimination from lungs approaches 0, PvCO2 (end pulmonary capillary blood) = PETCO2
How estimate cardiopulmonary shunting with indirect Fick principle?
Estimated via FIO2, SpO2
Summary of Indirect Fick Principle (NICO Unit)
–Essentially rate of CO2 elimination proportional to O2 consumption
–CO = rate of CO elimination (ETCO2)/(CvCO2-CaCO2) comparing normal breathing and rebreathing
–Change in CO2 elimination/ETCO2 change IRT rebreathing
–Q3min: rebreathing valve prevents normal volumes of CO2 from being eliminated, patient’s inhaled/exhaled gases diverted through NICO loop for 50s
–CO2 elimination drops, [CO2] in PA increases but CO unchanged
Limitations of NICO
–ETT/CMV – need for constant CO2 removal precludes use in SpV with SA patients
–CMV >200mL/kg (12mL/kg so need p >20kg)
–Assumes perfect distribution with no shunting
–Cumbersome calculations, multiple levels of inaccuracy
Advantages of the NICO unit
No PAC
No invasive blood gas sampling
Accuracy of the NICO/Indirect Fick Principle in Dogs, Horses
- VT 12mL/kg: good correlation with thermodilution, lithium dilution
Dye Dilution
o Stewart, Hamilton: estimation of CO by knowing amount of injected indicator, calculating area under dilution curve measured downstream
Same reliability as Fick technique, better suitable in clinical setting
Became accepted method of reference
Thermodilution
o Same principles as dye dilution, heat as indicator
o Advantage: less accumulation since thermal
New gold standard
Swan Gantz Catheter
- Catheterization of RH by balloon-tipped catheter
–Proximal injection port in RA (~30cm), usually blue - CVP; thermodilution
–Proximal infusion port in distal RA (~31cm), - inj drugs, IVF
–Balloon: usually red
–Thermistor port for thermistor measurements in PA (~4cm)
–Distal PA, pressure transducer and MvB sampling
MOA Thermodilution
Indicator bolus: sterile saline (known vol, temp) injected into RA via SG PAC – change over time in blood temp in main PA used to calculate CO
Changes in blood temp detected by thermistor at distal end of PA catheter
Computer acquires thermodilution curve over time
Inj 2x, consistent phase of resp cycle – traditionally end of expiration
Thermodilution Injectate Characteristics
High volumes, lower temps: most accurate
Stewart Hamilton Equation (Simplified)
CO = (mg of dye injected * 60)/(avg concentration of dye * time)
How does CO affect AUC?
CO inversely proportional to derivative of derivative of temp dt
Low CO: bolus diffuses in RV, PA slowly – larger AUC
High CO: bolus diffuses more quickly – smaller AUC
Limitations of Dye and Thermodilution Techniques
no real time values, rapid accumulation of indicator clouds results with serial comparisons, cumbersome calibration (dye techniques), significant quantities of blood required for sampling
SG = primarily human products, narrow applicable sizes in vet med
Sources of Error with Dye and Thermodilution Techniques
Lower vol injected than entered: smaller AUC, CO falsely high
Lower temp: change in temp artificially large, CO falsely low
Complications Assoc with PAC Placement
10% human patients – arrhythmias, heart block, rupture of RH/PA, thromboembolism, pulmonary infarction, valvular damage, endocarditis
Transpulmonary Thermodilution, US Indicator Dilution: PiCCO, COstatus
Does not require PAC, same basic principles as PAC thermodilution
Estimation of CO via central venous, arterial catheter only (dedicated femoral AC)
PiCCO
Inj of ice-cold IVF, measures changes in temp over time by arterial thermistor tipped catheter in femoral artery
COstatus
Changes in blood viscosity following inj of small saline bolus (0.5-1mL/kg) warmed to room temp – changes in US velocity, quantified, measured
Roller pump, extracorporeal AV loop btw peripheral AC, distal lumen of CVC
Two reusable sensors: measure change in US velocity, BF through AV loop
SV derived from dilution curves
COstatus sensors
- Venous sensory
- Arterial sensory
Venous sensory for COstatus
inj of saline, records time/vol of inj
Arterial Sensory for COstatus
changes in concentration of saline in blood as a dilution, indicator travel time
COstatus Accuracy/Limitations
Good agreement in humans, +volumetric variables, no specific equipment for veterinary patients, more user friendly
COstatus: accurate, safe in patients <1kg
Restricted to patients <250kg
Lithium Dilution (LiDCO)
Dye dilution CO monitoring: IV injection of isotonic lithium chloride (0.002-0.004mmol/kg) as indicator
o [Lithium] in blood – lithium selective electrode connected to peripheral AC
o Lithium [ ] vs time curve via 4.5mL/min blood draw through disposable sensor
Computer converts voltage signal across lithium-selective membrane to [lithium]
