The arterial cannula inserted should have parallel walls in order to reduce the risk of interruption of blood flow to distal limbs.

It is essential that the monitor used to display the arterial pressure waves has a frequency capacity of 0.5-40Hz. This is because the pressure waves are a combination of different sine waves of varying frequencies and amplitudes.

The diameter of the catheter is directly proportional to the natural frequency which is the frequency at which the system responsible for monitoring the waves resonates and amplifies the signals. This should be at least ten fold in comparison to the fundamental frequency. The diameter of the catheter is also inversely proportional to the square root of the system compliance, the tubing length and the fluid density within the system.

The presence of an air bubble, a clot or an easily malleable diaphragm and tube can result in wave damping. Increased damping will cause a reduction in the systolic pressure, and an increase in diastolic pressure. The maximum damping value of an appropriate monitoring system would be 0.64.

A rigid, non-malleable diaphragm and tubing can cause a resonance within the system. This resonance will result in an increase in the systolic pressure and a reduction in the diastolic pressure

The following table shows the compartments and their relative oxygen reserve:
Compartment Factors Room air (mL) 100% O2 (mL)
Lung FAO2, FRC 630 2850
Plasma PaO2, DF, PV 7 45
Red blood cells Hb, TGV, SaO2 788 805
Myoglobin 200 200
Interstitial space 25 160

Oxygen reserves in the body, with room air and after oxygenation.

FAO2-alveolar fraction of oxygen rises to 95% after administration of 100% oxygen (CO2 = 5%)
FRC- Functional residual capacity – (the most important store of oxygen in the body) – 2,500-3,000 mL in medium sized adults
PaO2-partial pressure of oxygen dissolved in arterial blood (80 mmHg breathing room air and 500 mmHg breathing 100% oxygen)
DF -dissolved form (0.3%)
PV-plasma volume (3L)
TG-total globular volume (5L)
Hb-haemoglobin concentration
SaO2-arterial oxygen concentration (98% breathing air and 100% when preoxygenated)

The common peroneal (fibular) nerve is a peripheral nerve in the lower limb. It arises of the L4-S2 nerve roots and has sensory and motor innervations:

Sensory: Provides innervation of the lateral leg and foot dorsum.

Motor: Provides innervation of the short head of the biceps femoris, as well as muscles of the anterior and lateral leg compartments.

It is the most commonly damaged nerve in the lower extremity, as it is easily compressed by a plaster cast or injured when the fibula is fractured.

Damage to the common peroneal nerve will result in loss of dorsiflexion at ankle (footdrop, as feet are permanently plantarflexed), with the accompanying high stepping gait.

The saphenous and sural nerve only provide sensory innervation.

The tibial nerve arises from the sciatic nerve (like the common peroneal), but it provides motor innervation to the posterior leg compartments and intrinsic foot muscles. Injury to the tibial nerve will cause loss of plantar flexion, toe flexion and weakened foot inversion.

Extreme hip flexion into the lithotomy or Lloyd-Davies position can result in stretch damage to the neurones (sciatic and obturator nerves) or by applying direct pressure (femoral nerve compression).

The laryngeal folds are comprised of two types of folds; the vestibular fold and the vocal fold. The vocal folds are mobile, and concerned with voice production. They are formed by the mucous membrane covering the vocal ligament. They are avascular, hence, are white in colour.

The internal branch of the superior laryngeal nerve provides sensation above the vocal cords. Lesions to this nerve may lead to loss of sensation above the vocal cords and loss of taste on the epiglottis.

The recurrent laryngeal nerve supplies the lateral and posterior cricoarytenoid, the thyroarytenoid. It also provides sensation below the vocal cords. Lesions to this nerve may cause respiratory obstruction, hoarseness, inability to speak and loss of sensation below the vocal cords.

The external branch of the superior laryngeal nerve supplies the cricothyroid muscle.

