Chp. 13: CO Measurement

Question 1

Cardiac Output (L/min)

Answer 1

Volume of blood ejected from each of the ventricles per minute

CO = HR x SV

Question 2

What five primary variables determine CO?

Answer 2

HR, rhythm, preload, afterload, contractility

Question 3

When should CO monitoring be considered?

Answer 3

In a hypotensive patient where clinical signs are difficult to interpret or the cause of hypotension is not obvious and likely multifactorial

Question 4

Cardiac Index

Answer 4

CO reference to the body surface area or body weight of the patient

Question 5

Cause of low CO in hypotensive patient

Answer 5

Likely hypovolemia or decreased cardiac function

Question 6

Cause of high CO in hypotensive patient

Answer 6

Decreased SVR

Question 7

What is the ONLY technique that allows direct measurement of CO?

Answer 7

Electromagnetic flowmetry

Question 8

What is the “reference standard” for CO measurement?

Answer 8

PAC thermodilution technique

Question 9

Fick Principle

Answer 9

Based on law of conservation of mass.

States that over a given time period, the quantity of O2 or CO2 entering or leaving the lungs is equal to the quantity of the gas taken up or expelled by the blood flowing in the pulmonary circulation.

Question 10

Fick Principle-derived CO equation

Answer 10

[VO2] / [CaO2 - CvO2]

True ONLY in absence of cardiopulmonary shunting

Question 11

What is the most problematic step of the direct Fick method for CO?

Answer 11

Measurement of VO2

Question 12

What is the indirect Fick technique?

Answer 12

Utilizes elimination of CO2 rather than oxygen update to avoid PAC and invasive blood gas sampling. Arterial and mixed venous CO2 content is estimated from measurements of ETCO2 during normal breathing and rebreathing maneuvers. VCO2 is calculated from minute ventilation. CaCO2 is estimated from ETCO2 during periods of ventilation. During rebreathing, PetCO2 rises to a plateau corresponding to partial pressure of CO2 in the blood entering the lungs, as a surrogate for CvCO2.

Requires ETT and mechanical ventilation.

Question 13

Indirect Pick Principle-derived CO equation

Answer 13

[VCO2N - VCO2R] / [CaCO2N - CaCO2R]

N = normal breathing
R = rebreathing conditions

Question 14

What is the basis of PAC thermodilution?

Answer 14

A saline bolus of known volume and temperature is injected into the RA via the proximal port of a Swan.

Question 15

What factors influence accuracy of PAC thermodilution?

Answer 15

Higher volumes and lower temperatures of saline.

Question 16

When should saline be injected for PAC thermodilution?

Answer 16

At a consistent phase of respiration, conventionally at the end of expiration, due to changes in CO during the respiratory cycle.

Question 17

Why does the CO temperature curve not return to baseline?

Answer 17

Recirculation

Question 18

How is CO related to the AUC?

Answer 18

When CO is high, indicator crosses the sensor quickly and the AUC is smaller.

When CO is small, indicator crosses sensor more slowly, and AUC is larger.

Question 19

What hemodynamic parameters can be measured with a PAC?

Answer 19

PA pressures, right-sided and left-sided filling pressures, mixed venous oxygen saturation (SvO2).

Question 20

Describe important sources of error associated with thermodilution measurements.

Answer 20

If volume injected is lower than entered, detected AUC is artificially small and estimates of CO are artificially high.

If temperature is lower (colder) than entered, temperature change is artificially large and CO artificially low.

Question 21

What are the common complications of PAC placement?

Answer 21

Arrhythmias, heart block, rupture of right heart or pulmonary artery, thromboembolism, pulmonary infarction, valvular damage, endocarditis

Question 22

What are other methods of CO measurement?

Answer 22

Transpulmonary thermodilution (CVC and femoral arterial catheter, temperature based); ultrasound indicator dilution (CVC and peripheral arterial catheter); lithium dilution (peripheral IV and peripheral arterial catheter).

Question 23

What is the benefit of arterial waveform analysis for CO measurement?

Answer 23

Allows continuous determination of CO

Question 24

What is the basis of arterial waveform analysis for CO measurement?

Answer 24

SV is equal to the sum of systolic and diastolic flows, which are proportional to the systolic and diastolic areas in the arterial pressure waveform

Question 25

How is CO determined using echocardiography?

Answer 25

Measurement of the cross-sectional area of the LVOT is performed (essentially a circle). Doppler cursor is aligned with LVOT and a profile of velocity over time is generated. Stroke distance (distance blood travels during one beat) is determined (called velocity time integral or VTI). Then, CO = HR x CSA x VTI, where CSA x VTI is SV.

Question 26

What is the basis of bio impedance used for CO measurement?

Answer 26

Uses changes in conductivity of a high-frequency, low-magnitude alternating current passing across the thorax to derive SV. Changes in electrical conductivity are produced by variations in intrathoracic blood flow during each cardiac cycle. Changes in voltage (“bio impedance”) is measured and converted to SV using an algorithm.

Not a valid or reproducible method in veterinary species at present.

Question 27

What is the role of ETCO2 as an indicator of CO? What are the appropriate therapeutic interventions?

Answer 27

A decrease in systemic blood pressure and ETCO2 may reflect a primary reduction in CO. Interventions should optimize preload and myocardial contractility to support SV.

A decrease in blood pressure with no reduction in ETCO2 suggests a decrease in SVR rather than CO. Vasopressor therapy is indicated.