Case 2- ECG and imaging Flashcards
ECG change in MI
ST elevation if there is a full thickness infarction. If the ST elevation is not present then its not full thickness
What does the ECG record
The electrical depolarisation of the myocardial tissue produces a dipole current which can be detected by an electrode pair on a body surface, the signals are then amplified on a monitor. Measures the potential difference between two different areas on the skin
ECG- P wave
Atrial depolarisation
ECG- QRS complex
Ventricular depolarisation, should be between 80-100ms, over 120 is a wide QRS may be due to a pacemaker or drug toxicity
ECG- PR interval
The time between the onset of the P wave and the QRS complex, should be 120-200ms. Look at its relation to the QRS complex, is each P wave followed by a QRS
ECG- T wave
Ventricular repolarisation
ECG- ST segment
The interval between the S wave and the T wave
ECG- QT interval
From the start of the QRS complex to the end of the T wave. Affected by heart rate so need to adjust it to create the QTc (corrected QT). Should be under 440ms in men and 460ms in women
Wires in an ECG
Positive electrode, negative electrode and earth wire
ECG- lead 1
The negative electrode is attached to your right wrist and the positive electrode is attached to the left wrist. Earth wire on the ankle
ECG- lead 2
Negative electrode is on the right wrist and the positive electrode is on the left ankle, the lead is about the same angle as the heart in the body. Produces bigger spikes on an ECG so is used more commonly.
ECG- lead 3
When the negative electrode is on the left wrist and the positive electrode is on the left ankle.
12 lead ECG
The 12 angles an ECG can be taken from
Working out heart rate
You count the squares, the small squares are 0.04 seconds, the medium squares are 0.2 seconds
ECG changes in left ventricular hypertrophy
More muscle needs to be depolarised so you will have a tall R wave. The S wave will be deeper and there may be an inverted T wave.
ECG first degree heart block
The PR interval is >200ms. Each P wave is followed by a QRS. Due to a delay in the signal reaching the ventricles
ECG second degree heart block (Mobitz 1)
The PRS interval extends till the QRS complex is dropped, the system will then rest itself to start again
ECG second degree heart block (Mobitz 2)
The PR does not extend but the QRS is occasionally dropped. Not every P wave is associated with a QRS complex. A ventricular contraction is skipped due to the signal not conducting through the AV node each time.
ECG third degree heart block
No correlation between the P wave and QRS. They can be regular but they are independent of each other. There is no communication between the atria and ventricles
ECG atrial fibrillation
No P wave and an irregular rhythm
Four initial features of an ECG
Clinical context, rate, rhythm and axis
Bradycardia
Low heart rate below 60bpm
Tachycardia
High heart rate above 100 bpm
Equation you use to measure heart rate on an ECG
To measure rate, you have 300 divided by the large boxes between the R waves. Only do this if the heart rate is regular.
Normal heart rhythm
When the QRS complex’s has the same number of box’s between them, ignore sinus arrhythmia
Ectopic beat
When someone misses a heartbeat
Basis of defibrillation
Used as treatment for ventricular fibrillation and non-perfusing ventricular tachycardia. It delivers a high dose of electric current to the heart depolarising the heart muscle and ending the dysrhythmia. The SA node is then able to re-establish normal sinus rhythm.
ECG indications of ischaemia
A depressed ST segment
ECG indications of a STEMI
An elevated ST segment
ECG indications of atrial fibrillation
No P wave and it will be irregular, QRS complex is irregularly spaced. It will be an irregularly irregular rhythm
ECG indications of ventricular fibrillation
Completely irregular ECG with no coordinated ventricular activity, no cardiac output. The ventricle is unable to coordinate muscular contraction
ECG indications of the first 5 to 30 minutes in ischaemic heart disease
Hyperacute T wave
ECG indication from the first hour to 24 hours in ischaemic heart disease
ST elevation
ECG indications of myocardial ischaemic angina
ST depression
How is odema formed
In people with heart failure there is increased hydrostatic pressure in the capillaries, because there is blood backing up in the venous system as the heart is not able to pump it all. The hydrostatic pressure in the capillaries is bigger then the oncotic pressure in the venous end of the capillaries, meaning fluid is unable to move into the capillaries from the tissues. There is now a build up of fluid in the interstitium
Echocardiography
A type of ultrasound which is used to assess cardiac function. You can see the 4 heart chambers, assess muscular contractions, assess direction of blood flow in real time. You place the ultrasound probe between the patients ribs, or down the oesophagus. By using the Doppler function, flow of blood through valves can be assessed and backflow or turbulence from vascular disease identified.
