The sacral hiatus is shaped by incomplete midline fusion of the posterior elements of the distal portion of S4 and S5. This inverted U shaped space is covered by the posterior aspect of the sacrococcygeal membrane and is an important landmark in caudal anaesthetic block. Distal most portion of the dural sac and the sacral hiatus usually terminate between levels S1 and S3. The dural sac ends at the level of S2 in adults and S3 in children.

An equilateral triangle is formed between the apex of the sacral hiatus and the posterior superior iliac spines. This triangle is used to determine the location of the sacral hiatus during caudal anaesthetic block.

Intra-arterial blood pressure monitoring is a common place procedure in the ICU. It is used to provide accurate beat-to-beat information using a pressure wave displayed on a monitor.

It involves catheter insertion in a peripheral artery (most commonly the radial, brachial and dorsalis pedis arteries). Each subsequent contraction of cardiac muscles results in pressure wave which induces a mechanical motion of flow in the catheter. This mechanical motion is then passed on to a transducer through a rigid fluid-filled tubing. The transducer is the able to process this mechanical motion into electrical signals which are displayed as arterial waves and pressure represented numerically on the monitor.

The transducer should be placed at the same level as the heart on the phlebostatic axis, and at the level of the atria (the 4th intercostal space, in the mid-axillary line).

Air bubbles and catheter tubing with longer lengths result in wave dampening (rounding of the resulting pressure waves). This dampening causes a decrease in systolic pressure, and an increase in diastolic pressure.

The external carotid artery has eight important branches:
Anterior surface:
1. Superior thyroid artery (first branch)
2. Lingual artery
3. Facial artery
Medial branch
4. Ascending pharyngeal artery
Posterior branches
5. Occipital artery
6. Posterior auricular artery
Terminal branches
7. Maxillary artery
8. Superficial temporal artery

The external carotid has eight branches, 3 from its anterior surface ; thyroid, lingual and facial. The pharyngeal artery is a medial branch. The posterior auricular and occipital are posterior branches.

Pulmonary vascular resistance (PVR) refers to the resistance to blood flow to the left atrium from the pulmonary artery.
It is derived mathematically by:

PVR = MPAP – PCWP
CO
where,
MPAP: Mean pulmonary artery pressure
PCWP: Pulmonary capillary occlusion pressure
CO: Cardiac output

For this patient:
PVR = 10 – 5 = 1mmHg
5

Remember, multiply by correction factor 80 to change units:

PVR = 1mmHg x 80 = 80 dynes·s·cm-5

Normal values range between 20-130 dynes·s·cm-5

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.

Normally, the oesophagus is lined by non-keratinized stratified squamous epithelium. This epithelium can undergo metaplasia and convert to the columnar epithelium (stomach’s lining) in long-standing GERD that leads to Barret’s oesophagus.

Cardiopulmonary exercise testing (CPX, CPEX, CPET) is a non-invasive testing method used to determine the performance of the heart, lungs and skeletal muscle. It measures the exercise tolerance of the patient.

The parameters measured include:

ECG and ST-segment analysis and blood pressure
Oxygen consumption (VO2)
Carbon dioxide production (VCO2)
Gas flows and volumes
Respiratory exchange ratio (RER)
Respiratory rate
Anaerobic threshold (AT)

The anaerobic threshold (AT) is an estimate of exercise ability. Any measurement below 11 ml/kg/min is usually related with an increase in mortality, especially when there is a background of myocardial ischaemia occurring during the test.

Peak VO2 <20 mL/kg with a low AT have a correlation with postoperative complications and a 30 day mortality. The CPX test is used for risk-testing patients prior to surgery to determine the appropriate postoperative care facilities. The V slope measured in CPX testing represents VO2 versus VCO2 relationship. During AT, the ramp of V slope increases, but does not provide a picture of postoperative mortality.

The ventilatory anaerobic threshold (VAT), formerly referred to as the anaerobic threshold, is an index used to estimate exercise capacity. During the initial (aerobic) phase of CPET, which lasts until 50–60% of Vo2max is reached, expired ventilation (VE) increases linearly with Vo2 and reflects aerobically produced CO2 in the muscles. Blood lactate levels do not change substantially during this phase, since muscle lactic acid production is minimal.

