Resistance pulmonary arteries constrict in response to alveolar and airway hypoxia, diverting blood to better-oxygenated alveoli.

In atelectasis, pneumonia, asthma, and adult respiratory distress syndrome, hypoxic pulmonary vasoconstriction optimises O2 uptake. Hypoxic pulmonary vasoconstriction helps maintain systemic oxygenation during single-lung anaesthesia.

A redox-based O2 sensor within pulmonary artery smooth muscle cells is involved in hypoxic pulmonary vasoconstriction. The production of reactive oxygen species by smooth muscle cells in the pulmonary artery varies in proportion to PaO2. Hypoxic removal of these redox second messengers inhibits voltage-gated potassium channels, depolarizing smooth muscle cells in the pulmonary artery.

L-type calcium channels are activated by depolarization, which raises cytosolic calcium and causes hypoxic pulmonary vasoconstriction. Some anaesthetics suppress this response, increasing the risk of further deterioration in ventilation perfusion mismatch.

Agents that inhibit HPV are ether, halothane, and desflurane (>1.6 MAC).
Agents with no effect on HPV include thiopentone, fentanyl, desflurane (1MAC), isoflurane (<1.5MAC), sevoflurane(1MAC), and propofol.

The correct answer is the Purkinje fibres, which conducts at a velocity of about 4m/sec.

The electrical conduction system of the heart starts with the SA node which generates spontaneous action potentials.

This is conducted across both atria by cell to cell conduction, and occurs at around 1 m/s. The only pathway for the action potential to enter the ventricles is through the AV node in a normal heart.
At this site, conduction is very slow at 0.05ms, which allows for the atria to completely contract and fill the ventricles with blood before the ventricles depolarise and contract.

The action potentials are conducted through the Bundle of His from the AV node which then splits into the left and right bundle branches. This conduction is very fast, (,2m/s), and brings the action potential to the Purkinje fibres.

Purkinje fibres are specialised conducting cells which allow for a faster conduction speed of the action potential (,2-4m/s). This allows for a strong synchronized contraction from the ventricle and thus efficient generation of pressure in systole.

The data is ideal for chi square test evaluation as it will help determine if observed outcomes are in line with expected outcomes, and also if results are significant or due to chance.

The student’s t test is not ideal as it requires comparison of means from different populations, rather than proportions.

Pearson’s coefficient of linear regression is not ideal as it requires the plotting of a linear regression.

The numbers should be analysed before determining if there are any statistical conclusions that can be drawn from the population.

Statistical analysis is always required to determine the performance of any treatment during a clinical drug trial. Conclusions cannot be drawn simply by looking at the data.

This may be the classical case of blood transfusion reaction due to ABO incompatibility.

Here red cells are destroyed in the bloodstream with the release of haemoglobin in circulation (causing haemoglobinuria). Here, IgM or IgG anti-A or anti-B antibody can cause rapid activation of complement cascade usually the classical pathway. This is called intravascular haemolysis.

There may be extravascular haemolysis by cells of the mononuclear phagocyte system situated in the liver and spleen. Extravascular red cell destruction can increase breakdown products of haemoglobin, such as bilirubin and urobilinogen.

Adenosine receptors are expressed on the surface of most cells.
Four subtypes are known to exist which are A1, A2A, A2B and A3.

Of these, the A1 and A2 receptors are present peripherally and centrally. There are agonists at the A1 receptors which are antinociceptive, which reduce the sensitivity to a painful stimuli for the individual. There are also agonists at the A2 receptors which are algogenic and activation of these results in pain.

The role of adenosine and other A1 receptor agonists is currently under investigation for use in acute and chronic pain states.

The mitral valve is the valve between the left atrium and left ventricle. It opens when the heart is in diastole (relaxation) which allows blood to flow from the left atrium to the left ventricle. In systole (contraction), the mitral valve closes to prevent the backflow of blood from the left ventricle to the left atrium.

The mitral valve is located posterior to the sternum at the level of the 4th costal cartilage. It is best auscultated over the cardiac apex, where its closure marks the first heart sound.

