Breathing system is an assembly of components which connects patient’s airway to anaesthesia machine through which controlled composition of gas mixture is dispensed. It delivers gas to the patient, removes expired gas and controls the temperature and humidity of the inspired mixture. It allows spontaneous, controlled, or assisted respiration. It may also provide ports for gas sampling, airway pressure, flow and volume monitoring.

Breathing systems have been classified by Conway and Mapleson.
Conway suggested a functional classification:
– Circuits requiring a CO2 absorber
– Circuits not requiring a CO2 absorber

William Mapleson designated varying arrangements of breathing system components (masks, breathing tubes, fresh gas flow inlets, adjustable pressure-limiting valves, and reservoir bags) as Mapleson A-E circuits.
Mapleson A: Arranged as FGF inlet, reservoir bag, APL valve, mask.
In this circuit, because the reservoir bag is between the FGF inlet valve and the APL valve, expired gas from the patient may re-enter the system and fill the reservoir bag during controlled ventilation. This is the most efficient system for spontaneous breathing as the FGF must only be equal to a patient’s minute ventilation to prevent rebreathing.

Mapleson B: Arranged as reservoir bag, FGF inlet, APL valve, mask.
In this circuit, the FGF inlet is closer to the APL valve, which helps prevent the rebreathing concern in the Mapleson A circuit as above during controlled ventilation.

Mapleson C: Arranged as reservoir bag, FGF inlet, APL valve, mask.
In this circuit, the arrangement is the same as the Mapleson B circuit. However, this circuit is shorter as it does not contain elongated corrugated tubing. This circuit also has the FGF inlet close to the APL valve to aid in preventing rebreathing.

Mapleson D: Arranged as reservoir bag, APL valve, FGF inlet, and mask.
In this circuit, the arrangement interchanges the FGF inlet and APL valve of the Mapleson A circuit. This system prevents rebreathing by directing FGF towards the APL valve rather than towards the patient during exhalation.

Mapleson E: Arranged as corrugated tubing, FGF inlet, and mask.
In this circuit, there is no reservoir bag and no APL valve. Given the inability to alter the pressure of the circuit, this is ideal for spontaneously ventilating neonates or paediatric patients where low-pressure ventilation is desired. The system prevents rebreathing, similar to the Mapleson D circuit.

Jackson Rees later modified the Mapleson E by adding an open ended bag, which has since become known as the Mapleson F.
Mapleson F: Arranged as APL valve directly connected to reservoir bag, corrugated tubing, FGF inlet, and mask.
The system prevents rebreathing similarly to Mapleson D by directing FGF towards the APL valve.

The principle by which a strain gauge works is that when a wire is stretched, it becomes longer and thinner, and as a result, its resistance increases.

A strain gauge, which is used in pressure transducers, acts as a resistor. When the pressure in a pressure transducer changes, the diaphragm moves, changing the tension in the resistance wire and thus changing the resistance.

Changes in current flow through the resistor are amplified and displayed as a pressure change measure.

A Wheatstone bridge, on the other hand, is frequently used to measure or monitor these changes in resistance.

A paediatric 19G Tuohy catheter is available that is 5cm in length and has 0.5cm markings

18G Tuohy catheters are generally 9 to 10cm to hub

Distal end of catheter is angled (15 to 30 degrees) and closed to avoid puncturing the dura

Epidural mesh are usually 0.2 microns and are used to filter bacteria and viruses to ensure sterility of procedure

Transparent catheters are 90cm long with diameters depending on gauge size. It has 1cm graduations from 5 to 20cm to ensure they have been inserted amply and removed completely. Distal end is smooth which can be open or closed (with lateral openings).

According to the Ohm’s law, the potential difference is defined as

V(potential difference) = I(current) x (R) resistance

So, for one resistor of 5 ohms, a 10 ampere current will generate:

V = I x R
V = 10 x 5
V = 50 volts

The formula for resistors in series can be defined as:

R(total) = R1+R2

Hence, when a current of 10 amperes passes through two 5 ohms resistors that are connected in series, the potential difference is:

V = I x (R1+R2)
V = 10 x (5+5)
V = 10 x 10
V = 100 volts

The formula for resistors that are connected in a parallel circuit is:

1/ Rtotal = 1/R1 + 1/R2

Hence, when a current of 10 amperes passes through two 5 Ω resistors that are connected in a parallel circuit, the potential difference is:

Rtotal = R1 × R2/ R1 + R2
Rtotal = 25/10
Rtotal = 2.5
V = I x R
V = 10 x (R1xR2 / R1 + R2)
V = 10 x (25/10)
V = 10 x 2.5
V = 25 volts

Capacitors are electronic components that have the ability to store energy and charge (Q). The derived SI unit of capacitance (C) is the farad (F), which is equivalent to one coulomb per volt (V). The typical capacitors usually have a very small capacitance range, that ranges from pico to microfarads. On the contrary, supercapacitors can have a capacitance of up to 1-5000 F.

