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What are the basic principles of thermo-dilution?
- A pulmonary artery catheter that includes a balloon at its tip and a thermister 10 cm from the tip.
- Cold saline injectate to create a temperature shift.
- Bolus of cold (room temp.) injectate passes by the temperature sensor, creating a shift in the time-temp. curve.
What does a typical time-temp curve for an ideal cardiac output determination look like?
How is cardiac output determined in regards to the time-temp curve?
It is determined as the area under the curve with corrections for the tail of the curve.
What is the typical cardiac output for a resting adult?
Male ~ 5 l/m
Female ~ 4.5 l/m
What is body surface area and how is it calculated?
It is the total surface area of a person's body calculated by:
How is cardiac index determined/calculated?
CI = cardiac output / body surface area
What is the normal range of cardiac index?
2.6 - 4.2 L/min per m2
What are three possible sources of error in cardiac output determinations?
- inconsistent injection
- respiratory interference
- motion artifact
What is the magnetic blood flow meter?
What is a doppler ultrasound blood flow meter?
Explain qualitative measuring:
Measurement is subjective, where data can be observed but not measured.
Explain the doppler shift example:
- If the signal source is approaching, the higher frequency detected by the observer shifts higher.
- If the signal source is receding, the frequency detected by the observer continuously shifts lower.
What is the purpose of using gel in flow detection?
Air gaps or bubbles will significantly attenuate the signal.
In a Doppler reading, the more peripheral the vessel has what effect on the frequency?
The more peripheral the vessel, the higher the frequency of the Doppler.
In a Doppler reading, the deeper the vessel has what effect on the frequency?
The deeper the vessel, the lower the frequency needed.
What frequency is used for arteries less than 1 cm deep?
9 - 10 MHz
What frequency is used for vessels roughly 2 cm deep?
What frequencies are needed larger, deeper vessels and fetal hearts?
2 to 4 MHz
What is the diameter of a 10 MHz probe?
less than 1 cm in diameter
What is the diameter of a 2 MHz probe?
roughly 4 cm in diameter
Is a Doppler probe used for direct or indirect measurements of flow in a vessel?
Explain electromagnetic flow detection in regards to a schematic:
- the magnet is AC driven
- the generator creates a feedback voltage to balance-out transformer voltage on the voltage created by flow
- the voltage is amplified, filtered and displayed as flow
Explain a magnetic blood flow meter in regards to Faraday's law:
- Blood behaves like a conductor
- an alternating magnetic field is created across the blood vessel
- an electrode at the surface of the blood vessel detects a small induced voltage
What is deep vein thrombosis?
formation of a blood clot in a deep vein.
What is deep vein thrombosis associated with...occurs from?
occurs from post surgical, smoking, birth control (Rx), and air travel
What is edema?
accumulation of fluid within and under the skin associated with heart failure or vascular disease. May also be pulmonary.
How does a sequential compression device work?
- the application of a low pressure to a compartmented cuff on the foot or leg.
- pressure is exerted at the distal compartment first then to the more proximal compartment to promote the return of venous blood and prevent the formation of clots.
List 5 items to check when troubleshooting:
- disposable cuffs (connectors and joints)
- tubing connectors (at the pump)
- user interface (membrane switches & power switch)
Explain Bernoulli's principle (gas and fluid):
- As the speed of a moving fluid increases, the pressure within the fluid decreases.
- as the rate of gas flow increases, the lateral pressure within the tube decreases. As the rate of gas flow increases, the pressure decreases such that the total energy of the gas remains constant.
- Flow is directly proportional to the pressure difference between point1 and point2
Explain Dalton's Law of partial pressures:
Gas - the total pressure of a mixture of gases is equal to the sum of the pressures of all its constituent gases . . . PT = P1 + P2 + Pn
What is the difference between respiration and ventilation?
- - Respiration refers to the entire system of O2 delivery and CO2 elimination.
- i.e. upper airway, lungs, vascular system, and metabolism at the cells.
- Ventilation refers to the movement of airway gases which only includes the mechanics of inspiration and expiration.
What are the typical gases and percentages of exhaled in ambient pressure?
- CO2 ~ 5%
- O2 ~ 16%
- N2 ~ 79%
Define: Tidal Volume
volume o fair that is inhaled or exhaled from the lungs during a breath
Define: Minute Volume
total amount of gas moving in and out of the lung during 1 minute
Define: I:E Ratio
ratio of the duration of inspiration to the duration of expiration. A range 1:15 to 1:2 for an adult is considered acceptable for mechanical ventilation
Define: Inspired Oxygen Percentage (FiO2)
FiO2 - the assumed percentage of oxygen concentration participating in gas exchange in the alveoli
Define: Peak Airway Pressure
resistive pressure + elastic pressure + PEEP
- peak airway pressure is measured at the airway opening, it represents the total pressure needed to push a volume of gas into the lungs
Define: Positive End Expiratory Pressure (PEEP)
alveolar pressure present at the beginning of a breath
Define: Continuous Positive Airway Pressure
CPAP - an invasive or noninvasive form of ventilation assisted by a flow of air delivered at a constant pressure throughout the respiratory cycle
What does Pressure-Volume loops show?
used by respiratory therapists and anesthesiologists to monitor and optimize ventilation in critical care patients or during surgery.
What is the use/reason for humidification of vent circuits?
- artificial ventilation removes moisture from the airway and can be very detrimental to airway and alveoli
- the upper airway provides 75% of the heat and moisture supplied to the alveoli
- when bypassed, it needs to be supplied by a humidifier
What is the role of hemoglobin in respiration?
to carry one oxygen molecule on each heme group (4 oxygen molecules total in each hemoglobin)
What is the basic principle of how SpO2 is determined by absorption of red and infra-red
- non-oxygenated hemoglobin partially absorbs light at 660nm where oxygenated hemoglobin absorbs light at 940nm.
- after measuring the absorption of both wavelenths at any given instant to determine the ratio, you then can determine the total homoglobin that is carrying oxygen (SpO2)
oxygen saturation SpO2 vs oxygen content in the blood
i.e. the ‘oxygen dissociation curve’
oxygen saturation (SpO2) - the percentage amount of how many oxygen molecules are being carried to the amount of oxygen molecules that can be carried by x amount of hemoglobin
tone modulation in pulse oximeters - why?
Explain Perfusion Index (PI)
What are the sources of inaccuracy in SpO2 measurement
Explain the detection of cerebral O2 saturation