The mitral valve anatomy is composed of five main structures:
1. Left atrial wall – the myocardium of the left atrial wall extends over the posterior leaflet of the mitral valve. (left atrial enlargement is one of the causes for mitral regurgitation)
2. Mitral annulus – a fibrous ring that connects with the anterior and posterior leaflets. It functions as a sphincter that contracts and reduces the surface area of the valve during systole (Annular dilatation can also lead to mitral regurgitation)
3. Mitral valve leaflets (cusps) – The mitral valve is the only valve in the heart with two cusps or leaflets. One anterior and one posterior.
i. The anterior leaflet is located posterior to the aortic root and is also anchored to the aortic root.
ii. The posterior leaflet is located posterior to the two commissural areas.
4. Chordae tendinae – The chordae tendinae connects both the cusps to the papillary muscles.
5. Papillary muscles – These muscles and their cords support the mitral valve, allowing the cusps to resist the pressure developed during contractions (pumping) of the left ventricle

The anterior and posterior cusps are attached to the chordae tendinae which itself is attached to the left ventricle via papillary muscle.

Structural isomers have a similar molecular formula, but they have a different structural formula as their atoms are arranged in a different manner. Such small changes lead to the differential pharmacological activity. Enflurane and isoflurane are two prime examples of structural isomers.

Stereoisomers are those substances that have a similar molecular and structural formula, but the arrangement spatially of atoms are different and have optical activity.

Enantiomers are a pair of stereoisomers, which are non-superimposable mirror images of each other. They also have chiral centres of molecular symmetry. Ketamine is considered as an example of racemic mixture (contain 50% R and 50% S enantiomers)

Geometric isomers contain a carbon-carbon double bond (i.e. C=C) or a rigid carbon-carbon single bond in a heterocyclic ring. Cis-atracurium is one example.

Dynamic isomers or Tautomers are a pait of unstable structural isomers, which are present in equilibrium. One isomer can easily change after the change in pH. Midazolam and thiopentone are their examples.

The magnitude of x determines the rate of change of x. First-order drug kinetics is a good example. Most drugs’ plasma levels are controlled by an exponential process. The rate of change in drug metabolism is proportional to the current plasma concentration (so-called non-linear kinetics).

A tear-away function is just one type of exponential relationship (y = ex), in which e is Euler’s number, x is the power, and e is the base. Natural logarithms rely on Euler’s number.

Euler’s number is a mathematical constant, not a mathematical principle. It’s referred to as an irrational number. This is a number that cannot be expressed as a simple fraction or a ratio.

A line or curve that acts as the limit of another line or curve is known as an asymptote. A washout exponential curve, for example, where the value y represents the plasma concentration of a drug in a single compartment model against time on the x axis. This descending curve approaches but never touches the x axis. This curve is asymptotic to the x axis, which is the curve’s asymptote. An asymptote isn’t just a characteristic of exponential curves.

There are different types of temperature measurement. These include:

Thermistor – this is a type of semiconductor, meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. There are small beads of semiconductor material (e.g. metal oxide) which are incorporated into a Wheatstone bridge circuit. As the temperature increases, the resistance of the bead decreases exponentially

Thermocouple – Two different metals make up a thermocouple. Generally, in the form of two wires twisted, welded, or crimped together. Temperature is sensed by measuring the voltage. A potential difference is created that is proportional to the temperature at the junction (Seebeck effect)

Platinum resistance thermometers (PTR) – uses platinum for determining the temperature. The principle used is that the resistance of platinum changes with the change of temperature. The thermometer measures the temperature over the range of 200°C to1200°C. Resistance in metals show a linear increase with temperature

Tympanic thermometers – uses infrared radiation which is emitted by all living beings. It analyses the intensity and wavelength and then transduces the heat energy into a measurable electrical output

Gauge/dial thermometers – Uses coils of different metals with different co-efficient of expansion. These either tighten or relax with changes in temperature, moving a lever on a calibrated dial.

If there is no change in preload and myocardial contractility, there will be decrease in end-diastolic volume and stroke volume. So there must be increase in end-systolic volume.

Anticholinergic agents are a group of drugs that blocks the action of the neurotransmitter called acetylcholine at synapses in the central and peripheral nervous system.