There are a number of factors that eventually determine the capacitance (C). They are as follows:

– Larger plate area (A)
– Closer plate spacing (d)
– Permittivity (ε) of the material (dielectric) between the plates (vacuum<<<glass), – C = ε × A/d

The units of stored charge are coulombs (Q), which is equal to the pathway of one ampere of current per second.
Stored charge, capacitance and voltage can be defined by the following equation:

V (potential difference across capacitor) = Q(charge) / C (capacitance)

In a parallel circuit, the formula of capacitors is:

Ctotal = C1 + C2

Hence, two 5 farad capacitors in a parallel circuit arrangement with a charge of 100 coulomb and capacitance 10 F will give a potential difference of::

V = 100/10
V = 10 volts

In a series circuit, the formula for capacitors is:

1/Ctotal = 1/C1 + 1/C2

Hence, two 5 farad capacitors with a charge of 100 coulomb will give:

Ctotal = C1 × C2/C1 + C2

In the example total capacitance = 25/10 = 2.5 F

V = 100/2.5
V = 40 volts.

There are two types of defibrillators
AC defibrillator
DC defibrillator

AC defibrillator,
consists of a step-up transformer with primary and secondary winding and two switches. Since secondary coil consists of more turns of wire than the primary coil, it induces larger voltage. A voltage value ranging between 250V to 750V is applied for AC external defibrillator. And used to enable the charging of a capacitor.

DC defibrillator,
consists of auto transformer T1 that acts as primary of the high voltage transformer T2. Is an iron core that transfers energy between 2 circuits by electromagnetic induction. Transformers are used to isolate circuits, change impedance and alter voltage output. transformers do not convert AC to DC.

Diode rectifier composed of 4 diodes made of semiconductor material allows current to flow only in one direction. Alternating current (AC) passing through these diodes produces direct current (DC). Capacitor stores the charge in the form of an electrostatic field.

Capacitor is used to convert the rectified AC voltage to produce DC voltage but capacitors do not directly convert AC to DC.

Inductor induces a counter electromotive force(emf) that reduces the capacitor discharge value.

In step-down transformer primary coils has more turns of wire than secondary coil, so induced voltage is smaller in the secondary coil.

Flowmeters measure the rate at which a specific gas, that the flowmeter has been calibrated for, passes through. This calibration is done at room temperature and standard atmospheric pressure with an accuracy of +/- 2%.

Reading the flowmeter is done from the top of a bobbin (the midpoint of a ball). Oxygen is the last gas to be added downstream to the mixture delivered to the back bar as a safety feature. This prevents delivery of a hypoxic mixture.

Inaccurate flow measurements occur when the bobbin sticks to the inside wall of the flowmeter. Stannic oxide has been used as a successful antistatic substance thus, reducing the aforementioned risk.

Carbon dioxide being easily delivered is found on some older machines, but those attached flowmeters are limited by a maximum flow of 500 ml /min. Thus avoiding the delivery of a hypercarbic mixture.

The body of the Entonox cylinder is usually blue (occasionally white), with blue and white shoulders. Entonox contains a 50:50 mixture of oxygen and nitrous oxide, with a full cylinder pressure of 13700 KPa (137 bar). The cylinder is equipped with a two-stage pressure regulator for safe operation.

The cylinder body and shoulder of nitrous oxide are (French) blue.

In today’s anaesthetic workstations, carbon dioxide cylinders are no longer used.

The body of an oxygen cylinder is black, with a white shoulder.

The white Heliox (21 percent oxygen and 79 percent helium) cylinder has a brown and white shoulder. The administration of this gas mixture, which is less dense than air, is used to reduce turbulence (stridor) of inspiratory flow in patients with upper airway obstruction.

Two bonded wires of dissimilar metals, iron/constantan or copper/constantan, make up a thermocouple (constantan is an alloy of copper and nickel). At the tip, a thermojunction voltage is generated that is proportional to temperature (Seebeck effect).

