The correlation coefficient gives us the idea about relation between two parameters. i.e. to what extent change in parameter A could prompt a change in parameter B. The numerical value of correlation coefficient could not be greater than 1.

A person remaining at high altitudes for days, weeks, or years becomes more and more acclimatized to the low PO2, so it causes fewer deleterious effects on the body.

After acclimatization, it becomes possible for the person to work harder without hypoxic effects or to ascend to still higher altitudes. The principal means by which acclimatization comes about are (1) a great increase in pulmonary ventilation, (2) increased numbers of red blood cells, (3) diffusing capacity of the lungs, (4) increased vascularity of the peripheral tissues, and (5) increased ability of the tissue cells to use oxygen despite low PO2.

The cardiac output often increases as much as 30% immediately after a person ascends to high altitude but then decreases back toward normal over a period of weeks as the blood haematocrit increases, so the amount of oxygen transported to the peripheral body tissues remains about normal.

The inferior vena cava is formed by the union of the right and left common iliac veins. The inferior vena cava has no functional valves like the one-way valves commonly found in many veins. The forward flow to the heart is driven by the differential pressure created by normal respiration.

The absence of functional valves has an important clinical role when cannulating during cardiopulmonary bypass.

There is a valve that is non-functioning called the eustachian valve that lies at the junction of the IVC and the right atrium. This valve has a role to help direct the flow of oxygen-rich blood through the right atrium to the left atrium via the foramen ovale during fetal life. It has no specific function in adult life.

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.

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.

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.

The ideal volatile agent for a day case surgery inhalational induction should have the following characteristics:

It has a pleasant scent that is not overpowering.
Breathing difficulties, coughing, or laryngeal spasm are not caused by this substance.
The action has a quick onset and a quick reversal.

The blood:gas partition coefficient is a physicochemical property of a volatile agent that determines the onset and offset of anaesthesia. The greater an agent’s insolubility in plasma, the faster its alveolar concentration rises.

The blood gas partition coefficients of the most commonly used volatile anaesthetic agents are as follows:
Halothane 2.3
Desflurane 0.45
Sevoflurane 0.6
Nitrous oxide 0.47
Isoflurane 1.4

Although halothane has a pleasant odour, it has a slower offset than sevoflurane.

Sevoflurane also has a pleasant odour and is less likely than desflurane to cause airway irritation and breath-holding.

The choice of agent for inhalational induction is unaffected by potency/lipid solubility measures such as the oil: gas partition coefficient and MAC.

In this case, an agent’s saturated vapour pressure is irrelevant.

The relationship between bacterial growth and time is a tear-away exponential. The mathematical relationship between y and x in this case is:

y = ex

Where: the power is x, and the base is e.

Euler’s number (e) is a mathematical constant that is the base for all logarithms occurring naturally. Its value is 2.718.

The statement X increasing with an increase in Y is proportional to Y refers to the change in y in terms of x when considering any exponential relationship.

This is not a build-up exponential, and that is mathematically stated as y = 1-e-kt.

The negative x axis being a horizontal asymptote and the y intercept being 0, 1 are examples of tearaway exponentials , but do not describe an exponential process.

Hiatus is an opening in the diaphragm. A hiatal hernia is a protrusion of the upper part of the stomach through an opening in the diaphragm, the oesophageal hiatus, into the thorax. The oesophageal hiatus occurs at the level of T10 in the right crus of the diaphragm.

Other important openings in the diaphragm:
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

An opening in the diaphragm is called a hiatus. The oesophageal hiatus is at vertebral level T10. A hiatus hernia is where the stomach bulges through the oesophageal hiatus hence the name – hiatus hernia.

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.

The ascending cervical artery lies media the phrenic nerve on scalenus anterior and can easily be mistaken for the phrenic nerve at operation.

The phrenic nerve passes across scalenus anterior and medius inferiorly.

The subclavian artery is separated from the vein by the scalenus anterior.

The brachiocephalic vein is formed at the medial border of scalenus anterior by the subclavian vein and the internal jugular vein.