During the latter half of exercise, anaerobic metabolism occurs because oxygen supply cannot keep up with the increasing metabolic requirements of exercising muscles. At this time, there is a significant increase in lactic acid production in the muscles and in the blood lactate concentration. The Vo2 at the onset of blood lactate accumulation is called the lactate threshold or the VAT. The VAT is also defined as the point at which minute ventilation increases disproportionately relative to Vo2, a response that is generally seen at 60–70% of Vo2max.

The VAT is a useful measure as work below this level encompasses most daily living activities. The ability to achieve the VAT can help distinguish cardiac and non‐cardiac (pulmonary or musculoskeletal) causes of exercise limitation, since patients who fatigue before reaching VAT are likely to have a non‐cardiac problem.

When VAT is detected, patients with PVo2 of ⩽10 ml/kg/min have a high event rate.

Its known that 68.3% of the total values of a normally distributed variable are found within a range of 1 standard deviation from the mean which makes the range to be 4.3 to 4.9 mmol/L.

Interleukin-4 is a cytokine which acts to regulate the responses of B and T cells.

Erythropoietin is responsible for the signal that initiated red blood cell production.

Granulocyte-colony stimulating factor stimulates the bone marrow to produce granulocytes.

Interleukin-5 is a cytokine that stimulates the proliferation and activation of eosinophils.

Thrombopoietin is the primary signal responsible for megakaryocyte and thus platelet production.
Platelets are also called thrombocytes. They, like red blood cells, are also derived from myeloid stem cells. The process involves a megakaryocyte developing from a common myeloid progenitor cell. A megakaryocyte is a large cell with a multilobulated nucleus, this grows to become massive where it will then break up to form platelets.

Immune cells are generated from haematopoietic stem cells in bone marrow. They generate two main types of progenitors, myeloid and lymphoid progenitor cells, from which all immune cells are derived.

Charles’ Law states that there is a direct correlation between temperature and volume, where pressure and amount gas are constant. As temperature increases, volume also increases.

Boyle’s Law states that Pressure is inversely proportional to volume, assuming that temperature and amount of gas are constant. As volume increases, pressure decreases. In Dalton’s law of partial pressure, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of the gases in mixture.

According to Henry’s Law for concentration of dissolved gases, at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. An equivalent way of stating the law is that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Gay-Lussac’s Law states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant. This law is very similar to Charles’ Law, with the only difference being the type of container. Whereas the container in a Charles’ Law experiment is flexible, it is rigid in a Gay-Lussac’s Law experiment.

The basal metabolic rate (BMR) is the amount of energy required to maintain basic bodily functions in the resting state. The unit is Watt (Joule/second) or calories per unit time.

Indirect calorimetry measures O2 consumption and CO2 production where gases are collected in a canopy which is the gold standard, Douglas bag, face-mask dilution technique or interfaced with a ventilator.

The BMR can be calculated using the Weir formula:

Metabolic rate (kcal per day) = 1.44 (3.94 VO2 + 1.11 VCO2)

The BMR should be measured while lying down and at rest with the following conditions met:

It should follow a 12 -hour fast
No stimulants ingested within a 12-hour period
It should be done in a neutral thermal environment (between 20°C-25°C)

The superficial inguinal ring is an opening in the aponeurosis of the external oblique muscle, just above and lateral to the pubic crest.

The superficial ring resembles a triangle more than a ring with the base lying on the pubic crest and its apex pointing towards the anterior superior iliac spine. The sides of the triangle are crura of the opening in the external oblique aponeurosis. The lateral crura of the triangle is attached to the pubic tubercle. The medial crura of the triangle is attached to the pubic crest.

The external oblique aponeurosis forms the anterior wall of the inguinal canal and also the lateral edge of the superficial inguinal ring. The rectus abdominis lies posteromedially, and the transversalis posterior to this.

