CO Flashcards
CO
qty of blood delivered to systemic circulation/unit of time → L/min
Factors influencing CO
Body’s O2 consumption/metabolic rate → correlated with BSA
Age: ↓CO with aging
Posture: ↓ by 10% when lying → sitting and by 20% when sitting → standing
T, anxiety, heat/humidity
Extraction reserve depend on
o Nutrient extraction from blood: depend on
Delivery rate
Ability of tissue to extract from circulation
How does extraction reserve is expressed
o Expressed in arteriovenous difference across tissue
Arterial blood: 95% O2 saturation (on 1L of blood that can carry 200mL of O2) = 190ml O2
Venous blood: 75% O2 saturation = 150ml O2
* Normal arteriovenous difference is 40ml/L of O2
Extraction reserve: ↑ arteriovenous difference with stable blood flow
* ↑ tissue metabolic demand
* Normal extraction reserve = 3 → can extract up to 120ml of O2/L
CO limits
Lower limit: in cardiac dz → CO can fall to 1/3 of normal until tissue hypoxia, acidosis
Upper limit: largest ↑ → athletes up to 600% of resting CO
Techniques to determine CO in cath lab
o Fick O2 technique
o Dilution technique
Fick’s principle
Total uptake release of a substance = Bloodflow to organ x arteriovenous [difference] of substance
Determination of O2 content
vol%, ml/dL, ml/L
Related to Hb content:
Numerator: measured O2 content
Denominator: theorical O2 carrying capacity of Hb
Volume %: expression of solution concentration
= (Volume of solute)/(Volume of solution) x 100 (ml of O2/100ml blood
Normal Arteriovenous O2 difference = 3-5ml O2/100ml of blood = 3-5vol%
O2 sat equation
(O2 content)/([Hb]x 1.34)
Fick O2 method
Pulm blood flow: determined by measuring
o Arteriovenous difference of O2 across lungs
o Rate of O2 uptake from lungs
Pulmonary = systemic blood flow in absence of intracardiac shunt CO = (O2 consumption)/((Arterial - venous O2 saturation)(1.36)(10)[Hb]) O2 consumption → uptake of O2 from room air if measured for lungs O2 uptake Arteriovenous difference → PA blood (arterial) + LV or arterial (venous) blood sampled Bronchial and Thebesian venous drainage → ↓O2 content of 2-5ml/L Calculate O2 content of blood samples and arteriovenous O2 difference with spectrophotometric method (see image)
Limitations of Fick O2
o Assume steady state → constant CO and O2 consumption during measures
Most accurate in low CO with wide arteriovenous O2 difference
Less accurate with irregular rhythms
o Spectrophotometric method: assume normal Hb for blood O2 saturation
o Improper collection of mixed venous samples (air bubbles)
o Calculation average error: 6% for O2 consumption, 5% for arteriovenous O2 difference, total 10%
Indications for shunting during KT
o R to L intracardiac shunt: unexplained arterial desaturation <95% → alveolar hypoventilation
o Le to R intracardiac shunt: ↑O2 content in PA >80%
how to detect L to R intracardiac shunt
Oximetry run
Qp/Qs
Oximetry run
o Blood O2 saturation/content in R heart: blood samples in PA → RV → RA → CrVC → CaVC
Shunt detected + localized if ↑O2 content in specific chamber
* Normal max variation in O2 content is <1vol%, PA<0.5vol%
RA has variable O2 content → receive blood from CrVC, CaVC and CS
* Could vary up to 2vol%
Depend on [Hb]
o Simplest method: sample CrVC and PA → if O2 saturation difference >8% → shunt may be present
o Step up >7-9%
CrVC → anomalous PV connection (partial or total)
Atria → ASD, LV → RA, VSD with TR, anomalous PV connection
Ventricle → VSD, ruptured sinus of valsalva, PDA with PI, primum ASD
PA → PDA, AP window, subpulmonary VSD, aberrant CA
Qp/Qs calc
o L → R shunt = Qp – Qs (L/min)
o Qp: systemic arterial O2 content can be used if >95%
If <95%, suspect R to L shunt
Use 98% for Qp calculcation
o Qs: mixed venous sample should be measured in chamber just proximal to flow
Qp equation
(O2 consumption (mL/min))/([PV O2 content (mL/L)]- [PA O2 content (mL/L)] )
Qs equation
(O2 consumption (mL/min))/([SA O2 content (mL/L)]- [MV O2 content (mL/L)] )
Where is shunt: O2 step up in PA
PDA
Where is shunt: O2 step up in RV
VSD
Where is shunt: O2 step up in RA
ASD
Flow ratio Qp/Qs
: information about degree of shunt
o <1 → R to L shunt
o <1.5 → small L to R shunt
o 1.5-2.0 → moderate L to R shunt
o >2.0 → large L to R, >3 = massive
Qp/Qs= ((SA O2-MV O2))/((PV O2-PA O2))
Effective blood flow calc
bidirectional shunts
o Flow that would exist in absence of shunting
Recirculated pulmonary flow: L to R shunt = Qp – Qeff
Recirculated systemic flow: R to L shunt = Qs – Qeff
Net shunt = (L → R) – (R → L)
Qeff= (O2 consumption (mL/min))/([PV O2 content (mL/L)]- [MV O2 content (mL/L)] )
Limitations of shunt calc
