ECG Arrhythmias (a.k.a. non-sinus rhythm) can be quite difficult to distinguish. As a starting point it is always ideal to identify the QRS complex and its rate (in relation to the ventricles), identify the P wave and its rate (in relation to the atria), and the relationship between the P wave and the QRS complex.
Identifying ECG Arrhythmias
STEP 1: The Ventricle
is the heartbeat FAST (>100bpm) or SLOW (<60bpm)?
is it REGULAR or IRREGULAR? can you hear any extra beats?
is it NARROW <120ms) or WIDE (>120ms)
STEP 2: The Atrium
focus on the P wave in the II and VI ECG lead reading
note the rate
note the morphology
Step 3: The Relationship between the Ventricle and the Atrium
is there any relationship between the P Wave and the QRS Complex?
is every P Wave followed by a QRS Complex?
is every QRS Complex preceded by a P Wave?
what is the ratio of P:QRS?
can you note an AV dissociation where the atria and ventricles beat independently of each other?
determine the PR interval – does it change?
determine the RR interval – does it change?
Normal Sinus Rhythm
A normal sinus rhythm features a good relationship between the ventricles and the atria, with a heart rate between 60-100bpm.
Bradycardia
Bradycardia presents with a heart rate of less than 60bpm.
Tachycardia
Tachycardia presents with a heart rate of over 100bpm; may present as:
NARROW COMPLEX, REGULAR OR IRREGULAR:
Regular Tachycardia: sinus tachycardia OR atrial flutter
Irregular Tachycardia: atrial fibrillation with no P Waves OR atrial flutter with variable AV conduction
WIDE COMPLEX, REGULAR OR IRREGULAR:
Regular Tachycardia: sinus tachycardia (VT with aberrant conduction)
Atrial Fibrillation presents with absent, very hard to identify P wave, indicating issues within the atrial chambers functionality.
Ventricular Tachycardia presents with an absent P wave, high heart rate, and with a wide and somewhat weird-looking QRS complex.
Retrieved from https://www.grepmed.com/images/12601/tachycardia-narrow-differential-cardiology-diagnosis on 9th January 2023
AV Block
1° AV Block = Delayed Block – PROLONGED but CONSTANT PR interval (>200ms) + P wave with every QRS complex.
2° AV Block Mobitz Type 1 a.k.a. Wenchenbach = Intermittently Blocked – PROGRESSIVELY LENGTHENING PR intervals until a P Wave fails to conduct, leading to a DROPPED QRS complex (missed beat); next cycle restarts with a normal PR.
2° AV Block Mobitz Type 2 = Intermittently Blocked – P waves NOT ALWAYS FOLLOWED by QRS complex + CONSTANT NORMAL or PROLONGED PR interval.
High Grade AV Block = Intermittently Blocked – consecutive P Waves are not followed by QRS complex + CONSTANT NORMAL or PROLONGED PR interval.
High Grade AV Block (right) – Retrieved from https://www.youtube.com/watch?v=uzMdL2HFEZY on 9th January 2023
3° AV Block = Completely Blocked – NO RELATIONSHIP a.k.a. dissociation BETWEEN P wave and QRS complex; PR interval is different with each beat and P Waves are usually faster than QRS complexes.
The Escape Rhythm
In complete heart block (3° AV Block), the heart functions through the:
Junctional escape beating @ 40-50bpm indicating block in the AV node;
Ventricular escape beating @ 20-40bpm indicating block in the His-Purkinje site. This is unreliable and results in asystole.
Ventricular Fibrillation happens when the ventricles fibrillate without prefilling, pushing no blood volume out to circulation = no cardiac output. This is a SHOCKABLE RHYTHM.
Asystole presents with an absence of electrical impulses as an almost flat line on an ECG. Prior to this rhythm, the patient may present with agonal breathing. CPR should be performed. Asystole is NOT A SHOCKABLE RHYTHM.
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An ECG is a ‘snapshot’ of the electrical activity of the heart presented on a graph. When interpreting ECG one can note the heart rate and rhythm, normal/abnormal conduction of both the atria or ventricles, structural changes within the heart such as atrial or ventricular enlargement, as well as an indication of a past Myocardial Infarction.