The liver receives blood supply from two sources.
1. Hepatic artery proper
It arises from the celiac trunk via the common hepatic artery and brings oxygenated blood to the liver.
It contributes to approximately 30% of the blood supply of the liver.
2. Hepatic portal vein – supplies the liver with partially deoxygenated blood, carrying nutrients absorbed from the small intestine. It gets tributaries from the inferior mesenteric vein, splenic vein, and superior mesenteric vein

The inferior mesenteric artery supplies the hindgut.
The superior mesenteric artery supplies the pancreas and intestine up to the proximal two-thirds of the transverse colon.
The inferior phrenic artery supplies the inferior surface of the diaphragm and oesophagus.

The hypothalamus is primarily responsible for the regulation of the basal metabolic rate. It releases thyrotropin releasing hormones (TRH) in response to low levels of triiodothyronine (T3) and thyroxine (T4). The TRH acts on the pituitary gland to release thyroid stimulating hormone, which will stimulate the thyroid gland to synthesize more T3 and T4.

Basal metabolic rate refers to the energy expended by an individual in a resting, post-absorptive state. It represents the energy required to carry out normal body functions, such as respiration.

Ferromagnetism is the property of a substance that is magnetically attracted and can be magnetised indefinitely. A material is said to be paramagnetic if it is attracted to a magnetic field. A substance is said to be diamagnetic if it is repelled by a magnetic field.

Cobalt, iron, gadolinium, neodymium, and nickel are ferromagnetic.

Gadolinium is a ferromagnetic rare earth metal that is ferromagnetic below 20 degrees Celsius (its Curie temperature). MRI scans are enhanced with gadolinium-based contrast media.

When ferromagnetic materials are exposed to a magnetic field, they can cause a variety of issues like magnetic field interactions, heating, and image artefacts.

Titanium, lead, chromium, copper, aluminium, silver, gold, and tin are non ferromagnetic.

The femoral triangle is a wedge-shaped area found within the superomedial aspect of the anterior thigh. It is a passageway for structures to leave and enter the anterior thigh.

Superior: Inguinal ligament
Medial: Adductor longus
Lateral: Sartorius
Floor: Iliopsoas, adductor longus and pectineus

The contents include: (medial to lateral)
Femoral vein
Femoral artery-pulse palpated at the mid inguinal point
Femoral nerve
Deep and superficial inguinal lymph nodes
Lateral cutaneous nerve
Great saphenous vein
Femoral branch of the genitofemoral nerve

Femoral nerve lesion (L2,L3 and L4) is characterised by weakness of the quadriceps femoris muscle. This results in weakness of extension of the knee, loss of sensation over the front of the thigh, and loss of the knee jerk reflex.

The skin over the lateral aspect of the thigh and knee, and the lower lateral quadrant of the buttock is supplied by the lateral cutaneous nerve of the thigh (L1,2).

The adductors of the hip are supplied by the obturator nerve (L2-4). This nerve also supplies sensation to the inner thigh.

When the convulsion lasts for five or more than five minutes, or if there are recurrent episodes of convulsions in a 5 minute period without returning to the baseline, it is termed as Status Epilepticus.
The first priority in the patient with seizures is maintaining the airway, breathing, and circulation.

Guideline for the management of Status Epilepticus in children by Advanced Life Support Group is as follow:

Step 1 (Five minutes after the start of seizures):

If intravascular access is available start treatment with lorazepam 0.1 mg/kg IV
If no intravascular access then give buccal midazolam 0.5 mg/kg or rectal diazepam 0.5 mg/kg.

Step 2 (Ten minutes after the start of seizure):

If the convulsions continue then a second dose of benzodiazepine should be given. Senior should be called on-site and phenytoin should be prepared.
No more than two doses or benzodiazepines should be given (including any doses given before arrival at the hospital)
If still no IV access then obtain intraosseous access (IO).

Step 3 (Ten minutes after step 2)

Senior help along with anaesthetic/ICU help should be sought
Phenytoin 20 mg/kg IV over 20 minutes
If the seizure stops before the full dose of phenytoin is given then the infusion should be completed as this provides up to 24 hours of anticonvulsant effect
In children already receiving phenytoin as treatment for epilepsy then an alternative is phenobarbitone 20 mg/kg IV over five minutes
Once the phenytoin is started, senior staff may wish to give rectal paraldehyde 0.4 mg/kg although this is no longer included in the routine algorithm recommended by APLS.