Emerging from the lateral border of scalenus anterior are the trunks of the brachial plexus .

Approximately 20% of halothane and 0.02% desflurane undergo hepatic biotransformation. Desflurane, halothane, and isoflurane are metabolised in the liver by cytochrome p450 to trifluoroacetate. Through an immunological mechanism involving trifluoroacetyl hapten formation, trifluoroacetate is thought to be responsible for hepatotoxicity.

Potency of inhaled anaesthetic agents is measured using the minimal alveolar concentration (MAC). The MAC of halothane is 0.74% while that of desflurane is 6.3%. The potency can also be compared using the oil: gas partition coefficient (224 and 18.7 for halothane and desflurane respectively).

Onset of action of volatile agents depends on the blood:gas partition coefficient. A lower blood:gas partition coefficient and insolubility in blood means faster onset and offset of action. The blood gas coefficient for halothane is 2.4 while that of desflurane is 0.42. Desflurane is less soluble than halothane in blood. Halothane has a pungent smell that can irritate the airway which limits its use for a gaseous induction especially in paediatric anaesthesia. desflurane is not pungent.

Desfluranes boiling point is only slightly above normal room temperature (22.8°C) making it extremely volatile while the boiling point of halothane is approximately 50.2°C.

Each kidney is composed of about 1.2 million uriniferous tubules. Each tubule consists of two parts that are embryologically distinct from each other. They are as follows:
a) Excretory part, called the nephron, which elaborates urine
b) Collecting part which begins as a junctional tubule from the distal convoluted tubule.

There are two types of nephrons in the kidney:
The cortical nephron comprises 80% of the total nephron and its major function is the excretion of waste products in urine whereas the juxtamedullary nephron comprises 20% of the total nephron and its major function is the concentration of urine by counter current mechanism.
In the superficial (cortical) nephrons, peritubular capillaries branch off the efferent arterioles and deliver nutrients to epithelial cells as well as serve as a blood supply for reabsorption and secretion. In juxtamedullary nephrons, the peritubular capillaries have a specialization called the vasa recta, which are long, hairpin-shaped blood vessels that follow the same course as a loop of Henle. The vasa recta serve as osmotic exchangers for the production of concentrated urine.

The kidney receives about 25% of cardiac output and about 20% of this is filtered at the glomeruli of the kidney. Thus, renal blood flow is 1200 ml/minute and renal plasma flow is 650 ml/minute

The upper five cervical segments of the spinal cord give rise to the XI cranial nerve. They connect with a few smaller branches before exiting the skull through the jugular foramen. The sternomastoid and trapezius muscles get their motor supply from the accessory root. Except for the palatoglossus, the hypoglossal nerve supplies motor supply to all tongue muscles.

The inability to shrug the shoulder on the affected side and rotate the head to the side against resistance is caused by damage to the spinal accessory nerve. This is due to the trapezius and sternomastoid muscles’ weakness.

The hypoglossal nerve is damaged, resulting in tongue wasting and inability to move from side to side.

The stylopharyngeus receives motor supply from the glossopharyngeal nerve. It also carries taste sensory fibres from the back third of the tongue, as well as the carotid sinus, carotid body, pharynx, and middle ear.

Motor supply to the larynx, pharynx, and palate; parasympathetic innervation to the heart, lung, and gut; and sensory fibres from the epiglottis and valleculae are all provided by the vagus nerve.

Dystonia is characterised by repetitive twisting movements or abnormal postures. They are classified as either primary or secondary.

Primary dystonia is a genetic disorder that is inherited in an autosomal dominant pattern.
Secondary dystonia can be caused by focal brain lesions, Parkinson’s disease, or certain medications.

The following drugs cause the most common drug-induced dystonic reactions:
Antipsychotics, antiemetics (especially prochlorperazine and metoclopramide), and antidepressants.

Following the administration of the neuroleptic prochlorperazine, 16 percent of patients experience restlessness (akathisia) and 4% experience dystonia.