All of the other connections are non-linear.

For a single compartment model, the relationship between a decrease in plasma concentration of an intravenous bolus of a drug and time is a washout exponential.

A sine wave is the relationship between current and degrees or time from a mains power source.

A sigmoid curve represents the relationship between efficacy and log-dose of a pure agonist on mu receptors.

The pressure of a fixed mass of gas and its volume (Boyle’s law) at a fixed temperature are inversely proportional, resulting in a hyperbolic curve.

A higher frequency ultrasound comes with a better resolution of the digital image. Ultrasound with a frequency of 15 MHz is best used in imaging of superficial organs such as the thyroid gland, muscles, tendons and breasts whereas deep organs are better imaged using a lower frequency of 2-7MHz because of its ability for deeper penetration but lower resolution. These low frequency probes are also used to diagnose ascites, pleural effusions or can be used in echocardiography.

The US probe emits and then absorbs a reflected wave. Similar to brightness control, increasing the gain will amplify the return signal which is then attenuated by the tissue. This increases the signal to noise ratio.
A high frame rate, which basically means the number of times an image is updated onto the screen per second, improves the resolution of a moving 3D image which has become more accurate as the computing power has increased.

Widening of the image field can be obtained by altering the penetration depth which is obtained by changing the frequency of the US beam.

Mapleson classified breathing systems into A, B, C, D and E. Jackson-Rees subsequently modified the Mapleson E by adding a double-ended bag to the end of the reservoir tubing, creating the Mapleson F. A Mapleson E or T-piece does not have a reservoir bag.

A Mapleson A system is a very efficient system for use during spontaneous ventilation. However, it is not suitable for use with patients less than 25 kg, due to the increased dead space at the distal / patient end. This system can be modified into a Lack system or coaxial Mapleson A, where the fresh gas flows through an outer tube (30 mm) and exhaled gases flow through the inner tube (14 mm).

The adjustable pressure limiting valve (APL) or expiratory valve allows exhaled gas and excess fresh gas to leave the breathing system. It is a one-way, adjustable spring-loaded valve, and gases escape when the pressure in the system exceeds the valve opening pressure. During spontaneous ventilation a pressure of less than 1 cm of water (0.1 kPa) is needed when the valve is in the open position (not 2 cm of H2O).

The reservoir bag is highly compliant and when over inflated, the rubber bag can limit the pressure in the system to about 40 cm of H2O.

This is due to the law of Laplace, which states that the pressure will fall as the radius of the bag increases:

Pressure = 2 x tension/radius.

Dinamap continuously measures the systolic, diastolic and mean arterial pressure along with pulse rate, thereby providing a continuous monitoring of the blood pressure using the osscillitonometric principle of measurement.

The device loses accuracy towards the extremes of BP and is more accurate with systolic compared with diastolic pressure. In arrhythmias such as AF, the devices are also inaccurate due to the major fluctuations associated with the individual pulse pressure variations.

The manual BP device is still the gold standard for BP measurement and monitoring.

The nerve stimulators deliver a stimulus lasting for 1-2 milliseconds (not second) to perform nerve blockage.

There are just 2 leads (not 3); one for the skin and other for the needle.

Prior to the administration of the local anaesthesia, a current of 0.25 – 0.5 mA (not 1-2mA) at the frequency of 1-2 Hz is preferred.

If the needle tip is close to the nerve, muscular contraction could be possible at the lowest possible current.

Insulated needles have improved the block success rate, as the current is only conducting through needle tip.

Stimulus to the femoral nerve which is placed in the mid lingual line causes withdrawer of the quadriceps and knee extension, that’s the dancing patella ( not plantar flexion).

Among the breathing circuits, the Lack circuit is the most efficient for spontaneous breathing.

An outer coaxial tube is present to deliver fresh air; exhaust air is routed to an inner tube, which is then delivered to a scavenging system. An expiratory valve is seen at the patient end, which is an advantage over other circuits. Moreover, the Lack circuit prevents rebreathing slightly greater than the alveolar minute ventilation at 4-5 litres per minute.

The Bain circuit prevents rebreathing at 160-200ml/kg per minute, and is a co-axial version of the Mapleson D circuit.