Step 4 (20 minutes after step 3)

If 20 minutes after starting phenytoin the child remains in status epilepticus then rapid sequence induction of anaesthesia with thiopentone and a short acting paralysing agent is needed and the child transferred to paediatric intensive care.

Mid-shaft fractures of the humerus usually occur after a direct blow to the upper arm, which can occur after a fall or RTAs.
The most important clinical significance of a mid-shaft humeral fracture is an injury to the radial nerve. The radial nerve originates from the brachial plexus and has roots of C5-T1. It crosses the spiral groove on the posterior side of the shaft of the humerus.
On examination, the patient may have a wrist drop, loss or weakness of finger extension, and decreased or absent sensation to the posterior forearm, digits 1 to 3, and the radial half of the fourth digit.

The humeral shaft has two compartments:
1. Anterior:
Brachial artery and vein
Biceps brachii, brachialis, coracobrachialis
Musculocutaneous, median, and ulnar nerves
2. Posterior:
Radial nerve
Triceps

Other significant nerve injuries are:
1. Axillary nerve – surgical neck fracture of the humerus
2. Brachial Artery – supracondylar fracture of the humerus
3. Axillary artery – surgical neck fracture of the humerus, but is relatively uncommon

The deep inguinal ring is the entrance of the inguinal canal. It is an opening in the transversalis fascia around 1 cm above the inguinal ligament. Therefore, the superolateral wall is made by the transervalis fascia.

The inferior epigastric vessels run medially to the deep inguinal ring forming its inferomedial border.

The inguinal canal extends obliquely from the deep inguinal ring to the superficial inguinal ring.
An indirect inguinal hernia arises through the deep inguinal ring lateral to the inferior epigastric vessels.

The cricoid cartilage is a hyaline cartilage ring surrounding the trachea. It provides support for key phonation muscles.

The inferior border of the cricoid cartilage is attached to the thyroid cartilage and the inferior border is attached to the first tracheal ring through the cricotracheal ligament.

Application of pressure to the cricoid cartilage to reduce risk of aspiration of gastric contents (Sellick manoeuvre) does not stop tracheal aspiration and cannot stop regurgitation into the oesophagus.

A force of 44 newtons to the cricoid cartilage is needed to control regurgitation.

Tracheal tubes are made of either disposable plastic or reusable red rubber.

The tube size refers to the internal diameter (ID) in mm which is marked on the outside of the tube (some manufacturers mark the external diameter on the outside).

Plastic tubes have a radiopaque line spanning the entire length of the tube, which allows their position to be identified on x-rays. The bevel located at the end of the tube is left-facing and oval in shape, which improves the view of the vocal cords during intubation.

Oxford tubes are L-shaped and have a bevel that faces posteriorly. They have thick walls that increase the external diameter, making for a wider internal diameter.

RAE (Ring, Adair, and Elwyn) tubes are preformed and can either be north or south facing and cuffed or uncuffed. The cuffed RAE tubes have one Murphy eye, whereas the uncuffed has two Murphy eyes. Uncuffed tubes are primarily used in paediatric anaesthesia and the two Murphy eyes ensure adequate ventilation- should the tube be too long.

The ideal gas equation makes the following assumptions:

The gas particles have a small volume in comparison to the volume occupied by the gas.
Between the gas particles, there are no forces of interaction.
Individual gas particle collisions, as well as gas particle collisions with container walls, are elastic, meaning momentum is conserved.
PV = nRT
Where:

P = pressure
V = volume
n = moles of gas
T = temperature
R = universal gas constant

Helium is a monoatomic gas with a small helium atom. The attractive forces between helium atoms are small because the helium atom is spherical and has no dipole moment. Because helium atoms are spherical, collisions between them approach the ideal state of elasticity.

