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 passive leg raising (PLR) manoeuvre is a method of altering left and right ventricular preload and it is done with real-time measurement of stroke volume. It is a simple, quick, relatively unbiased, and accurate bedside test to guide fluid management and avoid fluid overload.

Pulse pressure variation (PPV), Stroke volume variation (SVV), superior vena cava diameter variation (threshold 36%) and end-expiratory occlusion test are used for dynamic tests of fluid responsiveness.

PPV is derived peripherally from the arterial pressure waveform.

Stroke volume variation (SVV) can be derived peripherally through pulse contour analysis of the arterial waveform. PPV and SVV have a threshold of 12% but since they are not used in patients who have cardiac arrhythmias, are spontaneous breathing, and in ventilated patients with low lung compliance and tidal volumes, they are of limited value.

The tests of fluid responsiveness’ accuracy is determined by calculating the area under the receiver operating characteristic curve (UROC) obtained by plotting the sensitivity of the parameter in predicting fluid responsiveness vs. 1-specificity.

Under optimal conditions, the ability to determine the need for fluid is best with PPV>SVV>LVEDA>CVP.

Central venous pressure (CVP) is a static test of preload (not preload responsiveness) and a key determinant of cardiac function. The left ventricular end-diastolic area (LVEDA) a static test of fluid responsiveness, is derived using echocardiography

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

Closing capacity refers to volume of gas within the lungs at which the conducting small airways begin to close, that is, the point during expiration when small airways close.

It is calculated mathematically as:

Closing capacity = Closing volume (CV) + Residual volume (RV)

Functional residual capacity (FRC) is the volume of gas still present within the lungs post expiration.

Closing capacity is lower than the functional residual capacity in younger adults, but begins to rise to eventually equal, and then exceed it with increasing age (at about middle age), increasing intrabdominal pressure, decreasing blood flow in the pulmonary system and parenchymal disease within the pulmonary system.

The bispectral index (BIS) monitors works to determine the level of consciousness of a patient by processing electroencephalographic (EEG) signals to obtain a value between 0 and 100, where 0 reflects no brain activity, and 100 reflects a patient is completely awake.

The general meaning of BIS values are:

>95: Patient is in an awake state.
65-85: Patient is in a sedated state.
40-65: Patient is in a state that is optimal for general surgery.
<40: Patient is in a deep hypnotic state It is important in measuring the depths of anaesthesia to prevent haemodynamic changes or patient awareness during surgery. The nature of anaesthetic agent used is a determinant factor in resultant BIS values. Intravenous agents, such as propofol, thiopental and midazolam, result in a deeper hypnotic state, whilst inhalation agents have a lesser hypnotic effect at the same BIS values. Certain agents result in inaccurate BIS values such as ketamine and nitrous oxide (NO). These two agents appear to increase the BIS value, whilst putting the patient in a deeper hypnotic state, and should therefore not be used with BIS monitoring. Hypothermia also affects the BIS value as it causes a 1.12 per °C decrease in body temperature.

Mixed venous oxygen content (CvO2) is the oxygen concentration in 100mL of mixed venous blood taken from the pulmonary artery. It is usually 12-17 mL/dL (70-75%). It is represented mathematically as:

CvO2 = (1.34 x Hgb x SvO2 x 0.01) + (0.003 x PvO2)

Where,

1.34 = Huffner’s constant
Hgb = Haemoglobin level (g/dL)
SvO2 = % oxyhaemoglobin saturation of mixed venous blood
PvO2 = 0.0225 = mL of O2 dissolved per 100mL plasma per kPa, or 0.003 mL per mmHg

Therefore,

CvO2 = (1.34 x 10 x 70 x 0.01) + (0.003 x 50)

CvO2 = 9.38 + 0.15 = 9.53 mL/100mL

A patient presenting with symptomatic asthma should be assessed for severity to determine appropriate management options. Indications of acute severe asthma are:

Peak expiratory flow rate (PEFR): 33-50% best/predicted
Respiratory rate: ≥25/min
Heart rate: ≥110/min
Inability to finish a complete sentence in a single breath.

Oxygen saturation should be measured. Any measurement of an oxygen saturation of 92% or less, either on air or on oxygen, indicates severe, life threatening asthma, and requires an arterial blood gas (ABG) to detect normo- or hypercarbia.