Coronary angiography
A peripheral artery such as the femoral artery is punctures by a hallow needle. A coronary catheter wire is then passed through the needle and into the artery, it is threaded into the coronary vasculature. A radiopaque dye is injected into each of the coronary roots. The radiopaque dye is a dense liquid which dilutes in the blood, the x-ray cannot penetrate it. The coronary branches are subsequently imaged via plain radiography and any blockages in the coronary vasculature can be visualised.
Plain radiography
X-rays are created in a generator, the electrons are propelled down a tube, as they collide x-ray is produced. They are then directed at the patient as a beam, targeting the area of interest. They can penetrate soft tissue but not bone or metal, which is why doctors wear lead plated gown to protect themselves when doing it. A photographic film or plate is used for detection
CT- scan
A series of x-rays at different levels, this is then fed to a series of plain film in a processor in order to generate 3D image. CT stands for computed tomography
MRI scan
Takes serial images of a patient, hydrogen is used because its present in water and allows us to map the majority of the body
How MRI is used
- The hydrogen atom has its own quantum “spin”, a magnetic orientation of sorts. The orientation of hydrogen atoms in a resting patient is random.
- The patient is placed in the MRI machine, a large magnet is turned on. The hydrogen atoms will be influenced by this magnet and their spins will either be orientated in a high energy state (pointing towards the magnetic force) or a low energy state (pointing away from the magnetic force).
- The MRI then pulses a burst of electromagnetic energy at the patient, this provides the energy for the low energy atom to change their spin to a high energy one. All the hydrogen atoms are in the same spin orientation (high-energy).
- The burst of energy is now stopped, all the atoms which are normally in the low energy configuration (they spun around) revert to their original position. As they go from high to low energy they release their excess energy, this is detected by the receiving element of the MRI machine and is translated to an image representing the structure of the body.
Ultrasonography
Uses ultrasound electromagnetic range. The operator uses a probe and places it above the patient’s body on the area they want to investigate. The probe emits ultrasound waves which penetrate the body, passing through air cavities and then bouncing of solid matter such as bone. The degree of penetration vs obstruction is known as acoustic impedance and different tissues have different values due to their density. The waves that are reflected back by denser matter are picked up by the ultrasound probe which contains a receiver. The ultrasound machine then converts the amount of waves sent, to those received back (and their relative strength) and maps these signals geographically to provide a cross-sectional image of the area scanned. You can move the probe around so you are observing an image live
Doppler effect
The phenomenon that when an object is moving towards or away from an observer, the sound generated by the object will change in frequency accordingly. Can be used to detect arterial and venous flow
Advantages of plain radiology
Simple, cheap and quick. Shows infections in lungs and brakes or dislocations in the bones
Disadvantages of plain radiology
Risks involved in radiation (causes cancer) especially if its repeated, areas of development or places with a high cellular turnover are particularly at risk. You would never expose a pregnant women to radiation, or certain tissues like the testicles and thyroids. Its 2D, pathologies can be missed or hidden if obstructed by areas of high density. Rarely used to look at soft tissues, hollow organs or the neurological system.
Advantages of a CT scan
Allows you to create a 3D image. Relatively simple and quick to perform. Less recourse heavy then MRI’s. Good view of most of the body, produces the best image in bones
Disadvantages of CT scans
Difficult to create an image in soft tissue like muscles, ligaments and tendons. Lots of ionized radiation produced
Advantages of MRI scans
No ionizing radiation used, so no radiation exposure and the risks associated. Can therefore be used on a pregnant woman. There is an element of caution, as there are unknowns about subjecting developing tissue (foetus) to strong electromagnetic forces. Quality of detail in soft tissue is really good, so is used when looking at soft tissue injuries such as ligament and tendon tears or when looking at the nervous system
MRI scan disadvantages
Not very useful when looking at bones as they have a low water content. Due to the strong electromagnetic forces used, if the patient has metallic objects either surrounding the patient (oxygen tanks) or in them (pacemakers) it can cause fatal accidents. They are very expensive and can cost several millions of pounds. There are not many in hospitals and they are used rarely. Take a lot of time to perform and is recourse heavy. Certain patients find it very difficult to tolerate MRIs as they can be very claustrophobic, lengthy and to get a clear image, they require the patient to be as immobile as possible for the duration of the examination.
Advantages of ultrasonography
Cheap and portable, used in AE to conduct a fast scan of a patient to quickly assess for any signs of blood collection. They use acoustic waves so there is no risk of damage from ionizing radiation. However, there are some theorized unknowns of the effect of high frequency waves on developing tissues.
Disadvantages of ultrasonography
Very user dependent, depends on how good the person is giving the scan. Drastically effected by external factors such as a patients BMI. Good at looking at echogenic material in an otherwise empty hollow organ (foetus in uterus, fluid in lungs, thrombus in veins). If its high echogenicity (such as bone) it is not very useful.
