Respiratory Flashcards
1
Q
How can the approximate FiO2 being supplied by a venturi mask be calculated?
A
The venturi mask delivers a known FiO2 by allowing entrainment of air
To calculate the FiO2, the flow rate and entrainment ratio must be known
The entrainment ratio is the ratio of 100% FiO2 : room air being delivered
For example:
A patient is receiving oxygen therapy using a venturi mask with an entrainment ratio of 1:10 and oxygen flow rate of 6L/min, what is the approximate inspired oxygen concentration?
6 x 1.0 = 6
60 x 0.21 = 12.6
18.6 / 66 = 0.282 = 28% FiO2
2
Q
Piped gases and Schrader valves
A
- Supplied at a pressure of 4.1Bar
- The changeover valve incorporated in a cylinder bank works on a pneumatic shuttle mechanism - When a cylinder bank starts to run out the pressure will fall, and a passive shuttle will be pushed over by the higher pressure within an adjacent full cylinder bank so that continuous downstream pressure is maintained
- Collars of probes are pipeline specific and non-interchangeable, and pipeline connections also have non-interchangeable screw threads (NIST)
- The Schrader valve contains an internal non-return valve
3
Q
Peak expiratory flow rate
A
- Normal diurnal variation is <10%
- Wright’s peak flow meter works using the principle of a variable orifice with constant pressure
4
Q
Pneumotachograph
A
- Measures flow rate
- Bidirectional - can measure inspiratory and expiratory flow
- Constant orifice, variable pressure device
- Consists of a tube which widens in order to reduce the velocity of the gas in order to achieve laminar flow
- A mesh within the tube provides resistance to flow, which causes a pressure drop across the mesh
- The drop in pressure is proportional to the flow rate, therefore a differential pressure transducer across the mesh can provide the flow rate of the gas
5
Q
Anaesthetic gas cylinders
A
- Steel alloy cylinders are made of either manganese molybdenum steel, chromium molybdenum steel or nickel chromium molybdenum steel
- Filling ratio is the weight of the cylinder contents divided by the weight of water it could hold. In the UK this is 0.75, but in hotter countries may be 0.67
- Adiabatic expansion of a gas being released at high flow rate will result in the gas cooling and therefore the cylinder will also cool
6
Q
Volume of oxygen gas cylinders
A
C = 170L
D = 340L
E = 680L
7
Q
Pulse oximeter
A
- A pulse oximeter is a non-invasive method of measuring % haemoglobin saturation in peripheral arterial blood (SpO2) and heart rate
- It consists of a probe with two light-emitting diodes that emit pulses of red (660nm) and infrared (940nm) light, and a photodetector
- The diodes cycle approximately 30x per second, with a pause when both are off to account for ambient light
- Haemoglobin absorbs different amounts of light at varying wavelengths dependent on its oxygenation. Oxyhaemoglobin absorbs more infrared light, whereas deoxyhaemoglobin absorbs more red light
- The amount of light that reaches the photodetector at each wavelength of light is analysed by a microprocessor to produce an SpO2 reading
- Pulse oximetry relies on the Beer-Lambert law - which states that the intensity of light transmitted through a transparent substance decreases exponentially as concentration of the substance and distance travelled through it increases
- The non-pulsatile component from tissues and venous blood is ignored
- The isobestic point occurs at the point at which oxygenated and deoxygenated haemoglobin absorb a certain wavelength of light to the same extent - 590nm and 805nm. These may be used as a reference point
8
Q
Inaccuracies of pulse oximetry
A
- Poor peripheral perfusion
- Carboxyhaemoglobin - overestimates the reading
- Methaemoglobin - readings tend toward 85%
- Methylene blue - underestimates the reading
- Bright ambient light
- Diathermy
- Irregular pulse rate (AF), non-pulsatile flow (cardiopulmonary bypass), pulsatile venous flow (tricuspid regurgitation)
- Nail varnish
- Foetal haemoglobin has no effect