ECG Principles
The ECG’s value is magnified when recorded during a stress test eg. when the patient is running on a treadmill, or when recorded for a long period of time as in with a Holter.
The pumping action of the heart:
DEPOLARISATION – is initiated by an electrical activation of the myocardium
AUTOMATICITY – causes heart action
EXCITABILITY – responds to the electrical impulse
CONDUCTIVITY – conducts an electrical impulse
CONTRACTILITY – initiates contraction
Repolarisation in an ECG acts as an indication for diagnosis of ischaemia, myocardial stretch, pharmacological effects, electrolyte imbalance, and congenital ionic diseases able to cause a sudden cardiac arrest and imminent death.
Motor Unit Action Potential showing Depolarization, Repolarization and Resting Potential – Retrieved from https://www.researchgate.net/figure/Motor-Unit-Action-Potential-showing-Depolarization-Repolarization-and-Resting-Potential_fig2_340126449 on 8th January 2023
In both depolarisation and repolarisation, cardiac myocytes act like electric generators that cause electric currents to flow out into the body and back again into the heart. This produces various electrical potentials on the body’s surface, which are then recorded and represented on an ECG.
The ECG graph is usually set up at a speed of 25mm/s:
1 small square = 0.04 sec
1 large square = 0.2 sec
5 large squares = 1 sec
15 large squares = 3 sec
Retrieved from https://www.practicalclinicalskills.com/ekg-course-contents?courseid=301 on 8th January 2023
Each ECG lead used represents the heart from a different point of view on an ECG strip. The horizontal base line recorded is referred to as the iso-electric line, and a deflection from it signals electrical activity of the heart.
A normal ECG strip features the following:
P Wave = electrical activity within the atrial chamber
SA Node (Sinus Node a.k.a. sino-atrial node) – The pacemaker of the heart, firing about 60-100 times per minute;
AV Node (Atrio-Ventricular Junction) – Fires at a rate of 40-60 times per minute. The AV node takes charge whenever the SA node experiences impulse issues;
AV Bundle (Bundle of His), Left Bundle and Right Bundle Branches, and the Purkinje Fibres – Fire at 20-40 times per minute if both the AV and the SA node experience impulse issues.
Interpreting ECG
Heart Rate
The Rule of 300: when the rhythm is regular = 300 / (number of boxes between R to R wave)
Six Second Method: when the rhythm is irregular = number of R waves per 6 seconds X 10
ECG Recording
Limb Connection Points – Retrieved from https://www.pngegg.com/en/search?q=ecg+Monitor on 8th January 2023
The direction of the electrical current determines the upward or downward deflection of an ECG waveform.
Major deflections include:
P Wave – atrial depolarisation
QRS Complex – ventricular depolarisation
T Wave – ventricular repolarisation
Retrieved from https://ijdr.in/article.asp?issn=0970-9290;year=2014;volume=25;issue=3;spage=386;epage=389;aulast=Anoop;type=3 on 8th January 2023
Retrieved from https://aneskey.com/ecg-basics/ on 8th January 2023
P Wave should be small, rounded, and positive, visible through leads I, II, aVF, and V2-V6, with an amplitude of 0.5-2.5mm and duration of <120ms; there should be only 1 P Wave preceding the QRS Complex.
QRS Normal Interval should be less than 3 small squares on the ECG graph.
ST Segment is normally isoelectric and gently upsloping.
Retrieved from https://www.aclsmedicaltraining.com/basics-of-ecg/ on 8th January 2023
QT Prolonged could be indicating Hypokalaemia, Hypocalcaemia, Bradycardia, Drugs, issues with the CNS, Left Ventricular Hypertrophy and Pericarditis.
ST Elevation could be indicating MI or Myocardial Injury, Coronary Vasospasm or Pericarditis.
ST Depression could be indicating Ischaemia, Digitalis Glycocides use (eg. Digoxin), block in the left or right Bundle Branch, or left or right ventricular hypertrophy. ST Depression is a sign of a narrowed blood vessel.
