Chapter 184 - Cardiac output monitoring Flashcards
Define cardiac output. Give the unit and normal value range in dogs and cats.
Volume of blood transferred by the heart to the systemic circulation over time (mL/kg/min).
CO = stroke volume x HR
Normal value: 125-200 mL/kg/min (dogs) 120 mL/kg/min (cats)
Define cardiac index. Give the unit an normal value in dogs.
Volume of blood pumped over time to the animal’s surface body area (thought to correlate better with metablic rate compared to body mass).
Normal value: 3.5-5.5 L/min/m2 (dogs)
Define combined cardiac output
Total volume of blood ejected into the systemic circulation over time when both right and left ventricles can directly transfer blood to the arterial tree (e.g., fetal circulation, R-to-L patent ductus arteriosus)
What are the indications for cardiac output measurement?
Patients in sepsis, septic shock, SIRS, MODS as well as patients with severe compromise of the pulmonary and cardiovascular system are the ones for which more invasive measures of CO are likely to be required (early detection + contraindicatory information provided by PE, e.g. vascular extravasation suggesting a cautious fluid therapy associated with a low MAP suggestive a more aggressive fluid therapy)
Comment the sensitivity and specificity of physical exam trends (HR, mm, CRT, BP, mentation, T°) to detect alterations in a patient cardiovascular status.
Serial thorough PE are invaluable to the assessment of patients. Trends in serial PE findings typically provide the best and reliable measure of alterations in his cardiovascular status (very specific). However, the converse is not true: unchanged PE findings could only be the result of compensatory mechanisms (poorly sensitive).
What information does the measure of CO give?
- Insights into bulk blood delivery to the body
- Determination of whole body oxygen delivery (when taken together with measurements of the oxygen content of blood)
- Determination of the stroke volume (when taken together with HR)
- Determination of pysiologic indicators such as intrapulmonary shunts, systemic and pulmonary vascular resistance, oxygen consumption
Name 3 invasive methods to determine CO
- Fick oxygen consumption method
- Carbon dioxide partial rebreathing method (with Fick equation)
- Indicator dilution method
Name 3 non-invasive (or minimally invasive) methods to determine CO and briefly give their principle
- Transesophageal echocardiography (measurement of blood velocity using the doppler and aortic diameter using echocardiography)
- Pulse contour analysis (analysis of the arterial pressure waveforms using an algorithm)
- Thoracic bioimpedance (a small voltage is applied to the thorax and the conductivity and impedence of the thorax (determined by its air and fluid content) is measured using distant sensing electrodes)
Describe the Fick O2 consumption principle/formula (gold standard)
Give limitations
CO = O2 consumption / Arteriovenous O2 content difference
Principle: the total uptake or release of a substance by the peripheral tissues is equal to the product of the blood flow to the peripheral tissues and the arteriovenous concentration difference of the substance.
O2 consumption is determined by measuring the difference in the inhaled air and the exhaled air collected from patients over time (3min)
Oxygen content = ([Hb] x 1.36 x SO2) + (0.003 x PO2)
Blood gas analyzer gives:
SO2 = hemoglobin saturation
PO2 = O2 partial pressure
[Hb] = hemoglobin concentration
Limitations:
- Not a continuous real-time measure
- Reliable collection of respiratory gases requires intubation
- Implies a cardiovascular stability throughout the period of gas and blood collection
- Invalidity in the presence of significant intracardiac or intrapulmonary shunting of blood
Describe the carbon dioxide partial rebreathing method/formula
Give limitations
CO = (difference in CO2 elimination and end-tidal CO2) / (difference in arterial CO2 between baseline and rebreathing phase)
Measures the carbon dioxide production using an adjustable dead-space breathing loop in the circuit between the patients and the Y piece, and a carbon dioxide sensor.
Limitations:
- not accurate when low tidal volumes are used (e.g. lung-protective ventilation strategies)
- not accurate in small dogs
- the size of the rebreathing circuit limits its use in small dogs and cats
Describe the thermodilution method to measure CO.
Give limitations
A bolus of saline at a known temperature is injected into the right atrium, and dilution is measured in the pulmonary artery using a pulmonary artery catheter and a thermistor.
(transpulmonary thermodilution uses a central venous catheter and a thermistor inserted into the femoral artery, and is potentially as accurate)
Limitations: placement of a pulmonary arterial catheter associated with increased mortality, cost and hospital stay length in humans
Risk for catheter-related sepsis, pulmonary artery rupture, damage to cardiac structures, catheter knotting, hemorrhage, embolization.
Give 6 sources of error in CO measurement using the thermodilution method
- Respiratory cycle (pulm. artery blood cools during inspiration)
- Arrhythmias (rapid and marked changes in stroke volume)
- Altered intracardiac flow (shunting or regurgitation cause some of the injectate to bypass the thermistor or delay)
- Low cardiac output (slow ejection causes warming of the injectate before it reaches the thermistor)
- Injectate factors (wrong solution, volume, temperature)
- Thermistor factors (thrombus at the tip, migration defect)
- Additional infusions (simultaneous boluses of crystalloids, colloid, etc)
What are the disease states which could alter the CO?
Reduced stroke volume:
- Decreased preload: reduced filling pressure (hemorrhage, dehydration, etc), reduced ventricular compliance (pericardial tamponade, etc)
- Increased afterload: increased tone of the vasculature, aortic stenosis, arteriovenous fistulas, hyperviscosity syndromes
- Decreased contractility: depressed by circulating mediators (sepsis, pancreatitis, etc)
