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.

Adrenaline acts on ?1, ?2,?1, and ?2 receptors and also on dopamine receptors (D1, D2) and have sympathomimetic effects.

Natural catecholamines are Adrenaline, Noradrenaline, and Dopamine

Adrenaline is a sympathomimetic amine with both alpha and beta-adrenergic stimulating properties.
Adrenaline is the drug of choice for anaphylactic shock
Adrenaline is also used in patients with cardiac arrest. The preferred route is i.v. followed by the intra-osseous and endotracheal route.

Adrenaline is released by the adrenal glands, acts on ? 1 and 2, ? 1 and 2 receptors, and is responsible for fight or flight response.

It acts on ? 2 receptors in skeletal muscle vessels-causing vasodilation.

It acts on ? adrenergic receptors to inhibit insulin secretion by the pancreas. It also stimulates glycogenolysis in the liver and muscle, stimulates glycolysis in muscle.

It acts on ? adrenergic receptors to stimulate glucagon secretion in the pancreas
It also stimulates Adrenocorticotrophic Hormone (ACTH) and stimulates lipolysis by adipose tissue

A mercury barometer can be used to determine absolute pressure. A glass tube with one closed end serves as the barometer. The open end is inserted into a mercury-filled open vessel. The mercury in the container is pushed into the tube by atmospheric pressure exerted on its surface. Absolute pressure is the distance between the tube’s meniscus and the mercury surface.

Pressure is defined as force in newtons per unit area (F) (A). 

Mass of mercury = area (A) × density (ρ) × length (L)
Pressure = ((A × ρ × L) × 9.8 m/s2)/A
Pressure = ρ × L x 9.8
Pressure is proportional to L

The numerator and denominator of the above equation, area (A), cancel out. The constants are density and the gravitational acceleration value.

The length is proportional to the applied pressure.

Oxygen is formed by a molecule of oxygen and two molecules of hydrogen with a molecular formula of H2O

The critical temperature is defined as a temperature above which the substance cannot be liquefied, no matter how much pressure is applied.
Water has a critical temperature of -118.6oC. So, it cannot be liquified at room temperature.

Medical oxygen cylinder is stored in a cylinder with a white shoulder and black body. Meanwhile, medial air is stored in cylinders with a white and black shoulder and a French grey body.

The partial pressure of air at a high altitude is less but the relative concentration remains constant.

Absolute risk reduction = (Control event rate) – (Experimental event rate)

Experimental event rate = 24 / 120 = 0.2

Control event rate = 60 / 240 = 0.25

Absolute risk reduction = 0.25 – 0.2 = 0.05 = 5% reduction

The external iliac artery is the larger of the two branches of the common iliac artery. It forms the main blood supply to the lower limbs. The common iliac bifurcates into the internal and external iliac artery anterior to the sacroiliac joint.

The external iliac artery courses on the medial border of the psoas major muscles and exits the pelvic girdle posterior to the inguinal ligament. Here, midway between the anterior superior iliac spine and the pubic symphysis, the external iliac artery becomes the femoral artery and descends along the anteromedial part of the thigh in the femoral triangle.

The pectineus forms the posterior border of the femoral canal.
The femoral vein forms the lateral border of the femoral canal.
The medial border of the adductor longus muscle forms the medial wall of the femoral triangle.
The medial border of the sartorius muscle forms the lateral wall of the femoral triangle.

Impaired ventricular relaxation reduces diastolic filling and therefore preload.

Decreased blood volume decreases preload due to reduced venous return.

Heart failure is characterized by reduced ejection fraction and therefore stroke volume.

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 (is increased by stroke volume) = 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
Aortic stenosis would decrease stroke volume as end systolic volume would increase.
This is because of an increase in afterload, an increase in resistance that the heart must pump against due to a hard stenotic valve.

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.

Fractures to the surgical neck of the humerus are common place as it is the weakest point of the proximal humerus bone.

The structures most likely to be damaged are the axillary nerve and the posterior circumflex humeral artery as they surround the surgical neck.

The radial nerve runs along the radial groove, so injury to it would likely occur with a mid-shaft fracture of the humerus.

The brachial artery is most likely to be injured as a result of a supracondylar fracture of the humerus which increases the risk of volkmaan’s ischemic contractures.

Injury to the musculocutaneous nerve is least likely to happen and it very uncommon.

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 .