o Assume steady state
o O2 step up method: do not consistently detect small shunts
o Depend on O2 [Hb]
What is the oxygen content of blood w/ a Hb 15g/dL & pO2 96
O2 saturation= (O2 content)/([Hb]x 1.34)
O2 content= O2 saturation x [Hb] x 1.34
O2 content= 0.96 x 15g/dL x 1.34ml O2⁄g Hb
=19.296 ml O2⁄dL of blood
What is the principle of the indicator dilution technique
- Adaptation of Fick principle: known amount of indicator
o Indicator substance injected upstream
o Average [indicator] through passage of sampling site determined by downstream measure
o Rate of blood flow calculated - Usually: injection of indication into RA → continuous sampling into PA or peripheral artery
o Flow is calculated by injecting an indicator and measuring its concentration over time
o Fick O2 method: uses O2 as indicator
o 2 types: bolus or CRI
Indicators used for dilution technique
indocyanine green or thermodilution
Non toxic, mixes completely w blood
Concentration can be measured accurately/easy
Cannot be added/subtracted from blood
Majority must pass sampling site
Must pass through central circulation (mixing
Stewart-Hamilton equation
o Integral function = [mean] x time = area under the curve
o Blood flow (Q): determined from amount of indicator injected and area under the curve
Q = I/integral of indicator concentration over time
Normal indicator curve
Injection of indocyanine green into RA + continuous blood draw from femoral artery
* A: Appearance time → delay prior to initial appearance of dye
o Early appearance of indicator → R to L shunt
* U: Rapid upstroke
* PC: Peak concentration
* D: exponential downslope interrupted by recirculation
o E: extrapolated continued curve if no recirculation
* R: recirculation
What does AUC relate to
indicate [indicator] over time
o iα to CO → smaller area = ↑CO
Indocyanine green can…
cause allergic rx and turn patient greenétoxic
Conditions causing alteration of peak or distortion of recirculation curve
- L to R shunt: ↓ peak with gradual downslope
o Early recirculation - R to L shunt: early peak prior to primary curve
o Normal delay is 3-5s after any right chamber injection - ↓CO: low, delayed peak + ↑ area under curve
- ↑CO: early peak + ↓ area under curve
- Valvular regurgitation: low, delayed peak + ↑ area under curve
effect of location of indicator injection relative to sensor
- Farther location of sensor:
o ↑appearance time and delay peak
o No effect on area under curve - Shunt localization: series of curves w successively more distal injection sites (RA, RV inflow, RV outflow, MPA, PA) until dye no longer appear prematurely
Indicator dilution curves for TOF
- Depend on level of PS (changes in CO) and direction of VSD shunting
Indicator dilution curves for L to R VSD
dye appears after normal appearance time but have prolonged gradual downslope w/o separate recirculation phase after RA
o B-D: sampling in PA → premature appearance of dye with LA and LV injection, but not in ascending Ao
Localize shunt at ventricular level
Indicator dilution curves for R to L VSD
dye appears prematurely prior to primary curve (P) after RA and RV injection, but not after PA or LV
o Localize shunt to ventricular level
o Normal and quick rise after ventricular injection
Indicator dilution curves for TR
- Attenuated curve with prolonged upstroke and downstroke
- ↓ CO
- ↑time of appearance at sampling site
Steps for measure of CO w/ thermodilution
- KT in R heart with 2 thermistor located at site of
o Injection: in CrVC or RA
o Sampling: in MPA or distal PA - Measure known T in bowl before injection → use correction factor
o Sometimes only 1 thermistor at distal site and injection T is assumed - Cold saline injection of known volume and T into venous site through proximal port
- Sample in PA
- Curve generated based on T over time
Advantages of thermodilution
- No recirculation
- No arterial puncture
- Cheap indicator
Limitations of thermodilution
- Inaccurate in several situations
- T fluctuation with respiration
- Loss of cold btwn KT and measurement site
Situations where thermodil is inaccurate
o Severe Tr with falsely ↓CO and ↑ area under curve
o ↑CO → overestimate in patient with low CO
o Intracardiac shunt
o Low flow or slow push
o If tip of KT against PA wall
o Correction factor → warming in syringe, KT
Hb units
g/dL
PCV units
% volume
O2 content units
ml/L
SV units
L
CO units
L/min
cardiac index unit
L/min/O2
Vascular resistance units
dynes-sec-cm-5
Woods or Hybrid units
mmHg/L/min
Plasma volume units
ml/kg
Intracardiac P units
mmHg
Central venous pressure units
cm of H2O
1mmHg =
1.36 cm of H2O