The Mapleson E circuit prevent rebreathing at a fresh gas flow (FGF) of approximately twice the patient’s normal minute volume. A modification of this, the Mapleson F, has a reservoir bag at the opposite end for the FGF. This circuit is appropriate for paediatric patients with a body weight less than 20 kg.

End-tidal carbon dioxide (ETCO2) monitoring has been an important factor in reducing anaesthesia-related mortality and morbidity. Hypercarbia, or hypercapnia, occurs when levels of CO2 in the blood become abnormally high (Paco2 >45 mm Hg). Hypercarbia is confirmed by arterial blood gas analysis. When using capnography to approximate Paco2, remember that the normal arterial€“end-tidal carbon dioxide gradient is roughly 5 mm Hg. Hypercarbia, therefore, occurs when PETco2 is greater than 40 mm Hg.

The most likely explanation for the changes in capnograph is either exhaustion of the soda lime and a progressive rise in circuit dead space.

Inspect the soda lime canister for a change in colour of the granules. To overcome soda lime exhaustion, the first step is to increase the fresh gas flow (FGF) (Option A). Then, if need arises, replace the soda lime granules. Other strategies that can work are changing to another circuit or bypassing the soda lime canister, but remember that both these strategies are employed only after increasing FGF first. Exclude other causes of equipment deadspace too.

There are also other causes for hypercarbia to develop intraoperatively:
1. Hypoventilation is the most common cause of hypercapnia. A. Inadequate ventilation can occur with spontaneous breathing due to drugs like anaesthetic agents, opioids, residual NMDs, chronic respiratory or neuromuscular disease, cerebrovascular accident.
B. In controlled ventilation, hypercapnia due to circuit leaks, disconnection or miscalculation of patient’s minute volume.
2. Rebreathing – Soda lime exhaustion with circle, inadequate fresh gas flow into Mapleson circuits and increased breathing system deadspace.
3. Endogenous source – Tourniquet release, hypermetabolic states (MH or thyroid storm) and release of vascular clamps.
4. Exogenous source – Absorption of CO2 from pneumoperitoneum.

Ambient pressure has no effect on a volatile agent’s saturated vapour pressure (SVP). At a temperature of 20°C, the SVP of sevoflurane is approximately 21 kPa, or 21% of atmospheric pressure (100 kPa).

The SVP of sevoflurane remains the same when the ambient pressure is doubled to 200 kPa, but the output of the vaporiser is halved, now 21 percent of 200 kPa, equalling 10.5 percent. The vaporiser’s output has increased to 1%, but the partial pressure output has remained unchanged. The splitting ratio will not change because it is determined by temperature changes.

Calculations can be made as follows:

Vaporizer output % (ambient pressure) = % volatile (calibrated) x 100 kPa calibrated pressure/ambient pressure
2% = 2% (dialled) × 100/100
2% of 100 = 2 kPa

Altitude, pressure 50 kPa
4% = 2% (dialled) × 100/50
4% of 50 = 2 kPa

High pressure at 200 kPa
1% = 2% (dialled) × 100/200
1% of 200 = 2 kPa

Sevoflurane has a boiling point of 58°C and, unlike desflurane (which has a boiling point of 22.8°C), does not need to be heated and pressurised with a Tec 6 vaporiser.

A thermocouple, or a thermal junction, is temperature measuring device consisting of a pair of dissimilar metal (bimetallic) wires or strips joined together. Typically, copper and constantan (an alloy of 55% copper and 45% nickel) are used. When there is contact between these metals, a small voltage is generated in the order of millivolts. The magnitude of the thermojunction electromotive force (emf) is proportional to applied temperature (the Seebeck effect). This physical principle is applied in the measurement of temperature. The electromotive force at the measuring junction is proportional to temperature.

Two wires with different coefficients of expansion, joined together, can be used as a switch for thermostatic control.

Semiconductors are NOT used in thermocouple. The resistance of the measuring junction of a thermocouple is irrelevant.

A Wrights Respirometer measures the volume of air exhaled over the course of one minute of normal breathing

It is unidirectional and measures tidal volume and minute volume of gas flow in one direction. It is placed at the expiratory side (lower pressure than inspiratory side therefore lower chances of gas leaks)

Slits are arranged such that incoming gas will rotate the vane at a rate of 150 revolutions per litre of flowing gas

The Wright respirometer tends to over-read at high flow rates and under-read at low flows because of mechanical causes like friction and inertia and the accumulation of water vapour

The ideal flow for accurate readings is 2 L/min for the respirometer. The respirometer reads the tidal volume and minute volume with a ±5€“10% accuracy within the range of 4€“24 L/min.