The glossopharyngeal nerve contains both sensory and motor components, and provides somatic innervation to the stylopharyngeus muscle, visceral motor innervation to the parotid gland, and carries afferent sensory fibres from the posterior third of the tongue, pharynx and tympanic cavity.

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.

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

Breathing 100% oxygen for three minutes will provide the best reservoir of oxygen during apnoea by oxygenating the functional residual capacity (FRC).

Sitting at 45 degrees might increase the FRC and improve oxygen reserve but not compared with 100% oxygenation.

The following table compares the oxygen reserves in the body following pre-oxygenation with room air and 100% oxygen:

Compartment Factors Room air (mL) 100% O2 (mL)
Lung FAO2, FRC 630 2850
Plasma PaO2, DF, PV 7 45
Red blood cells Hb, TGV, SaO2 788 805
Myoglobin – 200 200
Interstitial space – 25 160

FAO2 = alveolar fraction of oxygen.
FRC = Functional residual capacity.
PaO2 = partial pressure of oxygen dissolved in arterial blood
DF = dissolved form.
PV = plasma volume.
TG = total globular volume .
Hb = haemoglobin concentration.
SaO2 = arterial oxygen saturation

Stopping smoking one month prior to surgery will not be more effective than pre-oxygenation with 100% oxygen though it may reduce postoperative pulmonary complications. Note that both long term and short term abstinence reduces pulse rate and blood pressure thus reducing oxygen consumption and also reduce carboxyhaemoglobin levels.

Blood transfusion will not make a big difference in oxygen reserve, particularly if a blood transfusion is administered within 12-24-hours before surgery.

Heliox (79% helium and 21% oxygen) despite its lower viscosity is unlikely to be more effective than 100% oxygen .

Glyceryl trinitrate (GTN) has a significant first pass metabolism. About 90% of a dose of GTN is metabolised in the liver by the enzyme glutathione organic nitrate reductase.

An INSIGNIFICANT amount of metabolism occurs in the intestinal mucosa.

There is approximately 1% bioavailability after oral administration and 38% after sublingual administration.

GTN does NOT cause gastric irritation and it is well absorbed in the gastrointestinal tract.

The volume of distribution of GTN is 2.1 to 4.5 L/kg. This is HIGH.

The right coronary artery supplies the right ventricle, the right atrium, the sinoatrial (SA) node and the atrioventricular (AV) node.

The circumflex artery originates from the left coronary artery and is supplied by it.

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.

Systemic vascular resistance (SVR) is a derived value based on:

SVR = (MAP-CVP)/CO x 80

= (60 -10)/5 x 80 = 800 dynes.s.cm-5

A correction factor of 80 is needed in converting mmHg to dynes.s.cm-5
Normal values is between 700 -1600 dynes.s.cm-5

Pulmonary resistance (PVR) = (MPAP-PCWP)/CO x 80

= (10 – 5)/5 x 80 = 80 dynes.s.cm-5

To account for body size, cardiac index (CI) can be used instead of CO. CI = CO/body surface area (m2) or mL/minute/m2.
N/B: either MAP and CVP, or MPAP and PCWP are used in calculation to get dynes.s.cm-5

When the loading dose of Digoxin is given, the primary thing to consider is the volume of distribution. The volume of distribution is the proportionality factor that relates the total amount of drug in the body to the concentration. LD is computed as:

LD = Volume of distribution X (desired plasma concentration/bioavailability)

Digoxin is an anti-arrhythmic drug with a large volume of distribution and high bioavailability, and only a small percentage of Digoxin is bound to plasma proteins (,20%).

In the case, since the arrhythmia is not life-threatening, there is no need for the medication to work rapidly.

The basilar artery is a large vessel that is formed by the union of the vertebral arteries at the junction of the medulla and pons. It lies in the pontine cistern and follows a shallow groove on the ventral pontine surface, extending to the upper border of the pons.

The basilar artery then bifurcates into the two posterior cerebral arteries that form part of the Circle of Willis.

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.

