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
