Cradiac Output Monitoring

Question 1

How is cardiac output defined?

Answer 1

• amount of blood delivered from the left ventricle to systemic circulation on a per minute basis (ml/min)

CO = SV x HR

Question 2

What are the determinants of MAP?

Answer 2

CO and SVR

Question 3

What are the determinants of CO?

Answer 3

HR + Stroke Volume

Question 4

What are the determinants of stroke volume?

Answer 4

• Contracility
• Preload
• Afterload

Question 5

What are the determinants of HR?

Answer 5

Sympathetic vs. parasympathetic NS

Question 6

What are the local and systemic determinants of SVR?

Answer 6

Local:
- CO2
- PGs
- NO
- Histamine

Systemic:
- Vasopressin
- Angiotensin II
- Sympathetic NS

Question 7

What are the determinants of oxygen delivery (DO2)?

Answer 7

CO and CaO2

Question 8

Name the three determinants of CaO2?

Answer 8

ml of O2 per dl of blood:
- Hb
- SaO2
- PaO2

Question 9

How is DO2 defined?

Answer 9

Oxygen delivery in ml per minute

Question 10

Draw the “tree of life”?

Answer 10

Question 11

Name 6 factors affecting preload?

Answer 11

• circulating volume
• venous tone
• body position
• intrathoracic pressure
• pericardial pressure
• RAP

Question 12

How is preload defined? What law is important for it?

Answer 12

• stretching of the ventricle prior to contraction (systoly)

As end-diastolic volume increases, there is increased stretching of the ventricular muscle fibers (loading) that then enhances contractile force (at least up to a certain point)
- Frank-Starling law

Question 13

What is afterload?

Answer 13

force (pressure) needed to achieve ventricular out-flow
(amount of work the heart must do in order to move blood forward during systolic ejection)

Question 14

What is the effect of increased afterload on the heart?

Answer 14

• decrease in velocity of cardiac muscle fibre shortening
• greater pressures needed to open pulmonic/aortic valves
• reduction in SV
• greater residual ESVV

Question 15

How is the relationship beetween SV and afterload?

Answer 15

• inverse

Question 16

Name 3 factors affecting afterload?

Answer 16

• pulmonic and SVR (for the right and left ventricle, respectively)
• function and integrity of cardiac valves
• ventricular chamber size

Question 17

How is contracility defined?

Answer 17

ability of cardiac muscle fibers to develop tension at a given preload and afterload
–> impacts the force of ventricular contraction (inotropy)

Stronger contraction –> mroe significant reduction in ventricular volume during systole –> increase in SV

Question 18

How is the relationship beetween contracility and CO?

Answer 18

direct

Question 19

Name 4 factors influencing contracility (independent of preload and afterload?

Answer 19

• body temperature
• HR
• ischaemia
• balance between SNA and PNS activation

Question 20

Name 9 causes of preload reduction?

Answer 20

Hypovolemia:
- Haemorrhage
- Severe Dehydration
- Edema/Cavitary effusions

Obstructive:
- Pericardial effusion
- GDV
- Mesenteric volvulus
- Caval/portal venous occlusion
- Severe pleural space disease
- Pulmonary thromboembolism

Question 21

Name 7 causes of reduction in cardiac function?

Answer 21

Primary:
- Cardiomyopathy
- Valvular disease
- Tachy- or bradyarrythmias

Secondary:
- SIRS/Sepsis
- Electrolyte abnormalities
- Severe hypoxia
- Severe acidosis/alkalosis

Question 22

Name 4 causes of increased afterload?

Answer 22

• Sytemic hypertension
• Aortic stenosis
• Aortic regurgitation
• Peripheral vasoconstrictions

Question 23

Name one disease process where CO monitoring might be beneficial and why?

Answer 23

• Sepsis
• high CO and low CO possible in hypotensive states
• guide therapy for use of fluids or positive inotropes

Question 24

What is the formula for calculation of SVR?

Answer 24

SVR = (MAP-CVP)/CO

Question 25

What is the Cardiac Index?

Answer 25

CI = CO referenced against bodyweight (ml/min/kg) or body surface area (ml/min/m2)

Question 26

What are normal CI for dogs and cats?

Answer 26

dogs: 125-200 ml/kg/min cats: 120 ml/kg/min

Question 27

Name 6 categories of technique used for CO monitoring?

Answer 27

- Fick method (Oxygen consumption or CO2 rebreathing) - Indicator dilution (thermodilution, lithium dilution) - pulse contour analysis - echocardiography - transthoracic Doppler - Pulse wave transit time

Question 28

Desribe the technique of Fick Oxygen Consumption for CO monitoring? What are the cons? How does it compare to dilution techniques?

Answer 28

= original gold standard --> tissue oxygen consumption (VO2) depends on product of CO and the difference between arterial (CaO2) and venous oxygen (CvO2) content. CO=VO2 /(CaO2 xCvO2) VO2: measuring O2 concentration difference between inhaled and exhaled air in the amount of O2 inhaled over a period of time (3min) Cons: - Patient needs to be intubated - arterial sampling (ideally from the pulmonary artery, but cranial VC also acceptable) - no real-time assessment of CO - major intracardiac or intrapulmonary shunting will affect measured variables compares well to indicator dilution techniques in dogs and cats

Question 29

What is the formula for oxygen content in the blood (venous or arterial)?

Answer 29

CxO2 =1.36 x [Hb] x SO2 + 0.003 x PO2

Question 30

Desribe the technique of Fick's based carbon dioxide rebreathing methods for CO monitoring? What are the downsides? How does it compare to dilution techniques?