Several published reports have linked the anaesthetics thiopentone, fentanyl, and propofol to opisthotonos and other abnormal neurologic sequelae. Dystonias following a general anaesthetic are uncommon. Tramadol has been linked to serotonin syndrome, while remifentanil has been linked to muscle rigidity.

The following are some of the risk factors:

Positive family history
Male
Children
An episode of acute dystonia occurred previously.
Dopamine receptor (D2) antagonists at high doses and recent cocaine use

Dystonia is treated in a variety of ways, including:

Benztropine (as a first-line therapy):

1-2 mg intravenous injection for adults
Child: 0.02 mg/kg to 1 mg maximum

Benzodiazepines are a type of benzodiazepine (second line treatment).

Midazolam:

1-2 mg intravenously, or 5-10 mg IV/PO diazepam

Antihistamines with anticholinergic activity (H1receptor antagonists):

Promethazine 25-50 mg IV/IM, or diphenhydramine 50 mg IV/IM (1 mg/kg in children) are used when benztropine is not available.

Bucks fascia refers to the layer of loose connective tissue, nerves and blood vessels that encapsulates the penile erectile bodies, the corpa cavernosa and the anterior part of the urethra, including the entirety of the spongiose part of the urethra.

It runs with the external spermatic fascia and the penile suspensory ligament.

A single chamber pacemaker was implanted in the patient. In VVI mode, a pacemaker paces and senses the ventricle while being inhibited by a perceived ventricular event. The most likely electrical hazard from diathermy is electromagnetic interference (EMI).

EMI has the potential to cause the following: Inhibition of pacing
Asynchronous pacing
Reset to backup mode
Myocardial burns, and
Trigger VF.

Diathermy entails the implementation of high-frequency electrical currents to produce heat and either make incisions or induce coagulation. Monopolar cautery involves disposable cautery pencils and electrosurgical diathermy units. In typical monopolar cautery, an electrical plate is placed on the patient’s skin and acts as an electrode, while the current passes between the instrument and the plate. Monopolar diathermy can therefore interfere with implanted metal devices and pacemaker function.

Bipolar diathermy, where the current passes between the forceps tips and not through the patient and is less likely to generate EMI.

Whilst the presence of a CVP line may in theory predispose the patient to microshock, the use of prerequisite CF electrical equipment makes this very unlikely. The presence of a CVP line and pacemaker does not therefore unduly increase the risk of an electrical hazard.

Isolating transformers are used to protect secondary circuits and individuals from electrical shocks. There is no step-up or step-down voltage (i.e. there is a ratio of 1 to 1 between the primary and secondary windings).

A ground (or earth) wire is normally connected to the metal case of an operating table to protect patients from accidental electrocution. In the event that a fault allows a live wire to make contact with the metal table (broken cable, loose connection etc.) it becomes live. The earth will provide an immediate path for current to safely flow through and so the table remains safe to touch. Being a low resistance path, the earth lets a large current flow through it when the fault occurs ensuring that the fuse or RCD will quickly blow. Without an operating table earth, the patient is not at more risk of an electrical hazard because of the pacemaker.

Asthma is a lung condition that causes reversible narrowing and swelling of airway passages. It is classified by the frequency and severity of symptoms.

The following are symptoms of moderate asthma:

Symptoms include cough, wheezing, chest tightness, or difficulty breathing which occurs daily
Decreased activity levels due to flare-ups
Night-time symptoms 5 or more times a month
Lung function test FEV1 is 60-80% of predicted normal values
Peak flow has more than 30% variability

With moderate asthma attacks, the arterial pCO2 levels may decrease, but as severity increases, so does the pCO2, reaching normal levels, and then exceeding them in severe asthma attacks.

Airway obstruction increases the functional residual capacity.

Concentration of serum bicarbonate would not increase in moderate asthma, but it could possibly increase in life-threatening asthma via the same mechanism as what increases arterial PCO2.

FEV1 is a good measure of airway obstruction. and is reduced in acute asthma attacks.

In the case of a pneumothorax, a decrease in arterial PO2 is higher.

