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