Hyoscine, atropine, and glycopyrrolate are anticholinergic which acts at muscarinic receptors with little activity at the nicotinic receptors.

Hyoscine and atropine are naturally occurring esters. Since Glycopyrrolate is a synthetic quaternary amine, it does not cross the blood brain barrier. Noteworthy, hyoscine, butylbromide also does not cross the blood brain barrier significantly.

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)

The clinical potency of the anaesthetic agent is measured using minimal alveolar concentration(MAC).

MAC and oil: gas partition coefficient is inversely related. Anaesthetic agent Oil/gas partition coefficient and Minimal alveolar concentration (MAC) is given respectively as

Desflurane 18 6
Isoflurane 90 1.2
Nitrous oxide 1.4 104
Sevoflurane 53.4 2
Xenon 1.9 71

With these data, we can conclude Isoflurane is the most potent with the highest oil/gas partition coefficient of 90 and the lowest MAC of 1.2

Cardiac output = stroke volume x heart rate

Left ventricular ejection fraction = (stroke volume / end diastolic LV volume ) x 100%

Stroke volume = end diastolic LV volume – end systolic LV volume

Pulse pressure = Systolic Pressure – Diastolic Pressure

Systemic vascular resistance = mean arterial pressure / cardiac output
Factors that increase pulse pressure include:
-a less compliant aorta (this tends to occur with advancing age)
-increased stroke volume

Oxytocin is secreted by the posterior pituitary along with Antidiuretic Hormone (ADH). It increases the contraction of the upper segment (fundus and body) of the uterus whereas the lower segment is relaxed facilitating the expulsion of the foetus.

Oxytocin acts through G protein-coupled receptor and phosphoinositide-calcium second messenger system to contract uterine smooth muscle.

It has 0.5 to 1 % ADH activity introducing possibilities of water intoxication when used in high doses.

The sensitivity of the uterus to oxytocin increases as the pregnancy progresses.

It is used for induction of labour in post maturity and uterine inertia.

Selection of a chest drain will depend on the external circumference.

A cannula, whether intravenous or intra-arterial, are classified according to standard wire gauge, which refers to the number of wires that can fit into the same hole. If a cannula is labelled 22G, then 22 wires will fit into the standard size hole.

A more popular measurement than SWG nowadays is cross sectional area.

When the concern for selecting equipment is the rate of flow, then it is important to consider the diameter and the radius of a parallel sided tube. These, however, are not routinely considered when comparing sizes of a cannula.

The guidelines for the management of cardiac arrest in hypothermic patients published by the UK Resuscitation Council differ slightly from the standard algorithm.

In a patient with a core temperature of less than 30°C, do the following:

If you’re on the shockable side of the algorithm (VF/VT), you should give three DC shocks.
Further shocks are not recommended until the patient has been rewarmed to a temperature of more than 30°C because the rhythm is refractory and unlikely to change.
There should be no drugs given because they will be ineffective.

In a patient with a core temperature of 30°C to 35°C, do the following:

DC shocks are used as usual.
Because they are metabolised much more slowly, the time between drug doses should be doubled.

Active rewarming and protection against hyperthermia should be given to the patient.

Option e is false because there is insufficient information to determine whether resuscitation should be stopped.

When the eye is compressed or the extra-ocular muscles are tractioned, the oculocardiac reflex causes a decrease in heart rate.

The ophthalmic division of the trigeminal nerve provides the afferent limb. This synapses with the vagus nerve’s visceral motor nucleus in the brainstem. The efferent signal is carried by the vagus nerve to the heart, where increased parasympathetic tone reduces sinoatrial node output and slows heart rate.

The most common symptom is sinus bradycardia, but junctional rhythm and asystole can also occur.

Propofol is a short-acting medication used for starting and maintenance of general anaesthesia, sedation for mechanically ventilated adults, and procedural sedation.
The dose of propofol is 1-2 mg/kg.

Dose of some other important drugs are listed below:
Thiopental dose: 3-7 mg/kg
Ketamine dose: 1-2 mg/kg
Etomidate dose: 0.3 mg/kg
Methohexitone dose: 1.0-1.5 mg/kg

Hypoxic Pulmonary vasoconstriction (HPV) reflects the constriction of small pulmonary arteries in response to hypoxic alveoli (.i.e.; PO2 below 80-100mmHg or 11-13kPa).