Retrieved from https://www.cvphysiology.com/CAD/CAD012 on 8th January 2023
Retrieved from https://www.cvphysiology.com/CAD/CAD012 on 8th January 2023
Retrieved from https://www.washingtonhra.com/arrhythmias/long-qt-syndrome.php on 8th January 2023
NOTE: Some drugs such as antibiotics, anti-psychotic and anti-arrhythmic drugs, prolong depolarisation and repolarisation time.
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Blood pressure is the force that the blood exerts against a blood vessel wall. When measuring BP, both the systolic and the diastolic blood pressure are recorded. The systolic blood pressure records the peak arterial blood pressure reached in the arteries during ventricular contraction, while the diastolic blood pressure records the minimum arterial blood pressure reached in the arteries during ventricular relaxation…
Normal Blood Pressure = 120/80 mmHg
PULSE PRESSURE is the difference between systolic and diastolic blood pressure…
Normal Pulse Pressure = 40 mmHg
MEAN ARTERIAL PRESSURE (MAP) is the average pressure in the arteries within a cardiac cycle…
MAP = (Diastolic X2) + Systolic = Answer / 3
or
MAP = CO X TPR
A stroke volume increase or a heart rate increase result in an increase in cardiac output. Total Blood Volume affects MAP as well.
Blood Pressure is determined by 3 main principles:
Cardiac Output (CO)
Blood Volume (BV)
Total Peripheral Resistance (TPR)
Venous Return, which is the volume of blood flowing towards the heart through systemic veins, affects BV resulting in a change in CO and stroke volume. Venous return is affected by the pressure difference between the pressure in the venules and the pressure within the right ventricle.
The Skeletal Muscle Pump and the Respiratory Pump are responsible for pumping blood from the lower body back to the heart through the inferior vena cava thanks to the valves present within the veins.
At rest, the proximal and distal valves within the calf are open, allowing blood flow to move upwards towards the heart. In leg muscle contraction, veins are compressed, pushing blood through the proximal valve, leading to the distal valve to close due to blood pushing against it.
Within the respiratory pump, the diaphragm moves downwards during inhalation, leading to a decrease in the intrathoracic pressure and an increase in the intrabdominal pressure. This creates compression within the abdominal veins.
Total Peripheral Resistance (TPR) refers to the resistance to flow due to friction of blood against the vessel walls. TPR depends on:
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The term haemodynamics refers to the physical principles of blood flow, with ‘flow’ being the amount of blood flow in a given time (mL/min). The cardiovascular system’s main aim is to keep blood flowing throughout the capillaries so that capillary exchange can happen.
Capillary exchange is a two-way traffic type movement of substances that occurs between the blood plasma and the interstitial fluid.
It is important to note that tissue perfusion occurs through capillary exchange, making the blood within the capillaries (which is usually around 250-300ml at a given time) the most important blood within the body.
Chemicals pass through capillary walls through 3 possible routes:
endothelial cell cytoplasm
intercellular clefts
filtration pores of the fenestrated capillaries
Movement through the capillary walls happen by:
Diffusion: allows exchange through the use of the concentration gradient across a permeable membrane (eg. glucose, oxygen, carbon dioxide and waste);
Transcytosis: through pinocytosis, fluid droplets are picked up by endothelial cells. Vesicles move across the cell and releases fluid through exocytosis (eg. fatty acids, albumin and hormones);
Filtration: fluids and solutes from blood capillaries move into the interstitial fluid due to blood hydrostatic pressure (BHP), which is the pressure that water within the blood plasma exerts against blood vessel walls, and interstitial fluid osmotic pressure (IFOP), which is the opposing pressure of the interstitial fluid;
Reabsorption: fluids and solutes from the interstitial fluid move into the blood capillaries due to blood colloid osmotic pressure (BCOP).
Starling’s Law of the Capillaries refers to the near equilibrium existing between the volume of liquid reabsorbed and the volume filtered. The discrepancy in filtration and re-absorption is normally absorbed back into circulation through the lymphatic system.