A chest x-ray would not be routine as it will not provide any relevant information. It is only required in specific cases, including:
Diagnosis of a subcutaneous emphysema
Indications of a unilateral pneumothorax
Indications of a lobar collapse of consolidation
Treatment-resistance life-threatening asthma
If mechanical ventilation is indicated

A peak expiratory flow rate (PEFR) can provide relevant information to help distinguish between acute, moderate, severe and life threatening asthma. However, it is not necessary as other parameters exist that can also help make the same distinction.

An ECG is indicated in this case as the patient has tachycardia and tachypnoea which are indicative of acute severe asthma. The ECG would indicate if arrhythmia is also present which would suggest life-threatening asthma.

Severity of a reduction in restrictive defect (%FVC) or obstructive defect (%FEV1/FVC) predicted are classified as follows:

Mild 70-80%
Moderate 60-69%
Moderately severe 50-59%
Severe 35-49%
Very severe <35% This patient has a mixed deficit with a severe obstructive deficit as %FEV1/FVC predicted is 46.9% and a moderate restrictive deficit as %FVC of predicted is 61.3 FEV1/FVC ratio 80% < predicted and VC < 80% = mixed picture. FEV1/FVC ratio 80% < predicted and VC > 80% = obstructive picture.

FEV1/FVC ratio 80% > predicted and VC > 80% = normal picture.

FEV1/FVC ratio 80% > predicted and VC < 80% predicted= restrictive picture.

There are different types of temperature measurement. These include:

Thermistor – this is a type of semiconductor, meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. There are small beads of semiconductor material (e.g. metal oxide) which are incorporated into a Wheatstone bridge circuit. As the temperature increases, the resistance of the bead decreases exponentially

Thermocouple – Two different metals make up a thermocouple. Generally, in the form of two wires twisted, welded, or crimped together. Temperature is sensed by measuring the voltage. A potential difference is created that is proportional to the temperature at the junction (Seebeck effect)

Platinum resistance thermometers (PTR) – uses platinum for determining the temperature. The principle used is that the resistance of platinum changes with the change of temperature. The thermometer measures the temperature over the range of 200°C to1200°C. Resistance in metals show a linear increase with temperature

Tympanic thermometers – uses infrared radiation which is emitted by all living beings. It analyses the intensity and wavelength and then transduces the heat energy into a measurable electrical output

Gauge/dial thermometers – Uses coils of different metals with different co-efficient of expansion. These either tighten or relax with changes in temperature, moving a lever on a calibrated dial.

There are different classes of electrical equipment that can be classified in the table below:

Class 1 – provides basic protection only. It must be connected to earth and insulated from the mains supply

Class II – provides double insulation for all equipment. It does not require an earth.

Class III – uses safety extra low voltage (SELV) which does not exceed 24 V AC. There is no risk of gross electrocution but risk of microshock exists.

Type B – All of above with low leakage currents (0.5mA for Class IB, 0.1 mA for Class IIB)

Type BF – Same as with other equipment but has ‘floating circuit’ which means that the equipment applied to patient is isolated from all its other parts.

Type CF – Class I or II equipment with ‘floating circuits’ that is considered to be safe for direct connection with the heart. There are extremely low leakage currents (0.05mA for Class I CF and 0.01mA for Class II CF)

Spirometry measures the total volume of air that can be forced out in one maximum breath, that is the total lung capacity (TLC), to maximal expiration, that is the residual volume (RV).

It is conducted using a spirometer which is capable of measuring lung volumes using techniques of dilution.

During spirometry, the following measurements can be determined:
Forced vital capacity (FVC)/vital capacity (VC): The maximum volume of air exhaled in one single forced breathe.
Forced expiratory volume in one second (FEV1)
FEV1/FVC ratio
Peak expiratory flow (PEF): the maximum amount of air flow exhaled in one blow.
Forced expiratory flow (mid expiratory flow): the flow at 25%, 50% and 75% of FVC
Inspiratory vital capacity (IVC): The maximum volume of air inhaled after a full total expiration.

Anatomical dead space is measured using a single breath nitrogen washout called the Fowler’s method.

Residual volume and total lung capacity are both measured using the body plethysmograph or helium dilution

The functional residual capacity is usually measured using a nitrogen washout or the helium dilution technique.

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.

Malignant hyperthermia (MH) is a autosomal dominant inherited medical condition which predisposes affected individuals to a clinical syndrome of hypermetabolism which involves abnormal ryanodine receptors in skeletal muscle causing a deregulation of calcium in muscle.