Medical gas cylinders are made up of molybdenum steel but not cast iron. They are checked and assessed at a regular interval.

At least one cylinder in each hundred are tested for tensile, pressure, smash, twist and straightening.

Nitrous Oxide cylinders contain a mixture of liquid and vapour at a pressure of approx. 4500 kPa or 45 Bar. Carbon dioxide cylinder contain gas at the pressure of 5000kPa.

The filling ratio is the ratio of mass of liquified gas in the cylinder to the mass of water required to fill the cylinder at the temperature of 15ºC. In the united kingdom, filling ratio of liquid nitrous oxide is 0.75. The cylinders are usually attached to the anaesthetic machine. As nitrous oxide is an N-methyl-d-aspartate receptor antagonist that may reduce the incidence of chronic post-surgical pain.

Carbon dioxide (CO2), is a carbonic gas made up of two dissimilar atoms, namely one carbon atom and two oxygen atoms. Capnography is a technique used to measure carbon dioxide during a respiratory cycle, and it consists in calculating the concentration of the partial pressure of CO2, through the absorption of the infrared light, namely that CO2 absorbs infrared radiation at a wavelength of 4.28 µm.

End-tidal CO2 (ETCO2), referring to the level of the carbon dioxide released at the end of an exhaled breath, is required to be continuously monitored, especially in ventilated patients, as it is a sensitive and a non invasive technique that provides immediate information about ventilation, circulation, and metabolism functions. ETCO2 is normally lower than the arterial partial pressure and varies between 0.6 and 0.7 kPa.

There are two methods used to measure carbon dioxide. The sidestream capnometer method samples gases at a set flow rate (150-200 mL/min) from a sampling area through small diameter tubing, and the mainstream analyser method that uses a direct measurement of the patient exhaled CO2 by a relatively large and heavy sensors. Sidestram method allows the analysis of multiple gases and anaesthetic vapours comparing to the mainstream method that does not allow the measurement of other gases.

Control of Substances Hazardous to Health (COSHH) was established by government under the Health and Safety at Work act in 1989. Their employers work on identification and management of those substances that are dangerous to health. The implications for anaesthetists include gas scavenging, equipment contamination and environmental safety. Adequate ventilation is required in areas where anaesthetic gases are present. The minimum air supply that is legally required in each specific area is: Operating theatres: 0.65 m3/second. Anaesthetic rooms: 0.15 m3/s. Preparation rooms: 0.1 m3/s. Recovery rooms need 15 air changes per hour.

There are different models of flowmeters determined by the applied pressure and its orifice. For instance, the watersight flowmeter functions through applying variable pressure, and it has a variable orifice. In contrast, the bubble flowmeter is operated using a constant pressure and orifice. Flowmeters such as rotameters, Heidbrink and Peak have a constant pressure but variable orifice. On the other hand, flowmeters including a simple pressure gauge, water depression, and pneumotachograph have a constant orifice but variable pressure.

Electromagnetic waves are categorized according to their frequency or equivalently according to their wavelength. Visible light makes up a small part of the full electromagnetic spectrum.

Electromagnetic waves with shorter wavelengths and higher frequencies include ultraviolet light, X-rays, and gamma rays. Electromagnetic waves with longer wavelengths and lower frequencies include infrared light, microwaves, and radio and televisions waves.

Different electromagnetic waves according to their wavelength from shorter to longer are X-rays, ultraviolet radiations, visible light, infrared radiation, radio waves. X-ray among electromagnetic waves has the shortest wavelength and higher frequency with wavelengths ranging from 10*-8 to 10* -12 and corresponding frequencies.

The pulse oximeter provides a continuous non-invasive measurement of the arterial oxygen saturation. The light emitting diodes (LEDs) produce beams of red and infrared light at 660 nm and 940 nm respectively (not 640 and 960 nm), which travel through a finger (toe, ear lobe or nose) and are then detected by a sensitive photodetector.

The light absorbed by non-pulsatile tissues is constant (DC), and the non-constant absorption (AC) is the result of arterial blood pulsation. The DC and AC components at 660 and 940 nm are then analysed by the microprocessor and the result is related to the arterial saturation.