If filtration exceeds re-absorption in an excessive way, oedema becomes present due to the abnormal increase in interstitial fluid volume. Excessive filtration can be caused by an increase in capillary blood pressure and capillary permeability, while inadequate re-absorption can be caused by a decrease in the concentration of plasma proteins which in turn lowers the blood colloid osmotic pressure (BCOP).
Below you can find a collection of videos that can help provide a more visual approach to Capillary Exchange.
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The lymphatic system, which comprises of lymph, lymphatic vessels, and lymphatic tissue, has an important role within the body. It:
drains excess fluid and protein from the interstitial tissue back into the blood
transports fat from the GI tract to the blood
produces and circulates lymphocytes that help in keeping the body protected
Lymph
Lymph is the same as the interstitial fluid. Fluid that bathes the cells is referred to as interstitial fluid, while when it flows through the lymphatic vessels, it is called lymph.
Lymphatic Vessels
The lymphatic vessels are microscopic vessels in between the cells spaces known as lymph capillaries. They are slightly larger than blood capillaries and more permeable. Whilst we can find lymphatic capillaries throughout the whole body, they are not found in the avascular tissue, CNS, splenic pulp and bone marrow.
Lymph capillaries connect through larger lymph vessels known as lymphatics, which converge into two main channels, namely the thoracic duct and the right lymphatic duct. Lymphatics have thinner walls and valves (more valves than veins). Lymph nodes can be found at various intervals.
Retrieved from https://www.lymphedemablog.com/2012/02/15/comparison-of-blood-and-lymph-vessels/ on 29th May 2022
Drainage of the Lymphatic System
The left side of the head, neck, chest, upper left extremities and the entire body below the ribs all drain into the thoracic duct.
The upper right side of the body drains into the right lymphatic duct.
Retrieved from https://www.amboss.com/us/knowledge/male-reproductive-organs on 29th May 2022
Lymphatic Tissue
Lymphatic tissue is rich in lymphocytes and accessory cells such as macrophages and reticular cells. It is scattered in the linings of the GI tract, the respiratory tract, the urinary tract, the reproductive tract, and in the stroma a.k.a. core of multiple organs. Lymphatic tissue can also be found surrounded by a capsule within the lymphatic organs a.k.a. lymph nodes, spleen, and the thymus gland.
Lymph nodes
Lymph nodes are oval structures measuring between 1-25mm in length, commonly found in groups, located along the lymph vessels’ pathway.
Lymph passes through the nodes and is filtered from foreign substances by the reticular fibres within the node
Macrophages destroy foreign substances through phagocytosis
T Cells destroy foreign substances through the release of various products
B Cells produce antibodies to destroy them
Retrieved from https://pubs.rsna.org/doi/abs/10.1148/rg.2021210053?journalCode=radiographics on 29th May 2022
Lymphatic ORgans: Tonsils
Tonsils are a pair of soft tissue masses located at the pharynx. Their location helps protect against the invasion of foreign substances through the production of lymphocytes and antibodies.
Retrieved from https://www.aboutkidshealth.ca/Article?contentid=1918&language=English on 29th May 2022
Lymphatic Organs: Spleen
The spleen is an oval shaped organ measuring around 12cm in length which is made up of lymphatic tissue. It is located in the upper left side of the abdomen, next to the stomach and behind the left ribs. Functions of the spleen include:
B Lymphocyte Production – eventually develop into antibody-producing plasma cells
Phagocytosis – of bacteria and damaged or worn-out red blood cells and platelets
Blood Storage and Release – in cases such as haemorrhage
NOTE: the spleen does not filter lymph.
Retrieved from https://www.researchgate.net/publication/309457531_A_Brief_Survey_of_Spleen_Segmentation_in_MRI_and_CT_Images/figures?lo=1&utm_source=google&utm_medium=organic on 29th May 2022
Lymphatic Organs – Thymus Gland
The thymus gland is found in the superior mediastinum, between the lungs and behind the sternum. It reaches it’s maximum size during puberty, after which it starts to break down. The main function of the thymus gland is to produce T Lymphocytes.