It is a life threatening condition requiring immediate medical intervention. It often lies dormant until triggered in susceptible individuals mostly by volatile inhaled anaesthetic agents and succinylcholine which is a muscle relaxant.

The signs and symptoms of MH are related to this hypermetabolism, which includes an increase in carbon dioxide production, metabolic and respiratory acidosis, accelerated oxygen consumption, heat production, activation of the sympathetic nervous system, hyperkalaemia, disseminated intravascular coagulation (DIC), and multiple organ dysfunction and failure.

Early signs of MH to look out for in patients includes an uptick in end-tidal carbon dioxide (even with increasing minute ventilation), tachycardia, muscle rigidity, tachypnoea, and hyperkalaemia. Later signs include fever, myoglobinuria, and multiple organ failure.

In vitro muscle contracture test (IVCT) is the standard for determining individual susceptibility to MH. It is conducted by measuring the force of muscle contraction after exposing the patient’s muscle sample to halothane and caffeine., the sample is normally taken from the vastus medialis or lateralis under regional anaesthesia.

Since an arterial pressure transducer, and every other measuring apparatus, is prone to errors due to offset and gradient drifts, regular calibration is required to maintain accuracy of the instrument. The two-point calibration pressure values of 0 mmHg and 200 mmHg are within the physiologic range and can best compensate for the gradient drift.

A pulse oximeter is a piece of medical equipment used as a non-invasive method of measuring the oxygen saturation of blood.

It works by measuring the ratio of absorption of red and infrared light in a section of blood flow, as red light is largely absorbed by deoxygenated blood, and infrared light is largely absorbed by oxygenated blood.

Pulse oximetry relies on photoplethysmography (PPG) waveforms. The oximeter has 2 sides, with different functions. One side houses light-emitting diodes which are responsible for transmitting 2 light wavelengths, 660nm for red light and 940nm for near infrared light. The other side is a photodetector. The light emitted travels through the body and the amount that is not absorbed is measured by the photodetector.

Smokers often have increased levels of carboxy haemoglobin (COHb). This leads to artificial increases in pulse oximeter readings as it is unable to differentiate between COHb and oxyhaemoglobin (O2HB) as they both absorb red light at 660nm. Every 1% increase of circulating carboxyhaemoglobin, results in a correlative 1% increase in oximeter readings.

Prilocaine toxicity will cause an artificial decrease in oximeter readings. This is because prilocaine metabolites cause methemoglobinemia (MetHB), which are dysfunctional haemoglobins unable to properly transport oxygen. In this case, a laboratory multiwavelength co-oximeter is recommended for a more accurate reading.

Anaemia will not affect oximeter readings as long as haemoglobins in the blood are normal.

Sickle cell disease does not affect oximeter readings despite its ability to cause hypoxia and shift the oxygen dissociation curve to the right.

Brown-red fingernail polish will cause an underestimation of pulse oximeter readings.

There are different types of temperature measurement. These include:

Thermistor – this is a type of semiconductor, meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. There are small beads of semiconductor material (e.g. metal oxide) which are incorporated into a Wheatstone bridge circuit. As the temperature increases, the resistance of the bead decreases exponentially

Thermocouple – Two different metals make up a thermocouple. Generally, in the form of two wires twisted, welded, or crimped together. Temperature is sensed by measuring the voltage. A potential difference is created that is proportional to the temperature at the junction (Seebeck effect)

Platinum resistance thermometers (PTR) – uses platinum for determining the temperature. The principle used is that the resistance of platinum changes with the change of temperature. The thermometer measures the temperature over the range of 200°C to1200°C. Resistance in metals show a linear increase with temperature

Tympanic thermometers – uses infrared radiation which is emitted by all living beings. It analyses the intensity and wavelength and then transduces the heat energy into a measurable electrical output

Gauge/dial thermometers – Uses coils of different metals with different co-efficient of expansion. These either tighten or relax with changes in temperature, moving a lever on a calibrated dial.

Static lung compliance refers to the change in volume within the lung per given change in unit pressure. It is usually measured when air flow is absent, such as during pauses in inhalation and exhalation.

It is a combination of:

Chest wall compliance: normal value is 200 mL/cmH2O
Lung tissue compliance: normal value is 200 mL/ cmH2O

It is represented mathematically as:

1/Crs = 1/Cl + 1/Ccw

Where,

Crs = total compliance of the respiratory system
Cl = compliance of the lung
Ccw = compliance of the chest wall

Therefore in this case:

1/Crs = 1/200 + 1/200

1/Crs = 0.005 + 0.005 = 0.01

1/Ct = 0.01

Rearranging equation gives:

Ct = 1/0.01 = 100 mL/cmH2O.