Calculation for LiDCO
CO = (LiClx60)/[AUC(1-PCV)
PCV correct bc lithium only distributed in plasma, transform into total BF
Advantages of LiDCO
- As accurate as PAC thermodilution, more accurate when given via central line vs electromagnetic flowmetry (pigs)
- Easy to set up, operate
- Horses, dogs, pigs cats
- Uses lines already present in critically ill patients (inj via peripheral catheter)
Disadvantages of LiDCO
- Poor performance in presence of arrhythmias
- Interactions btw lithium, some ax drugs (rocuronium)*
- Blood loss assoc with withdrawal of arterial blood
Other Important Feature with CO Monitoring
ideally would paralyze patients for CO monitoring bc CO affected by resp, better able to standardize
Arterial Waveform Analysis
Requires arterial access, estimation of CO by measurement of AUC of pulse wave
Unreliable in dogs, horses
PiCCO, LiDCO, other devices
PiCCO for Arterial Waveform Analysis
arterial pulse contour analysis
Requires calibration: transpulmonary thermodilution
Repeat calibration needed to obtain adequate estimation of CO, whenever change in vasomotor tone/significant change in patient’s condition
calibration prior to CO measurement based on assumption that SV = sum of systolic, diastolic flows
Systolic, diastolic flows proportional to systolic, diastolic areas in AP waveform
LiDCO for Arterial Waveform Analysis
pulse power analysis for beat to beat estim of CO
Calibration via lithium dilution
calibration prior to CO measurement based on assumption that SV = sum of systolic, diastolic flows
Systolic, diastolic flows proportional to systolic, diastolic areas in AP waveform
Other Devices for Arterial Waveform Analysis
no baseline calibration, empirically calculate SV
Accuracy, precision questionable
Technical difficulties
Advantages of Echo/Doppler Based Techniques for CO Measurement
large amt of hemodynamic info obtained – contractility, chamber filling, assessment of valves/pericardium
Non-Doppler techniques for CO Measurements
Based on approximate volumetric reconstructions of LV chamber
Simpson’s rule: LV divided into series of disks stacked from base to apex
* LV volume: summing approximated volumes of individual disks
* SV: determining difference in vol btw systole, diastole
Simpson’s Rule for Non-Doppler Measurements of CO
LV divided into series of disks stacked from base to apex
* LV volume: summing approximated volumes of individual disks
* SV: determining difference in vol btw systole, diastole
Disadvantages of Non-Doppler Techniques for Measurement of CO
Disadvantages: time consuming, inadequate for rapid assessment
Rarely used clinically
Doppler for CO Measurement
Transthoracic, transesophageal – Doppler measurement of flow
Doppler Effect: shift in frequency as US beam directed along aorta, part of signal reflected back by moving RBCs at different frequency
* Determination of flow velocity
MOA Doppler for CO Measurement
measure cross sectional area (CSA) of LVOT, essentially a circle = (pi)r2
* CO = HR x CSA x VTI
* VTI = velocity time integral, represents distance that blood travels during one beat, ‘stroke distance’
* Subcostal view for SA – three or four chambered views, perfect parallel alignment of Doppler with LVOT
Advantages of Doppler for CO Measurement
Acceptable alternative to thermodilution for clinical purposes
Disadvantages of Doppler for CO Measurement
Not appropriate for continuous CO measurements – heat-induced injury
Expertise required: veterinary cardiologists, equipment
Image quality, sample site, angle of insonation, velocity signal to noise ratio, shape of aortic valve, ability to measure LVOT
Bioimpedance
changes in conductivity of high frequency, low magnitude alternating current passing across thorax to derive SV
Changes in electrical conductivity produced by variations in intrathoracic blood flow during each cardiac cycle
Bioimpedance MOA
Electrodes placed on thorax, neck - small, non-painful current passed btw electrodes, change in voltage (bioimpedance) measured
Measurements converted to SV using various equations, algorithms
Real time estimation of SV, CO
* Measures of thoracic fluid content, LV ejection time, SVR, L cardiac work index
Bioreactance
measures changes in frequency of electrical currents
Less prone to noise-derived errors
Advantages of Bioimpedance, Bioreactance
non-invasive, quick application
Disadvantages of Bioimpedance, Bioreactance
inaccurate in critically ill patients esp in presence of pulmonary edema/pleural effusion; electrical interference
Little to no evidence in vet med
Approximation of chest shape as cylinder, cone for SV determination
Unlikely that human algorithms applicable to veterinary patients