Retrieved from https://www.thoracic.theclinics.com/article/S1547-4127(10)00194-5/fulltext on 29th May 2022
Body Defences
The human body aims to maintain haemostasis by counteracting pathogens or related toxins in the environment.
Resistance = the body’s ability to keep off disease
Susceptibility = the body’s inability to resist disease
Body defences can be divided in two groups: Non-Specific Defence and Specific Defence a.k.a. Immunity
Retrieved from https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/63-defence-against-infectio/lines-of-defense.html on 30th May 2022
Non-Specific Defences
The non-specific defence mechanism provides an immediate response to protect the body from foreign substances. Components of the non-specific defence mechanisms include:
SKIN & MUCOUS MEMBRANES – MECHANICAL FACTORS include the epidermis‘ anatomy i.e. made up of closely packed cells, continuous layering and the presence of keratin; mucous membranes that secrete mucus to prevent cavities from drying up whilst trapping microbes at the same time (eg. in nose through hairs and in the upper respiratory tract through cilia); lacrimal apparatus; saliva which helps prevent microbe colonisation; epiglottis which helps prevent microbes from entering the lower respiratory tract; CHEMICAL FACTORS include sebum which forms a protective film over the skin’s surface and inhibits bacterial growth; perspiration which flushes microbes from the skin; gastric juice produced by the stomach glands which is highly acidic due to being made of hydrochloric acid, enzymes and mucus, all of which help preserve the stomach’s sterility whilst destroying bacteria and most bacterial toxins; lyzozyme (found in perspiration, tears, saliva, nasal secretions and tissue fluids) which is an enzyme that can break down cell walls of various bacteria;
ANTI-MICROBIAL SUBSTANCES – INTERFERON (IFN) (alpha, beta & gamma) which are produced by lymphocytes and other leucocytes and fibroblasts; COMPLEMENT – a group of 11 proteins found in normal blood serum which complements immune and allergic reactions involving antibodies – once activated, destroys microbes; PROPERDIN – a protein found in the serum which together with COMPLEMENT causes the destruction of several types of bacteria, enhances phagocytosis, and triggers inflammatory responses;
PHAGOCYTOSIS– the ingestion and destruction of microbes or other foreign particulate matter by phagocytes through the Adherence process and the Ingestion process;
INFLAMMATORY PROCESS – when cells are damaged by microbes, inflammation is triggered, characterised by redness, pain, heat, swelling, and loss of function; during inflammation, vasodilation increases permeability of blood vessels, neutrophils migrate to the injured area within one hour, nutrients are released to help support defensive cells and increased metabolic reactions of the affected cells, fibrin formation, and pus formation;
FEVER – inhibits microbial growth and speeds up body reactions which help the body to heal
HIGH FEVER => HIGH BODY TEMPERATURE => INCREASED RATE WITH WHICH THE BODY WORKS TO FIGHT OFF INFECTION, BACTERIA OR VIRUSES.
Specific Defences a.k.a. immunity
Immunity involves the production of a specificy cell type or molecule a.k.a. antibody that can destroy a particular antigen.
An antigen is a chemical substance which causes the body to produce specific antibodies which can react with the antigen. Antigens have immunogenicity (can stimulate the formation of specific antibodies) and reactivity (can react specifically with the produced antibodies). Antigens with both immunogenicity and reactivity are called complete antigens eg. proteins, nucleoproteins, lipoproteins, glycoproteins, and some polysaccharides. Non-microbial antigens include pollen, egg white, incompatible blood cells, and transplanted tissues and organs.
An antibody is a protein produced by the body in response to antigen presence. An antibody can combine with the antigen.
Antibodies belong to a group of proteins called globulins, hence the name immunoglobulins.