Spirometry measures the total volume of air that can be forced out in one maximum breath, that is the total lung capacity (TLC), to maximal expiration, that is the residual volume (RV).

It is conducted using a spirometer which is capable of measuring lung volumes using techniques of dilution.

During spirometry, the following measurements can be determined:
Forced vital capacity (FVC)/vital capacity (VC): The maximum volume of air exhaled in one single forced breathe.
Forced expiratory volume in one second (FEV1)
FEV1/FVC ratio
Peak expiratory flow (PEF): the maximum amount of air flow exhaled in one blow.
Forced expiratory flow (mid expiratory flow): the flow at 25%, 50% and 75% of FVC
Inspiratory vital capacity (IVC): The maximum volume of air inhaled after a full total expiration.

Anatomical dead space is measured using a single breath nitrogen washout called the Fowler’s method.

Residual volume and total lung capacity are both measured using the body plethysmograph or helium dilution

The functional residual capacity is usually measured using a nitrogen washout or the helium dilution technique.

Peak expiratory flow rate (PEFR) is the maximum speed of air flow generated during a single forced exhaled breath. It is most useful when expressed as a percentage of the best value obtained from the patient.

Forced expiratory volume over 1 second (FEV1) is a lung parameter measured using spirometry. It is the amount of air forced out of the lung in one exhaled breath. It is a more accurate measure of lung obstructions as it doesn’t rely on effort like PEFR

PEFR and FEV1 are usually similar, but become more different in asthmatic patients as airflow becomes increasingly obstructed.

Acute severe asthma is most often diagnosed on history taking and examinations:

Respiratory rate: >25 breaths/min
Heart rate: >110 beats/min
PEFR: 33 – 50% predicted (<200L/min)
Patient state: Unable to complete a sentence in a single breath.

A chest x-ray is not routinely required, and is only indicated in specific circumstances, which are:

If a pneumomediastinum or pneumothorax is suspected
Possible life threatening asthma
Possible consolidation
Unresponsive asthma
If ventilation is required.

An echocardiograph (ECG) is not necessary in this case

Routine haematological and biochemical investigations are not urgent in this case as any abnormalities they detect will be secondary to the patient’s presentation.

An arterial blood gas (ABG) will only be indicated if SPO2 was <92% or if patient presented with life threatening symptoms.

Coronary perfusion pressure(CPP), the difference between aortic diastolic pressure (Pdiastolic) and the left ventricular end-diastolic pressure (LVEDP), is mainly determined by the formula:

CPP = Pdiastolic -LVEDP
where
Pdiastolic is the lowest pressure in the aorta before left ventricular ejection and
LVEDP is measured directly during a cardiac catheterisation or indirectly using a pulmonary artery catheter. The pulmonary artery occlusion or wedge pressure approximates best with LVEDP.

Using this patient’s haemodynamic data:

CPP = Pdiastolic – MPAWP
COO = 70 – 12 = 58mmHg

Capnography is the non-invasive measurement and pictorial display of inhaled and exhaled carbon dioxide (CO2) partial pressure.

It is depicted graphically as the concentration of CO2 over time.

It is used in disease diagnosis, determining disease severity, assessing response to treatment and is the best method to for indicating when an endotracheal tube is placed in the trachea after intubation.

The wavelength of IR light usually absorbed by nitrous oxide is between 4.4-4.6?m (very close to that of CO2). Its absorption of wavelengths at 3.9 ?m is much weaker. It causes a measurable deficit of 0.1% for every 10% of nitrous oxide. The maximal wavelength of infrared (IR) light absorbed by carbon monoxide is 4.7 ?m. The volatile agents have strong absorption bands at 3.3 ?m and throughout the ranges 8-12 ?m.

IR light is not absorbed by oxygen (O2), but O2 and CO2 molecules are constantly colliding which interrupts the absorption of IR light by CO2. This increases the band of absorption, that is the Collison or pressure broadening). An oxygen percentage of 95 will result in a 0.5 percentage fall in CO2 measure.

IR light is also absorbed by water vapour which will result in an overlap of the absorption band, collision broadening and a dilution of partial pressure. This is why water trap and water permeable tubing is recommended for use as it reduces measurement inaccuracies.