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 patient’s diagnosis is microcytic hypochromic anaemia which is often as a result of iron deficiency and thalassaemia.

Macrocytic anaemia is often caused by folate and B12 deficiencies and alcohol abuse.

Normocytic normochromic anaemia is often caused by acute blood loss, haemolytic anaemia, anaemia of chronic disease and leucoerythroblastic anaemias.

The cardiac action potential has several phases which have different mechanisms of action as seen below:
Phase 0: Rapid depolarisation – caused by a rapid sodium influx.
These channels automatically deactivate after a few ms

Phase 1: caused by early repolarisation and an efflux of potassium.

Phase 2: Plateau – caused by a slow influx of calcium.

Phase 3 – Final repolarisation – caused by an efflux of potassium.

Phase 4 – Restoration of ionic concentrations – The resting potential is restored by Na+/K+ATPase.
There is slow entry of Na+into the cell which decreases the potential difference until the threshold potential is reached. This then triggers a new action potential

Of note, cardiac muscle remains contracted 10-15 times longer than skeletal muscle.

Different sites have different conduction velocities:
1. Atrial conduction – Spreads along ordinary atrial myocardial fibres at 1 m/sec

2. AV node conduction – 0.05 m/sec

3. Ventricular conduction – Purkinje fibres are of large diameter and achieve velocities of 2-4 m/sec, the fastest conduction in the heart. This allows a rapid and coordinated contraction of the ventricles

Recommendations from the British Thoracic Society for acute severe asthma or life-threatening asthma are:

1. Give controlled supplementary oxygen to all hypoxemic patients with acute severe asthma titrated to maintain a SpO₂ level of 94 98%.
2. Use high-dose inhaled ?₂ agonists as first-line agents in patients with acute asthma and administer them as early as possible. Reserve
intravenous ?₂ agonists for those patients in whom inhaled therapy cannot be used reliably.
3. Give steroids in adequate doses to all patients with an acute asthma attack.
4. Add nebulized ipratropium bromide (0.5 mg 4–6 hourly) to ?₂ agonist treatment for acute severe or life-threatening asthma or those with a poor initial response to ?₂ agonist therapy.
5. Consider aminophylline for children with severe or life-threatening asthma unresponsive to maximal doses of bronchodilators and steroids.

A review (including 12 case reports, three RCTs, and five other observational studies) of ketamine use in adults and children in status asthmaticus reported that ketamine is a potential bronchodilator. Still, prospective trials are needed before conclusions about effectiveness can be drawn.

Heliox has no place in the current guidelines issued by the British Thoracic Society.

Secreted T4 and T3 circulate in the bloodstream almost entirely bound to proteins. Normally only about 0.03% of total plasma T4 and 0.3% of total plasma T3 exist in the free state. Free T3 is biologically active and mediates the effects of thyroid hormone on peripheral tissues in addition to exerting negative feedback on the pituitary and hypothalamus. The major binding protein is thyroxine-binding globulin (TBG), which is synthesized in the liver and binds one molecule of T4 or T3. About 70% of circulating T4 and T3 is bound to TBGl 10% to 15% is bound to another specific thyroid-binding protein called transthyretin (TTR). Albumin binds 15% to 20%, and 3% to lipoproteins. Ordinarily only alterations in TBG concentration significantly affect total plasma T4 and T3 levels.

Two important biological functions have been ascribed to TBG. First, it maintains a large circulating reservoir of T4 that buffers any acute changes in thyroid gland function. Second, binding of plasma T4 and T3 to proteins prevents loss of these relatively small hormone molecules in urine and thereby helps conserve iodide. TTR transports T4 in CSF and provides thyroid hormones to the CNS.

The static-response defines how a measuring system behaves while it is in equilibrium (i.e. when the measured values are not changing). If the value being measured changes over time, the reaction of a measuring system will change as well which would be a dynamic response.
The dynamic response of a measuring system can be subdivided into zero-order, first-order and second-order responses:

Zero-order:
Consider a thermometer that has been left in a room for a week. The thermometer will display the current ambient temperature when you enter the room.