Retrieved from https://www.toppr.com/ask/content/concept/antibodies-intermediate-266269/ on 30th May 2022
Cellular immunity & Humoral Immunity
The body can defend itself against bacteria, toxins, viruses and foreign tissues thanks to 2 components:
CELLULAR IMMUNITY A.K.A. CELL-MEDIATED IMMUNITY – includes the formation of specially sentisised lymphocytes which can attach to the foreign agent and destroy it; T cells are responsible for cellular immunity
HUMORAL IMMUNITY A.K.A. ANTIBODY-MEDIATED IMMUNITY – includes the formation of circulating antibodies able to attack an invading agent; B cells are specialised plasma cells which produce antibodies and provide humoral immunity
Retrieved from https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/111-antibody-production-and/immune-pathways.html on 30th May 2022
Retrieved from https://www.pinterest.com/pin/729442470871870835/ on 30th May 2022
The Immune Response
The immune response of the body, be it cellular or humoral, is more intense after a second or subsequent exposure to an antigen than after initial exposure, as illustrated below…
Retrieved from https://www.elevise.co.uk/gab3q5.html on 3th May 2022
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Blood vessels can be divided into 3 principal categories:
ARTERIES are the efferent blood vessels of the CVS which carry blood away from the heart;
VEINS are the afferent blood vessels that carry deoxygenated blood back to the heart;
CAPILLARIES are microscopic thin-walled vessels that connect the smallest arteries to the smallest veins.
The artery and vein walls are composed by 3 layers called tunics. The tunica intima lines the inside of the vessel and acts as a selectively permeable barrier to materials entering or leaving the blood stream. The tunica media is the middle layer of the vessel (usually the thickest layer) which consists of smooth muscle, collagen and elastic tissue that help strengthen the vessels and prevent them from rupturing due to the blood pressure, and allows vasoconstriction or vasodilation of the vessels. The tunica adventitia, which is the outermost layer, consists of loose connective tissue that merges with neighbouring blood vessels, nerves or organs.
Arteries are resistance vessels of the cardiovascular system. Their muscular built allows them to retain their round shape even when they are empty. Due to the left ventricle pressure surge, arteries need to be able to resist the blood pressure surge, otherwise they would burst.
There are 3 types of capillaries:
Continuous capillaries: present in most tissues;
Fenestrated capillaries: found in kidneys, endocrine glands and small intestine;
Sinusoids a.k.a. Discontinued capillary: found in the liver, bone marrow and spleen.
Veins are the capacitance vessels of the cardiovascular system. They are relatively thin and flaccid, and collapse when empty. However they are able to expand easily to accomodate blood volume increase. Different type of veins include:
large veins (such as the vena cava)
venous sinuses
medium veins (containing venous valves, usually found in lower limbs)
muscular venules
post-vapillary venules
Below you can find a collection of videos that can help provide a more visual approach to Blood Vessels including Arteries, Veins and Capillaries.
blood vessel layers
3 types of capillaries
arteries vs veins
Post-capillary venules
Special thanks to the creators of the featured videos on this post, specifically Youtube ChannelsKhan Academy Medicineand Khan Academy.
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Electrolyte imbalance is a frequent and potentially hazardous complication in patients with heart failure.
Potassium keeps blood pressure levels stable, regulates heart contractions and helps with muscle functions…
HYPOKALAEMIA: HR slow and irregular, weak pulse, orthostatic hypotension, diminished breathing sounds. The heart may also arrest in diastole. ECG shows a depressed ST segment, flat or inverted T wave, and a U wave.
HYPERKALAEMIA: muscle weakness, low or absent urine production due to renal failure, respiratory failure, seizures, decreased cardiac contractility, low BP. ECG shows tall peaked T wave, flat P wave, wide QRS complex or prolonged QR interval.
Calcium helps with muscle contractions, nerve signaling, blood clotting, cell division, and the formation or maintenance of bones and teeth…
HYPOCALCAEMIA: heart beats slower due to the effect of calcium on heart contractility. Muscle spasms, arrhythmias. ECG shows prolonged QT interval or prolonged ST interval.
HYPERCALCAEMIA: heart beats faster due to the effect of calcium on heart contractility. Muscle weakness, absent reflexes, constipation, kidney stone formation. ECG shows shortened QT interval.