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

The ideal agent for this case should have low blood: gas coefficient, pleasant smell, and high oil: gas coefficient (potent with a low Minimum alveolar coefficient (MAC)). Among the given options, Sevoflurane is perfect with 0.692 blood: gas partition coefficient and is low pungency, and is sweet.

Other drugs with their blood: gas partition coefficient and their smell are given as:
Blood/gas partition coefficient MAC Smell
Enflurane 1.8 1.68 Pungent, ethereal
Desflurane 0.42 7 Pungent, ethereal
Halothane 2.54 0.71 Sweet
Isoflurane 1.4 1.15 Pungent, ethereal

During fetal development, blood oxygenated by the placenta flows to the foetus through the umbilical vein, bypasses the fetal liver through the ductus venosus, and returns to the fetal heart through the inferior vena cava.

Blood returning from the inferior vena cava then enters the right atrium and is preferentially shunted to the left atrium through the patent foramen ovale. Blood in the left atrium is then pumped from the left ventricle to the aorta. The oxygenated blood ejected through the ascending aorta is preferentially directed to the fetal coronary and cerebral circulations.

Deoxygenated blood returns from the superior vena cava to the right atrium and ventricle to be pumped into the pulmonary artery. Fetal pulmonary vascular resistance (PVR), however, is higher than fetal systemic vascular resistance (SVR); this forces deoxygenated blood to mostly bypass the fetal lungs. This poorly oxygenated blood enters the aorta through the patent ductus arteriosus and mixes with the well-oxygenated blood in the descending aorta. The mixed blood in the descending aorta then returns to the placenta for oxygenation through the two umbilical arteries.

An echocardiogram provides real-time visualisation of cardiac structures. The ejection fraction (EF) is normally measured using this system.

The ejection fraction (EF) can be deduced mathematically if the patient’s end-diastolic volume (EDV), end-systolic volume (ESV) and stroke volume (SV) are known, as:

SV = EDV – ESV, and

EF = SV/EDV x 100

The normal range for EF is >55-70%.

For this patient,

SV= 40 – 30 = 10 mL, therefore

EF = 10/40 x 100 = 25%

Type I – immediate hypersensitivity reaction

Examples are: Atopy, urticaria, Anaphylaxis, Asthma( IgE mediated).

Type II – Antibody mediated cytotoxic reaction

Examples are: Autoimmune haemolytic anaemia, Thrombocytopenia( IgM or IgG mediated).

Type III – Immune complex mediated reaction

Examples are: Serum sickness,SLE – IgG., Farmers lungs, rheumatoid arthritis

Type IV – Delayed hypersensitivity reaction

Examples are: Contact dermatitis, drug allergies.

Type V – Autoimmune

Graves’
Myasthenia – IgM or IgG.

The diaphragm divides the thoracic cavity from the abdominal cavity. Structures penetrate the diaphragm at different vertebral levels through openings in the diaphragm to communicate between the two cavities. The diaphragm has openings at three vertebral levels:

T8: vena cava, terminal branches of the right phrenic nerve
T10: oesophagus, vagal trunks, left anterior phrenic vessels, oesophageal branches of the left gastric vessels
T12: descending aorta, thoracic duct, azygous and hemi-azygous vein

The ?-2 adrenoceptor has three subtypes (2a, 2b and 2c). The receptors are generally presynaptic, meaning they prevent noradrenaline from being released at nerve endings. Both the central and peripheral nerve systems are affected by the ?-2 agonists. ?-2 agonists cause drowsiness, analgesia, and euphoria centrally in the locus coeruleus (in the brainstem), lower the MAC of volatile anaesthetic drugs, and are used to treat acute withdrawal symptoms in chronic opioid addicts.

The most common impact of ?-2 agonists on heart rate is bradycardia. The adrenoreceptors ?-1 and ?-2 are blocked by phenoxybenzamine.

Clonidine is a selective agonist for the ? -2 receptor, having a 200:1 affinity ratio for the ?-2: ?-1 receptors, respectively.