Answer 30

- based on CO2 production - amount of CO2 expired is a function of cellular metabolism and CO driving blood from the tissues to the lungs - ETCO2 reflects changes in perfusion (CO) relative to changes in inspired CO2 - also considers pulmonary shunt fraction by comparing inspired oxygen to arterial oxygen saturation CO=Difference in CO2 elimination and ETCO2/Difference in ETCO2 between rebreathing and non-rebreathing Downsides: - need to be intubated - more accureacte if on MV - volume of the rebreathing circuit limits the size of patient on which it can be used Gunkel et al (JVECC 2004) ound good agreement and correlation using this method when compared with an indicator dilution technique Yamashita et al (J Vet Med Sci 2007) found significant bias

Question 31

Desribe the indicator dilution method for CO monitoring?

Answer 31

- injection of a known quantity of a marker (cold saline, lithium) at one location and subsequent measurement at a remote location - curve is generated reflecting the change in concentration over time - area under that curve is inversely proportional to CO, as a higher CO state will result in faster dilution of the indicator and a smaller curve 2 possibilites: - measurement in the pulmonary artery after injection into the RA - transpulmonary: injection in venous circulation with detec- tion downstream in the arterial circuit

Question 32

Describe thermodilution method for CO monitoring?

Answer 32

Swan Ganz catheter is placed through the jugular vein and has a therm- istor tip that terminates in the pulmonary artery fixed amount of cold saline is injected through the proximal port that will mix in the RA and eventually pass through the PA to be detected by the thermistor conflicting evidence whether the use of PACs in clinical decision making has any beneficial impact on patient outcome

Question 33

Name two methods for confirmation of the right location in pulmonary artery catheterisation?

Answer 33

1. fluoroscopy 2. detection of pulmonary artery pressure waveform

Question 34

Name 3 complications of pulmonary artery catheter placement?

Answer 34

- pulmonary thromboembolism - pneumothorax - blood- stream infections

Question 35

What is transpulmonary thermodilution?

Answer 35

- indicator: temperature - administering cold saline into venous circulation, with detection by a thermistor-tipped arterial catheter (femoral artery) - comparable CO determination compared with PAC in humans and lithium dilution in dogs

Question 36

Describe the method of lithium dilution for CO monitoring? What are lmitations of this technique?

Answer 36

- indiator: lithium - administering lithium into a central or peripheral vein and then measuring arterial lithium levels (e.g., dorsal metatarsal) and using a peristaltic pump to draw blood across the lithium sensor - good correlation between lithium dilution and thermodilution, even in circumstances of varying hemody- namic states Limitations: - cost of equipment - lithium accumulation (especially in smaller patients) - amount of blood needed to be withdrawn across the lithium sensor

Question 37

Describe the method of Pulse Contour/Pulse Pressure Analysis for CO monitoring? What are the current limitations? How does it correlate to thermodilution?

Answer 37

- beat-to-beat CO assessment - utilize area under the pressure waveform and mathematical models to account for differences in vascular compliance to calculate CO - some sytems require repeated calibration with lithium dilution (PulseCO) or transpulmonary thermodilution (PiCCO) - reasonable correlation between thermodilution and PiCCO - femoral arterial catheter necesssary for PiCCO - other studies failed to show good correlation or trending between CO measured by thermodilution or lithium dilution and any of these systems (tendency to overestimate CO in low output states + underestimate in high output states - algorithms are based on people)

Question 38

Describe the method of transthoracic ultrasound for CO monitoring? How does it compare to thermodilution?

Answer 38

- blind assessment of aortic or pulmonic flow velocity waveforms across the chest via Doppler ultrasound --> in conjunction with the vessel cross-sectional area --> determination of SV --> multiplied by HR = CO USCOM: continuous-wave spectral Doppler probe is placed over the thoracic inlet (for aortic flow) or below the xyphoid (for pul- monary artery flow) device has an algorithm (based on people) to provide vessel cross-sectional area relative to body surface area animals: direct determination of this value must be made using echocardiography + manually entered - CO measurements exceeded the limits of agreement, but overall trended with hemodynamic changes when compared to thermodilution

Question 38

Name 2 possibilities to monitor CO through echocardiography? How does it correlate to thermodilution or cardiac MRI? What are the limitations?

Answer 38

1. measurement of flow velocity in the RV outflow track and aortic diameter --> product of these two = SV --> multiplied by HR = CO 2. determination of LVEDV and LVESV --> difference between these two values = SV --> multiplied by HR = CO - strong correlation when compared with CO determined by thermodilution or cardiac MRI Limitations: - extensive training - no real-time assessment

Question 39

Describe the method of pulse wave transit time for CO monitoring?

Answer 39

--> time from cardiac contraction to generation of pulse = inversely related to CO (the longer the time, the smaller the CO) - measured as the time from the ECG R-wave peak to the rise point of the pulse oximeter waveform (30% of the peak height is achieved) - proprietary software necessary - needs to be calibrated against another CO measurement (e.g. lithium or thermodilution) - PWTT has shown the ability to detect changes in CO, though limits of agreement in comparison to reference CO measurement were beyond acceptable range

Question 40

What is DO2?

Answer 40

Oxygen delivery to tissues = ml of O2 delivered per minute

Question 41

Explain the expected changes in CO, contractility and SCR in hypovolemic, obstructive, cardiogenic and maldistributive shock.

Answer 41