These blood vessels become independent of the nerve stimulus, when blood with a high PO2 flows through the lung which contains a low alveolar PO2.

Thus a low PO2 within the alveoli has been shown to impact on hypoxic pulmonary vasoconstriction (HPV) more than a low PO2 within the blood.

HPV results in the blood flow being directed away from poorly ventilated areas of the lung and helps to reduce the ventilation/perfusion mismatch (not increase).

In animals, volatile anaesthetic agents can diminish HPV, while in adults, the evidence proves less persuading, in spite of the fact that it certainly doesn’t strengthen the effects.

HPV response will be suppressed by 20 parts per million (ppm) of nitric oxide.

The above mentioned patient presentation is one of an acute coronary syndrome.

The elevations noted in the ST-segments of multiple heart leads on ECG is diagnostic of an ST-elevation myocardial infarction.

The pulmonary artery branches to give rise to the right and left pulmonary arteries, which supply deoxygenated blood to the right and left lungs from the right ventricle.

The pulmonary veins do not form any bifurcations, and therefore do not give rise to any vessels. They travel to the left atrium from the lungs, carrying oxygenated blood.

The descending aorta continues from the aortic arch, and bifurcates to give off many branches, including the right and left common iliac arteries.

The coronary sinus is formed from the combination of four coronary veins, receiving blood supply from the great, middle, small and posterior cardiac veins, and transporting this venous blood into the right atrium.

The right and left aortic sinus give rise to the right and left coronary arteries, respectively. They branch of the ascending aorta, in the area just superior to the aortic valve.

During carotid endarterectomy, injury to the vagus nerve or its branches can cause hoarseness. Injury to the vagus nerve can result in adductor vocal cord paralysis. It can also cause other symptoms like dysphagia or even vocal cord immobility.

Carotid endarterectomy is the procedure to relieve an obstruction in the carotid artery by opening the artery at its origin and stripping off the atherosclerotic plaque with the intima. Because of the internal carotid artery relations, there is a risk of cranial nerve injury during the procedure involving one or more of the following nerves: CN IX, CN X (or its branch, the superior laryngeal nerve), CN XI, or CN XII.

However, only damage to the vagus would account for speech difficulties.

The measure of drug potency or therapeutic response is the ED95. This is defined as the dose of a neuromuscular blocking drug required to produce a 95% depression of muscle twitch height. The ED50 and ED90 describe a depression of twitch height by 50% and 90% respectively.

The ED95 (mg/kg) of the commonly used neuromuscular blocking agents are:

suxamethonium: 0.27
rocuronium: 0.31
vecuronium: 0.04
pancuronium: 0.07
cisatracurium: 0.04
mivacurium: 0.08

The hormone parathyroid hormone (PTH) and the receptor parathyroid hormone type 1 (PTH1-Rc) are important regulators of blood calcium homeostasis.

PTH can cause a rapid release of calcium from the matrix in bone, but it also affects long-term calcium metabolism by acting directly on bone-forming osteoblasts (by binding to PTH1-Rc) and indirectly on bone-resorbing osteoclasts.

PTH causes changes in the synthesis and/or activity of several proteins, including osteoclast-differentiating factor, also known as TRANCE or RANKL, when it acts on osteoblasts.

RANK receptors are found on the cell surfaces of osteoclast precursors. The osteoclasts are activated when RANKL binds to the RANK receptors. Osteoclasts lack PTH receptors, whereas osteoblasts do. Osteoclasts are activated indirectly when the RANK receptor binds to the RANKL secreted by osteoblasts, resulting in bone resorption. PTH1 receptors are found in osteoclasts, but they are few.

PTH activates G-protein coupled receptors in all target cells via adenylate cyclase.

The PTH2 receptor is most abundant in the nervous system and pancreas, but it is not a calcium metabolism regulator. It is abundant in the septum, midline thalamic nuclei, several hypothalamic nuclei, and the dorsal horn of the spinal cord, as well as the cerebral cortex and basal ganglia. Expression in pancreatic islet somatostatin cells is the most prominent on the periphery.