Tizanidine is similar to clonidine but has a few key variances. It has the same sedative, anxiolytic, and analgesic characteristics as clonidine, although for a shorter period of time and with less effect on heart rate and blood pressure.

Dexmedetomidine, like clonidine, is a highly selective ?-2 adrenoreceptor agonist having a higher affinity for the ?-2 receptor. In the case of ?-2: ?-1 receptors, the affinity ratio is 1620:1. It has a biphasic blood pressure impact and induces a brief rise in blood pressure and reflex bradycardia (activation of ?-2b subtypes of receptors in vascular smooth muscles), followed by a reduction in sympathetic outflow from the brainstem and hypotension/bradycardia.

A prodrug is methyldopa. It blocks the enzyme dopa-decarboxylase, which converts L-dopa to dopamine (a precursor of noradrenaline and adrenaline). It is also converted to alpha-methyl noradrenaline, a centrally active agonist of the ?-2 adrenoreceptor. These two processes contribute to its blood pressure-lowering effect. Without a rise in heart rate, cardiac output is generally maintained. The heart rate of certain patients is slowed.

Phentolamine is a short-acting antagonist of peripheral ?-1 and ?-2 receptors that causes peripheral vascular resistance to reduce and vasodilation to increase. It’s used to treat hypertensive situations that aren’t life threatening (e.g. hypertension from phaeochromocytoma).

A baroreceptor reflex commonly causes reflex tachycardia when systemic vascular resistance drops.

Silent (S) is the most probable phenotype of the patient. In S phenotype, patients have significantly reduced levels of BChE, the lowest among the four phenotypes. Because of this, individuals with S phenotype are subjected to long periods of apnoea. In addition, their dibucaine number is very low.

Other BChE phenotypes are the following:

Usual (U)
Atypical (A)
Fluoride-resistant (F)

Dopamine (DA) is a dopaminergic (D1 and D2) as well as adrenergic ? and ?1 (but not ?2 )agonist.

The D1 receptors in renal and mesenteric blood vessels are the most sensitive: i.v. infusion of a low dose of DA dilates these vessels (by raising intracellular cyclic adenosine monophosphate).

Moderately high doses produce a positive inotropic (direct ?1 and D1 action + that due to NA release), but the little chronotropic effect on the heart.

Vasoconstriction (?1 action) occurs only when large doses are infused.

At doses normally employed, it raises cardiac output and systolic BP with little effect on diastolic BP. It has practically no effect on nonvascular ? and ? receptors; does not penetrate the blood-brain barrier – no Central nervous system effects.

Dopamine is less arrhythmogenic than adrenaline

Regarding dopamine part of the dose is converted to Noradrenaline in sympathetic nerve terminals.

The apex of an equilateral triangle completed by the posterior superior iliac spines is where the sacral hiatus or sacrococcygeal membrane can normally located. The failure of posterior fusion of the laminae of the fourth and fifth sacral vertebrae allows the sacral canal to be accessible via the membrane.

In adults, the spine of L4 usually lies on a line drawn between the highest points of the iliac crests (Tuffier’s line). A line connecting each anterior iliac spine, approximates to the L3/4 interspace in the sitting position. Both of these options are incorrect.

A line connecting the greater trochanters is also incorrect.

A line connecting the posterior superior iliac spines is correct, but in adults the presence of a sacral fat pad can still make identification of this landmark less straightforward.

The processes of S5 are remnants only and form the sacral cornua, which are also used to help identify the sacral hiatus.

Changes in the osmolality of body fluids (changes as minor as 1% are sufficient) play the most important role in regulating AVP secretion. The receptors that monitor changes in osmolality of body fluids (termed osmoreceptors) are distinct from the cells that synthesize and secrete AVP, and are located in the organum vasculosum of the lamina terminalis (OVLT) of the hypothalamus. The osmoreceptors sense changes in body osmolality by either shrinking or swelling. When the effective osmolality of the plasma increases, the osmoreceptors send signals to the AVP synthesizing/secreting cells located in the supraoptic and paraventricular nuclei of the hypothalamus, and AVP synthesis and secretion are stimulated. Conversely, when the effective osmolality of the plasma is reduced, secretion is inhibited. Because AVP is rapidly degraded in the plasma, circulating levels can be reduced to zero within minutes after secretion is inhibited.