Magnesium is needed for muscle contractions, proper heart rhythms, nerve functioning, bone building, bone strength, reducing anxiety, digestion, and keeping a stable protein-fluid balance.
HYPOMAGNESEMIA: muscle twitching, positive Trusseau sign, weak respirations, irritability, high BP, involuntary movements, low bowel mobility. ECG shows tall T waves and depressed ST segment. Prolonged PR and QT intervals with wide QRS complex in case of severe hypomagnesemia.
HYPERMAGNESEMIA: Signs are showing only in severe cases: lethargy, ECG shows PR and QT prolonged intervals and wide QRS complex, hypotension, bradycardia, GI issues, impaired breathing, cardiac arrest.
Sodium helps maintain fluid balance, and is needed for muscle contractions and nerve signaling.
HYPONATREMIA: seizures, lethargy, abdominal cramping, orthostatic hypotension, muscle spasms, trouble concentration, lack of urine, lack of appetite, shallow respirations.
Below you can find a collection of videos that can help provide a more visual approach to electrolyte imbalance affecting the heart.
Hypokalaemia
Hyperkalaemia
Hypocalcaemia
hypercalcaemia
hypomagnesemia
hypermagnesemia
hyponatremia
hypernatremia
Special thanks to the creator of the featured videos on this post, specifically Youtube Channel Registered Nurse RN.
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The cardiac cycle can be divided into 2 major events: systole and diastole, both of which sub-divide into smaller phases. Systole refers to the contraction of the heart muscle, whilst diastole refers to the relaxation of the heart muscle. Both are equally important for the normal functioning of the heart, as diastole allows filling with blood whilst systole causes the pumping out of the blood. It is important to note that:
blood flows from higher to lower pressure
contraction increases the pressure within the chamber while relaxation lowers the pressure
valves open/close according to pressure gradients
Phases of the cardiac cycle
Atrial depolarization/contraction
Isovolumetric Contraction
Rapid Ejection
Reduced Ejection
Isovolumetric Relaxation
Ventricular Filling
Atrial systole starts after the P wave of the ECG and lasts 0.1 seconds, which is then followed by atrial diastole that lasts 0.7 seconds.
Ventricular systole starts close to the end of the R wave and ends just after the T wave, lasting for about 0.3 seconds. It is then followed by ventricular diastole that lasts 0.5 seconds.
Heart Sounds
Heart sounds are caused primarily from the turbulence in the blood flow created by the closure of the valves. While there are 4 heart sounds per cardiac cycle, only the 1st and 2nd heart sounds are loud enough to be auscultated.
S1 HEART SOUND is a long booming sound caused by the closure of the atrioventricular valves soon after ventricular sistole begins.
S2 HEART SOUND is a short sharp sound caused by the closure of the semilunar valves towards the end of the ventricular systole.
The 3rd heart sound happens due to the blood turbulence during rapid ventricular filling, while the 4th heart sound happens due to blood turbulence during the atrial systole, both of which are not long enough to be auscultated.
Below you can find a collection of videos that can help provide a more visual approach to the cardiac cycle.
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Digoxin is a medication from the cardiac glycosides class. It helps the heart pump more efficiently by inhibiting the normal function of the sodium-potassium pump. It affects the heart by causing a:
Positive Inotropic Effect – helps the heart contract harder
Negative Chronotropic Effect – promotes a slower heartbeat
Negative Dromotropic Effect – causes the AV Node to send slower impulses
This causes the heart to squeeze more blood out, increasing stroke volume and cardiac output. This allows the heart to provide better perfusion throughout the body.
Digoxin is helpful with heart failure, cardiogenic shock, atrial fibrillation or atrial flutter, and problems within the heart pumping and emptying actions.
Digoxin Toxicity
Early signs and symptoms of Digoxin toxicity include nausea and vomiting as well as anorexia. Futher symptoms may arise such as changes in vision, including yellow/greenish halos, as well as dysrhythmias.
Toxicity risk increases if the patient is experiencing hypokalaemia (<3.5), hypomagnesmia (<1.5), or hypercalcaemia (>10.2).