First-order:
Consider the use of a mercury thermometer to check a patient’s temperature. It is comprised of a mercury column that expands as it warms up. The scale’s initial temperature is room temperature, but when it’s placed under the patient’s tongue, the temperature readings rise until they reach body temperature.

Second-order
Consider putting weights on a mechanical weighing scale. The weight as reported on the measuring dial, will wobble around the correct value at first until reaching equilibrium. An example of this is in clinical practice is the direct measurement of arterial pressure with a transducer. The value of the input fluctuates around a central point.

Drift is the progressive deterioration of a measurement system’s precision. With time, the measurement deviates from the genuine, calibrated value. The graph between this measurement and the real value should, ideally, be linear (e.g. on the y-axis the measured end-tidal CO2 against true value of the end-tidal CO2). Drift is split into three types: zero-offset, gradient, and zonal drift.

The pituitary gland plays a crucial role in the neuro-endocrine axis. It is located at the base of the skull in the sella turcica of the sphenoid bone. It is connected superiorly to the hypothalamus, third ventricle, and visual pathways, and laterally to the cavernous sinuses, internal carotid arteries, and cranial nerves III, IV, V, and VI.

Pituitary tumours make up about 10-15% of all intracranial tumours. The majority of adenomas are benign. Over-secretion of pituitary hormones (most commonly prolactin, growth hormone, or ACTH), under-secretion of hormones, or localised or generalised pressure effects can all cause symptoms.

Compression of the optic chiasm can result in visual field defects, the most common of which is bitemporal hemianopia. This is caused by compression of the nasal retinal fibres, which carry visual impulses from temporal vision across the optic chiasm to the contralateral sides before continuing to the optic tracts.

The interruption of the visual pathways distal to the optic chiasm causes a homonymous visual field defect. The loss of the right or left halves of each eye’s visual field is referred to as homonymous hemianopia. It’s usually caused by a middle or posterior cerebral artery territory stroke that affects the occipital lobe’s optic radiation or visual cortex.

Binasal hemianopia is a condition in which vision is lost in the inner half of both eyes (nasal or medial). It’s caused by compression of the temporal visual pathways, which don’t cross at the optic chiasm and instead continue to the ipsilateral optic tracts. Binasal hemianopia is a rare complication caused by the internal carotid artery impinging on the temporal (lateral) visual fibres.

A monocular visual loss (that is, loss of vision in only one eye) can be caused by a variety of factors, but if caused by nerve damage, the damage would be proximal to the optic chiasm on the ipsilateral side.

A central scotoma is another name for central visual field loss. Every normal mammalian eye has a scotoma, also known as a blind spot, in its field of vision. The optic disc is a region of the retina that lacks photoreceptor cells and is where the retinal ganglion cell axons that make up the optic nerve exit the retina. When both eyes are open, visual signals that are absent in one eye’s blind spot are provided for the other eye by the opposite visual cortex, even if the other eye is closed.

Scotomata can be caused by a variety of factors, including demyelinating disease such as multiple sclerosis, damage to nerve fibre layer in the retina, methyl alcohol, ethambutol, quinine, nutritional deficiencies, and vascular blockages either in the retina or in the optic nerve.

Bilateral scotoma can occur when a pituitary tumour compresses the optic chiasm, causing a bitemporal paracentral scotoma, which then spreads out to the periphery, causing bitemporal hemianopsia. A central scotoma in a pregnant woman could be a sign of severe pre-eclampsia.

Multiple regression analysis revealed that velocity of lactate accumulation (VOBLA) accounted for 92 percent of the variation in marathon running velocity (VM), and VOBLA plus training volume prior to the marathon accounted for 96 percent of the variation. Percent ST muscle fibre distribution (r = 0.55-0.69) and capillary density (r = 052-0.63) were found to be positively correlated with all performance variables. As a result, marathon running performance was linked to VOBLA and the ability to run at a pace close to it during the race. The percent ST, capillary density, and training volume were all related to these properties.