An isosbestic point is a point at which two substances absorb a wavelength of light to the same degree. In pulse oximetry the different absorption profiles of oxyhaemoglobin and deoxyhaemoglobin are used to quantify the haemoglobin saturation (in %). Isosbestic points occur at 590 and 805 nm (not 490 and 805 nm), where the light absorbed is independent of the degree of saturation, and are used as reference points.

The pulse oximeter is accurate to within +/- 2% in the range of 70% to 100% saturation, and below 70% the readings are extrapolated. Pulse oximeters average their readings every 10 to 20 seconds and thus they cannot detect acute desaturation events. Consequently, they are often referred to as ‘lag’ monitors, due to the time delay in identifying the desaturation episode.

Eye damage is the most common potential hazard associated with laser energy. Everyone in the laser treatment room has the risk of eye exposure when working with a Class 3b or Class 4 healthcare laser system, and damage to various structures in the eye depending on wavelength of the laser if they are unprotected.

Red and near-infrared light (400-1400 nm) has very high penetration power. The light causes painless burns on the retina after it is absorbed by melanin in the pigment epithelium just behind the photoreceptors.

Infrared radiation (IR), or infrared light (>1060 nm), is a type of radiant energy that’s invisible to human eyes and hence won’t elicit the protective blink.

Ultraviolet light (<400 nm) is also a form of electromagnetic radiation which is can penetrate the cornea and be absorbed by the iris or the pupil and cause burn injuries or cataract occur due to irreversible photochemical retinal damage.

Safety eyewear is the best method of providing eye protection and are designed to absorb light specific to the laser being used. Laser protective eyewear (LPE) includes glasses or goggles of proper optical density (OD). The lenses should not be glass or plastic. The LPE should withstand direct and diffuse scattered laser beams.

The laser protection supervisor (LPS) or LSO is an individual who is responsible for any clinical area in which lasers are used. They are expected to have a certain level of equipment and determine what control measures are appropriate, for each individual system, but their presence does not guarantee the chances of having an eye injury.

Class 1 lasers are generally safe under every conceivable condition and is not likely to cause any eye damage. Class 3b or Class 4 medical laser systems are utilized in healthcare which have their own safety precautions.

Polarized spectacles can make your eyes more comfortable by eliminated glare, however, they will not be able to offer any protection against wavelengths at which laser act.
Using short bursts to reduce energy is also not correct as it would still be harmful to eye.

Warming, humidifying, and filtering inspired anaesthetic gases is done by heat and moisture exchangers (HME) and breathing system filters. They are made of glass fibres materials and are supported by a sturdy frame. Pleating increases the surface area to reduce resistance to air flow and boost efficiency.

Filters’ effectiveness is determined by the amount and size of particles they keep out of the patient’s airway. The efficiency of filters might be classified as 95, 99.95, or 99.97 percent. Pores with a diameter of 0.2 µm are common. The following are examples of typical particle sizes:
Red blood cell – 5 µm
Lymphocyte – 5-8 µm
Viruses – 0.02-0.3 µm
Bacteria – 0.5-1 µm
Depending on particle size, gas flow speed, and charge, particles are collected via a number of processes. Mechanical sieve, interception, diffusion, electrostatic filtration, and inertial impaction are some of the options:

Sieve:
The diameter of the particle the filter is supposed to collect is smaller than the apertures of the filter’s fibres.

Interception:
When a particle following a gas streamline approaches a fibre within one radius of itself, it becomes attached and captured.
Diffusion:

A particle’s random (Brownian) zig-zag path or motion causes it to collide with a fibre.
By attracting and capturing a particle from within the gas flow, it generates a lower-concentration patch within the gas flow into which another particle diffuses, only to be captured. At low gas velocities and with smaller particles (0.1µm diameter), this is more common.

Electrostatic:

These filters use large diameter fibre media and rely on electrostatic charges to improve fine particle removal effectiveness.

Impaction due to inertia:

When a particle is too large to respond fast to abrupt changes in streamline direction near a filter fibre, this happens. Because of its inertia, the particle will continue on its original course and collide with the filter fibre. When high gas velocities and dense fibre packing of the filter media are present, this sort of filtration mechanism is most prevalent.

This patient is morbidly obese and has a high risk of developing hypoxia. This will be exacerbated by the patient’s supine position, as a result of:

Functional residual capacity has been reduced (FRC)
Increased closing capacity (CC)
Reduced tidal volume due to increased airway resistance, decreased thoracic cage compliance, and decreased respiratory muscle strength and endurance
Following induction of general anaesthesia, there is a tendency for atelectasis and increased O2 consumption due to the increased workload of respiratory muscles and the overall increase in metabolism.