Most real gases behave qualitatively like ideal gases at standard temperatures and pressures. When intermolecular forces and molecular size become important, the ideal gas model tends to fail at lower temperatures or higher pressures. It also fails to work with the majority of heavy gases.

Helium, argon, neon, and xenon are noble or inert gases that behave the most like an ideal gas. Xenon is a noble gas with a much larger atomic size than helium.

The following are the pharmacokinetic properties of drugs with a low hepatic extraction ratio:

Changes in liver blood flow have no effect on drug clearance.
When given orally, drug clearance is extremely sensitive to changes in protein binding, intrinsic metabolism, and excretion, and there is no first-pass metabolism.

Warfarin and phenytoin are two drugs with low hepatic extraction ratios.

The following are the pharmacokinetic properties of drugs with a high hepatic extraction ratio:

When taken orally, undergo extensive first-pass metabolism; drug clearance is dependent on liver blood flow, and drug clearance is less sensitive to changes in protein binding and intrinsic metabolism.

Morphine, lidocaine, propranolol, and etomidate are examples of drugs with high hepatic extraction ratios.

A hernia is a protrusion of the abdominal viscera through a defect in the abdominal wall. Inguinal hernias are of two types; Indirect inguinal hernia and Direct inguinal hernia.
– Indirect inguinal hernia is common at young age commonly due to a patent processes vaginalis and bowel passes through the deep inguinal ring lateral to the inferior epigastric artery.
– Direct hernia forms as a result of the weakening of the posterior wall of the inguinal canal more specifically within a region called ‘Hasselbach triangle. It is defined medially by the rectus abdominis muscle, laterally by the epigastric vessels, and inferiorly by the inguinal ligament.

Direct and indirect hernias can be differentiated based on their relation to the inferior epigastric artery. Direct inguinal hernia lies medial to it while indirect inguinal hernia lies lateral to the inferior epigastric artery.

The femoral ring is the site of the femoral hernia.

A major source of caloric expenditure and oxygen consumption in the body is work of breathing (WOB) and 70% of this is to overcome elastic forces. The remaining 30% is for flow-resistive work

In a normal patient breathing normally, the total area of hysteresis pressure volume curve represents the flow-resistive WOB.

The area of the expiratory resistive work increases during an asthma attack making the compliance curve larger in area. The larger the area the greater the work required to breathe.

The posterior tibial artery originates from the popliteal artery in the popliteal fossa. It passes posterior to the popliteus muscle to pierce the soleus muscle. It descends between the tibialis posterior and flexor digitorum longus muscles.

The posterior tibial artery supplies blood to the posterior compartment of the lower limb. The artery can be palpated posterior to the medial malleolus.

There are 4 main pulse points for the lower limb:

1. Femoral pulse 2-3 cm below the mid-inguinal point
2. Popliteal partially flexed knee to loosen the popliteal fascia
3. Posterior tibial behind and below the medial ankle
4. Dorsal pedis dorsum of the foot over the navicular bone

The prostate gland is primarily supplied by the inferior vesical artery, which branches off from the anterior division of the internal iliac artery. The inferior vesical artery supplies the base of the bladder, the distal ureters, and the prostate. The branches to the prostate communicate with the corresponding vessels of the opposite side.

The inferior vesical artery branches into two main arteries:
1. Urethral artery – supplies the transition zone and is the main arterial supply for the adenomas in BPH
2. Capsular artery – supplies the glandular tissue

The venous drainage of the prostate is from the prostatic venous plexus, which drains into the paravertebral veins.

The posterior cerebral arteries are the terminal branches of the basilar artery and are connected to the circle of Willis via the posterior communicating artery. The posterior cerebral artery supplies the visual areas of the cerebral cortex and other structures in the visual pathway.

The posterior cerebral artery is separated from the superior cerebellar artery near its origin by the oculomotor nerve (3rd cranial nerve) and, lateral to the midbrain, by the trochlear nerve.

PCA strokes will primarily cause a visual field loss or homonymous hemianopia to the opposite side. This large occipital or PCA stroke causes people to be “blind” on one side of the visual field. This is the most common symptom of a large occipital lesion or PCA stroke.