The use of multi-gas analysers of modern gases also help reduce the effects of collision broadening.

Beer’s law is also applied in this system as an increase in the concentrations of CO2 causes a decrease in the amount of IR able to pass through the gas. This IR light is what generated the signal that is analysed for display.

The capnograph can indicate oesophageal intubation, but cannot determine if it is endotracheal or endobronchial. For this, auscultation is used.

Certain skeletal muscles are more resistant to the effects of neuromuscular blocking agents, both non-depolarizing and depolarizing. The diaphragm is the most resistant. The muscles of the larynx and the corrugator supercilii are less resistant. The abdominal, orbicularis oris, and limb peripheral muscles are the most sensitive muscles.

Twitch stimulation patterns:

Supramaximal single stimulus:

The frequency ranges from 1 Hz to 0.1 Hz (one every second to one every 10 seconds)
The response is proportional to the frequency of the event.
It has limited clinical utility because it only tells you whether or not a patient is paralysed (no information on degree of paralysis).

Over the course of 0.5 seconds (2 Hz), four supramaximal stimulate were applied:

It is possible to see ‘fade’ and use it as a basis for evaluation.
This stimulation pattern is used to determine the degree of blockade (1-2 twitches is appropriate for abdominal surgery)
If the train of four (TOF) count is 1-2, reversal agents can be used in conjunction with medium-acting neuromuscular blocking agents.

Ratio of TOF:

This is the ratio of the 4th twitch amplitude to the 1st twitch amplitude.
The ratio decreases with non-depolarising block and is inversely proportional to the degree of block, allowing objective measurement of residual neuromuscular blockade.
To achieve adequate reversal, the ratio (as measured by accelerography) must be between 0.7 and 0.9.

Count of twitches after a tetanic experience(PTC):

50 Hz for 5 seconds, then a 3 second pause, followed by a single 1 Hz twitch stimulus.
When the TOF count is zero, this stimulation pattern is used to assess deep blockade (that is, in neurosurgery, microsurgery or ophthalmic surgery when even small movements of a patient will disturb the surgical field)
It gives an estimate of how long it will take for the response to return to single twitches, allowing assessment of blocks that are too deep for any other technique.
A palpable post-tetanic count (PTC) of 2 indicates no twitch response for about 20-30 minutes, and a PTC of 5 indicates no twitch response for about 10-15 minutes.

This is without a doubt the best way to keep track of paralysis in patients who need to avoid diaphragmatic movement. It’s best to use drug infusions and aim for a PTC of 2. After a tetanic stimulus, acetylcholine is mobilised, causing post-tetanic potentiation.

Stimulation in Two Bursts:

750 milliseconds between two short bursts of 50 Hz
This stimulation pattern is used to assess small amounts of residual blockade manually (tactile).

A damped sine wave is a smooth, periodic oscillation with an amplitude that approaches zero as time goes to infinity. In other words, the wave gets flatter as the x-values become larger.

Critical damping is defined as the threshold between overdamping and underdamping. In the case of critical damping, the oscillator returns to the equilibrium position as quickly as possible, without oscillating, and passes it once at most.

In overdamping, the system moves slowly towards the equilibrium. An underdamped system moves quickly to equilibrium, but will oscillate about the equilibrium point as it does so.

Optimal damping has a damping coefficient of around 0.64-0.7. It maximizes frequency response, minimizes overshoot of oscillations, and minimizes phase and amplitude distortion.

In an undamped system, the amplitude of the waves that are being generated remain unchanged and constant over time.

Tachyphylaxis is the swiftly declining response to successive drug doses which vastly reduces its effectiveness in a short space of time, mostly as a result of an acute consumption of neurotransmitters.

Tolerance or desensitisation is the slow decline in a person’s reaction to a drug due to continued use. It requires a longer time span than tachyphylaxis, usually over days or weeks.

Down- regulation is a reduction in the amount of receptors available on target cells which decreases the affinity of the agent to the cells. For this to occur, the down-regulation of receptors must occur at a rate faster than receptor synthesis. This down-regulation often occurs with beta1 receptors due to:

1) The transportation or receptors from the cell surface to the interior of the cell

2) Degradation of receptors occurring over time.