Another metabolic adaptation compared to normal people is the early selection of fat for oxidation by muscle, especially when glucose availability is limited during high-intensity exercise. This helps to delay the onset of muscle fatigue, but it does not prevent VOBLA.

For a given level of exercise, training can also result in cardiovascular adaptation, such as increased heart size, increased contractility, and a slower heart rate. All of these factors contribute to an increase in maximal oxygen consumption (VO2 max), but genetic factors, despite intensive training, play a large role in an athlete’s performance.

At the ankle joint, midway between the malleoli, the anterior tibial artery changes names, becoming the dorsalis pedis artery (dorsal artery of the foot).

The dorsalis pedis artery is palpated against the underlying tarsals, immediately lateral to the tendon of extensor hallucis longus, from the midpoint between the malleoli to the proximal end of the first intermetatarsal space.

The popliteal artery forms the anterior tibial artery.
The tibioperoneal trunk is a branch of the popliteal artery.
The peroneal artery (also known as the fibular artery) supplies the lateral compartment of the leg.
The external iliac artery is formed from the common iliac artery at the level of the pelvis.

Atypical antipsychotic drugs include risperidone and quetiapine. They not only inhibit dopamine receptors in the limbic system, but also histamine (H1) and alpha2 adrenoreceptors. When combined with general and/or central neuraxial block, this might result in severe hypotension.

Donepezil (Aricept) is an acetylcholinesterase (AChE) inhibitor that increases the neurotransmitter acetylcholine in the cerebral cortex and hippocampus in a reversible, non-competitive manner. It is used to reduce the advancement of Alzheimer’s disease symptoms (AD). Rivastigmine and galantamine are two more drugs that work in the same way.

Ginkgo Biloba contains anti-oxidant characteristics and is used to treat early-stage Alzheimer’s disease, vascular dementia, and peripheral vascular disease. It lowers platelet adhesiveness and decreases platelet activating factor (PAF) increasing the risk f bleeding, especially in individuals who are also taking anticoagulants and antiplatelet medication.

Memantine is an antagonist of the NMDA receptor. Synaptic plasticity, which is thought to be a critical component of learning and memory, can be inhibited at high doses. The use of ketamine is a relative contraindication since antagonism of this receptor can cause a dissociative state.

The following terms are used in medical testing:

True negative – The test is negative and the patient does not have the disease.
True positive – The test is positive and the patient has the disease.
False positive – The test is positive but the patient does not have the disease.
False negative – The test is negative but the patient has the disease.

The sensitivity of a predictive test = true positives / (true positives + false negatives).

The specificity of a test = true negatives / (false positives + true negatives).

The negative predictive value of a test = true negatives / (false negatives + true negatives).

Potassium maintains the resting potential of cardiac myocytes, with depolarization triggered by a rapid influx of sodium ions, and repolarization due to efflux of potassium. A slow influx of calcium is responsible for the longer duration of a cardiac action potential compared with skeletal muscle.

The cardiac action potential has several phases which have different mechanisms of action as seen below:

Phase 0: Rapid depolarisation – caused by a rapid sodium influx.
These channels automatically deactivate after a few ms.

Phase 1: caused by early repolarisation and an efflux of potassium.

Phase 2: Plateau – caused by a slow influx of calcium.

Phase 3 – Final repolarisation – caused by an efflux of potassium.

Phase 4 – Restoration of ionic concentrations – The resting potential is restored by Na+/K+ATPase.
There is slow entry of Na+into the cell which decreases the potential difference until the threshold potential is reached. This then triggers a new action potential

Of note, cardiac muscle remains contracted 10-15 times longer than skeletal muscle.

Different sites have different conduction velocities:
1. Atrial conduction – Spreads along ordinary atrial myocardial fibres at 1 m/sec

2. AV node conduction – 0.05 m/sec

3. Ventricular conduction – Purkinje fibres are of large diameter and achieve velocities of 2-4 m/sec, the fastest conduction in the heart. This allows a rapid and coordinated contraction of the ventricles