Pre-oxygenation with 100 percent oxygen via a tight-fitting mask can be done using either tidal volume breaths for three to five minutes or four vital capacity breaths in normal circumstances. In the head-up position, this patient is much more likely to be adequately pre-oxygenated, maximising the FRC and minimising the CC. In spontaneously breathing patients, the Mapleson A and circle systems are both effective, but the Mapleson D requires 160-200 ml/kg/minute to prevent rebreathing.

Pulse oximeters combine the principles of oximetry and plethysmography to noninvasively measure oxygen saturation in arterial blood. A sensor containing two or three light emitting diodes and a photodiode is placed across a perfused body part, commonly a finger, to be transilluminated. Oximetry depends on oxyhaemoglobin and deoxyhaemoglobin, and their ability to absorb the beams of light produced by the light emitting diodes: red light at 660 nm and infrared light at 960 nm.

The isosbestic point is the point wherein two different substances absorb light to the same extent. For oxyhaemoglobin and deoxyhaemoglobin, the points are at 590 nm and 805 nm. These are considered reference points where light absorption is independent of the degree of saturation.

Non-constant absorption of light is often due to the presence of an arterial pulsation, whilst constant absorption of light is seen in non-pulsatile tissues.

Most pulse oximeters are inaccurate at low SpO2, but is accurate at +/- 2% within the range of 70% to 100% SpO2. All pulse oximeters demonstrate a delay in between changes in SaO2 and SpO2, and display average readings every 10 to 20 seconds, hence they are unable to detect acute desaturation episodes.

It’s a minute volume divider – True
The Manley MP3 ventilator is classed as a minute volume divider. The entire fresh gas flow or minute volume is delivered to the patient, having been divided into readily set tidal volumes.

Can efficiently ventilate patients with poor pulmonary compliance – False
Ventilating patients with poor pulmonary compliance is not easily achieved, which makes it an unsuitable ventilator for a modern ICU.

Can generate tidal volume up to 1500ml – False
It can generate tidal volumes up to 1000 ml (not 1500 ml), and the inflation pressure can be adjusted by sliding a weight along a rail.

Functions like a Mapleson A system during spontaneous ventilation – False
The ventilator functions like a Mapleson D breathing system (not Mapleson A) during spontaneous ventilation.

Has three sets of bellows – False
The fresh gas flow drives the ventilator which allows rapid detection of gas supply failure. It has two sets of bellows (not three) and three unidirectional valves.

Laminar flow can be defined as the motion of a fluid where every particle in the fluid follows the same path of its previous particles. The following are properties of laminar flow of gas or fluids:

1. Smooth unobstructed flow of gas through a tube of relatively uniform diameter
2. Few directional changes
3. Slow, steady flow through straight smooth, rigid, large calibre, cylindrical tube
4. Outer layer flow slower than the centre due to friction, results in discrete cylindrical layers, or streamlines
5. Double flow by doubling pressure as long as the flow pattern remains laminar

Poiseuille’s Law relates the factors that determine laminar flow. It indicates the degree of resistance to fluid flow through a tube. The resistance to fluid flow through a tube is directly related to the length, flow and viscosity; and inversely related to the radius of the tube to the fourth power. This means that, when the radius is doubled, there is increase in flow by a factor of 16.

Direct laryngoscopy are performed using laryngoscopes and they can be classed according to the shape of the blade as curved or straight.

Miller, Soper, Wisconsin and Seward are examples of straight blade laryngoscopes. Straight blades are commonly used for intubating neonates and infants but can be used in adults too.

The tip of the miller blade is advanced over the epiglottis to the tracheal entrance then lifted in order to view the vocal cords.

The RIGHT-SIDED Macintosh blade is used in adults while the left-sided blade may be used in conditions that make intubation with standard blade difficult e.g. facial deformities.

The McCoy laryngoscope is based on the STANDARD MACINTOSH blade not Robertshaw’s. It has a lever operated hinged tip, which improves the view during laryngoscopy.

Polio blade is mounted at an angle of 120-135 degrees to the handle. Originally designed for use during the polio epidemic €‹in intubation patients within iron lung ventilators, it is now useful in patients with conditions like breast hypertrophy, barrel chest, and restricted neck mobility.