In this scenario, the osmolality of the plasma will decrease to an estimate of 2.5%, hence inhibition of AVP.

Stimulation of atrial stretch receptors is incorrect because the increase in plasma volume is still below the threshold for its activation.

Osmotic diuresis is incorrect because 5% dextrose is isotonic, hence osmotic diuresis is not probable.

Renin is inhibited when an excess of NaCl in the tubular fluid is sensed by the macula densa.

Compliance of the respiratory system describes the expandability of the lungs and chest wall. There are two types of compliance: dynamic and static.

Dynamic compliance describes the compliance measured during breathing, which involves a combination of lung compliance and airway resistance. Defined as the change in lung volume per unit change in pressure in the presence of flow.

Static compliance describes pulmonary compliance when there is no airflow, like an inspiratory pause. Defined as the change in lung volume per unit change in pressure in the absence of flow.

For example, if a person was to fill the lung with pressure and then not move it, the pressure would eventually decrease; this is the static compliance measurement. Dynamic compliance is measured by dividing the tidal volume, the average volume of air in one breath cycle, by the difference between the pressure of the lungs at full inspiration and full expiration. Static compliance is always a higher value than dynamic

Static compliance can be computed using the formula:

Cstat = Tidal volume/Plateau pressure – PEEP

Substituting the values given,

Cstat = 800/50-10
Cstat = 20 ml/cmH2O

From superior to inferior, the following structures enter the orbit through the superior orbital fissure:
1. Lacrimal nerve
2. Frontal nerve
3. Superior ophthalmic vein
4. Trochlear nerve
5. Superior division of the oculomotor nerve*
6. Nasociliary nerve*
7. Inferior division of the oculomotor nerve*
8. Abducent nerve*
9. Inferior ophthalmic vein.

The superior and inferior division of the oculomotor nerve, nasociliary nerve, and abducent nerve are within the tendinous ring.

The common origin of the four rectus muscles is the tendinous ring (also known as the annulus of Zinn). The tendinous ring’s lateral portion straddles the superior orbital fissure, while the medial portion encloses the optic foramen, through which the optic nerve and ophthalmic artery pass.

Sodium-chloride symporter inhibitor.

The thiazide sensitive sodium chloride symporter is inhibited by thiazides at the proximal portion of the distal convoluted tubule leading to increased sodium and water excretion. Increased delivery of sodium to the distal portion of the distal convoluted tubule promotes potassium loss. This is why thiazides are associated with hyponatraemia and hypokalaemia.

Carbonic anhydrase inhibitors are used mainly in the treatment of glaucoma. They act on the proximal convoluted tubule to promote bicarbonate, sodium and potassium loss.

Sodium potassium chloride symporter is inhibited by Loop diuretics.
Epithelial sodium channels are inhibited by Amiloride.
Drugs which lead to nephrogenic diabetes insipidus such as lithium and demeclocycline, are Inhibitors of vasopressin.

The umbilical vein closes once the umbilical cord is clamped following birth. This causes a rise in systemic vascular resistance, closing the ductus venosus.

Upon birth, the pulmonary vascular resistance is decreased as the lungs are aerated.

At birth, there is a rise in oxygen tension which causes the corresponding constriction of the ductus arteriosus. This prevents a left to right shunt as it stops aortic blood and blood from the pulmonary artery from mixing. The ventricles do no have an opening connecting them.

The foramen ovale closes soon after birth. It is the septum opening between the left and right atrium.

An adult’s cardiac output is expected to be 5 L/min

Correlation doesn’t justify causality. Correlation coefficient gives us an idea whether or not the two parameters provide have any relation of some sort or not i.e. does change in one prompt any change in other? It has nothing to do with predictions. For that purpose linear regression is used.