This medication should not be administered if the patient has an apical pulse of less than 60 (in adults). In case of a lesser pulse rate, hold medication and consult with MD.
Below you can find a collection of videos that can help provide a more visual approach to Digoxin and how to assess an apical pulse.
Digoxin
Assessing the Apical Pulse prior to Administration
Special thanks to the creator of the featured videos on this post, specifically Youtube Channel Registered Nurse RN
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Cardiac muscle cells contract spontaneously, independently, regularly and continuously. Autorhythmic fibres generate action potentials that trigger heart contractions repeatedly, acting as the natural pacemaker of the heart throughout the electrical activity of the heart.
SA Node a.k.a. Sinoatrial Node is the pacemaker of the heart, exactly where cardiac excitation begins. It fires 60-100 electrical impulses per minute (approx. one every 0.8 secs). The SA Node cells DEPOLARISE repeatedly to threshold spontaneously; SPONTANEOUS DEPOLARISATION = PACEMAKER POTENTIAL.
AV Node a.k.a. gatekeeper of the heart acts as an electrical gateway to the ventricles. It fires 40-60 electrical impulses per minute (approx. one every 0.5 secs), evidently having slowed down due to having thinner myocytes with fewer gap junctions over which signals are transmitted. This delay allows the ventricles to fill up with blood before contracting.
The Bundle of His a.k.a. AV Bundle is where action potentials can conduct from the atria to the ventricles, entering both the right and left bundle branches.
Here the Purkinje Fibres conduct the action potential from the apex of the heart up to the rest of they ventricular myocardium, causing the ventricles to contract at the fastest speed of the whole conduction system (4m/s), pushing blood towards the semilunar valves.
Ventricular Myocyte Action Potential
Cardiac myocytes have a stable resting membrane potential of -90mV, depolarising only when stimulated. Ventricular Myocytes’ action potential has 3 phases:
DEPOLARIZATION – a stimulus opens voltage-gated Na+ channels, causing depolarisation as they enter the cells. The threshold voltage opens additional Na+ channels triggering a positive feedback cycle, peaking at almost +30mV. In response the Na+ channels close abruptly, causing the rising phase of the action potential to be very short.
PLATEAU – here is where depolarisation is maintained while the myocytes contract. Voltage-gated slow Ca2+ channels open up allowing small amounts of Ca2+ ions from within the ECF to enter the myocytes. With the binding action of Ca2+ ions to the ligand-gated Ca2+ channels on the sarcoplasmic reticulum, more channels open allowing more Ca2+ ions into the cytoplasm, which then bind to troponin, causing the ventricular myocyte to contract by the stimulus. Finally Ca2+ channels close and K+ channels reopen, causing K+ ions to difuse out of the cell and the Ca2+ ions to return to the ECF.
REPOLARIZATION – The negative resting membrane potential is now restored to -90mV.
Baroreceptors in the Cardiovascular Centre
Baroreceptors are found in the aorta and the internal carotid arteries.
Increase in HR = Increase in CO = Increase in BP = Baroreceptors sense changes & signal to the cardiac centre = cardiac centre Decreases HR.
Decrease in HR = Decrease in CO = Decrease in BP = Baroreceptors sense changes & signal to the cardiac centre = cardiac centre Increases HR & re-stabilises CO & BP.
Chemoreceptors in the Cardiovascular Centre
Chemoreceptors are found in the aortic arch, carotid arteries and the medulla oblongata. These are sensitive to blood pH, Carbon Dioxide and Oxygen.
Chemoreceptors can sense Hypercapnia and Acidosis, which then stimulate the cardiac centre, increasing the HR and restores perfusion of the tissues. Accumulated Carbon Dioxide is then removed.
In response to Hypoxaemia, chemoreceptors lead to a slowing down of the HR.
Below you can find a collection of videos that can help provide a more visual approach to the electrical activity of the heart.
Electrical Activity of the Heart
Ventricular Myocyte Action Potential
Regulation of the Heart Activity Through the ans (autonomic nervous system)
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