The distribution of the receptor is being used to test functional hypotheses. It may play a role in pain modulation and hypothalamic releasing-factor secretion control.

The inferior mesenteric artery arises from the abdominal aorta at the level of the L3 vertebrae and supplies blood to the final third of the transverse colon, the descending colon, the sigmoid colon and the uppermost part of the rectum.

It is the artery most likely to affect the left colon.

Skeletal muscle blood flow is in the range of 1-4 ml/min per 100 g when at rest. Blood flow can reach 50-100 ml/min per 100 g during exercise. With maximal vasodilation, blood flow can increase 20 to 50 times.

The adrenal medulla releases catecholamines and increases neural sympathetic activity during exercise. Normally, alpha-1 and alpha-2 would cause vasoconstriction in the muscle groups being used, but vasodilatory metabolites override these effects, resulting in a so-called functional sympathectomy. Local hypoxia and hypercarbia, nitric oxide, K+ ions, adenosine, and lactate are some of the stimuli that cause vasodilation.

However, the splanchnic and cutaneous circulations, which supply inactive muscles, vasoconstrict.

Sympathetic cholinergic innervation of skeletal muscle arteries is found in some species (such as cats and dogs, but not humans). Vasodilation is induced by stimulating smooth muscle beta-2 adrenoreceptors, but at rest, the alpha-adrenoreceptor effects of adrenaline and noradrenaline predominate. During exercise, the skeletal muscle pump promotes venous emptying, but it does not necessarily increase blood flow.

BMR is lower in females than males.

It decreases with increasing age.

There is an increase in BMR with increased muscle (i.e. lean tissue)

BMR is increased in stress and illness. There is also an catabolic state in these conditions.

Accumulation of morphine-6-glucuronide is a risk factor for opioid toxicity during morphine treatment. Morphine is metabolized in the liver to morphine-6-glucuronide and morphine-3-glucuronide, both of which are excreted by the kidneys. In the setting of renal failure, these metabolites can accumulate, resulting in a lowering of the seizure threshold. However, it does not occur in all patients with renal insufficiency, which is the most common reason for accumulation of morphine-6-glucuronide; this suggests that other risk factors can contribute to morphine-6-glucuronide toxicity.

The active metabolites of codeine are morphine and the morphine metabolite morphine-6-glucuronide. The enzyme systems responsible for this metabolism are: CYP2D for codeine and UGT2B7 for morphine, codeine-6-gluronide, and morphine-6-glucuronide. Both of these systems are subject to genetic variation. Some patients are ultrarapid metabolizers of codeine and produce higher levels of morphine and active metabolites in a very short period of time after administration. These increased levels will produce increased side effects, especially drowsiness and central nervous system depression.

The structures that pass through the foramen magnum are:

Meningeal lymphatics
Spinal cord
Spinal meninges
Sympathetic plexus of vertebral arteries
Vertebral arteries
Vertebral artery spinal branches
The spinal roots of the accessory nerve.

The jugular foramen contains the vagus nerve, the accessory nerve and glossopharyngeal nerve.

The vertebral veins does not pass into the skull.

Once the decision to deliver a baby by caesarean section has been made, it should be carried out with a level of urgency commensurate with the baby’s risk and the mother’s safety.

There are four types of caesarean section urgency:

Category 1: A threat to the life of the mother or the foetus. 30 minutes to make a delivery decision
Category 2 : Maternal or foetal compromise that is not immediately life threatening. In most cases, the decision to deliver is made within 75 minutes.
Category 3 – Early delivery is required, but there is no risk to the mother or the foetus.
Category 4: Elective delivery at a time that is convenient for both the mother and the maternity staff.

There may be evidence of foetal compromise in the example above (late foetal pulse decelerations and a borderline pH).

Blood samples from the foetus:
normal: 7.25 or above
borderline: 7.21 to 7.24
abnormal: 7.20 or below

When a foetal deceleration occurs, the mother should be given oxygen, kept in a left lateral position, and given a tocolytic if the foetal deceleration is hyper stimulating. Maintaining adequate hydration will reduce the likelihood of a caesarean section.