In this case, dobutamine is prescribed to treat cardiogenic shock. It is able to function by binding to beta1-adrenergic receptors to increase the contraction of the heart, thereby improving cardiac output. It also binds to beta2- and alpha1-adrenergic receptors to balance out the effects produced by binding to beta1 receptors and reduce the risk of system vasculature responses.

Arterial pressure waveforms is an invasive form of monitoring cardiac parameters. It provides a lot of information on the performance of the heart from different sections, including:

Cardiac measurements:

Heart rate
Systolic pressure
Diastolic pressure
Mean arterial pressure
Pulse pressure
Change in pulse amplitude corresponding to respiratory changes
Slope of anacrotic limb associated with aortic stenosis

From the shape of the arterial waveform displayed:

Slope of anacrotic limb represents aortic valve and LVOT flow
Indications of aortic stenosis (AS): Slurred wave, collapsing wave
Rapid systolic decline in LVOTO
Bisferiens wave in HOCM
Low dicrotic notch in states with poor peripheral resistance
Position and quality of dicrotic notch as a reflection of the damping coefficient

For this question, the upstroke slope of the pressure wave is indicative of myocardial contractility and is mathematically represented as:

dP/dt, which represents a change of pressure with regards to time.

There are different classes of electrical equipment that can be classified in the table below:

Class 1 – provides basic protection only. It must be connected to earth and insulated from the mains supply

Class II – provides double insulation for all equipment. It does not require an earth.

Class III – uses safety extra low voltage (SELV) which does not exceed 24 V AC. There is no risk of gross electrocution but risk of microshock exists.

Type B – All of above with low leakage currents (0.5mA for Class IB, 0.1 mA for Class IIB)

Type BF – Same as with other equipment but has ‘floating circuit’ which means that the equipment applied to patient is isolated from all its other parts.

Type CF – Class I or II equipment with ‘floating circuits’ that is considered to be safe for direct connection with the heart. There are extremely low leakage currents (0.05mA for Class I CF and 0.01mA for Class II CF)

The STOP-BANG questionnaire is used to screen patients for obstructive sleep apnoea (OSA).

The scoring system assigns one point for each feature.

S: Snoring (louder than talking or loud enough to be heard through closed doors)
T: Feeling tired, fatigued, or sleepy during daytime
O: Observed apnoeas during sleep
P: Hypertension
B: BMI more than 35 kg/m2
A: Age 50-years of age or greater
N: Neck circumference (male 17 inches / 43cm or greater and female 16 inches / 41 or greater)
G: Gender: Male

Our patient has a score of 5 ( O, P, A, N, G)

The score helps clinicians stratify patients for unrecognized OSA and target appropriate clinical management. It can also help triage patients for further investigation. A STOP-BANG score of 5-8 will identify patients with high probability of moderate to severe OSA in the surgical population.

Arrhythmias and/or hypotension are the most common causes of death from tricyclic antidepressant (TCA) overdose.

The quinidine-like actions of tricyclic antidepressants on cardiac tissues are primarily responsible for their toxicity. Conduction through the His-Purkinje system and the myocardium slows as phase 0 depolarisation of the action potential slows. QRS prolongation and atrioventricular block are caused by slowed impulse conduction, which also contributes to ventricular arrhythmias and hypotension.

Arrhythmias can also be caused by abnormal repolarization, impaired automaticity, cholinergic blockade, and inhibition of neuronal catecholamine uptake, among other things.

Acidaemia, hypotension, and hyperthermia can all exacerbate toxicity.

The anticholinergic effects of tricyclic antidepressants, as well as the blockade of neuronal catecholamine reuptake, cause sinus tachycardia. Sinus tachycardia is usually well tolerated and does not require treatment. It can be difficult to tell the difference between sinus tachycardia and ventricular tachycardia with QRS prolongation.

A QRS duration of more than 100 milliseconds indicates a higher risk of arrhythmia and should be treated with systemic sodium bicarbonate.

The tricyclic is dissociated from myocardial sodium channels by serum alkalinization, and the extracellular sodium load improves sodium channel function.

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.

Damping is the reduction of energy in a system achieved by reducing the amplitude of oscillations. It is necessary to some degree to prevent wave overshoots.

Critical damping usually causes the system to be slow, so optimal damping is normally applied to provide a balance between speed and distortion.

Damping can cause errors if excessive (overdamping) or inadequate (Underdamping). The amount of damping in a system can be determined using the damping coefficient (D), where:

Undamped: 0
Critically damped: 1
Optimally damped: 0.64

An overdamped system will cause an artificial decrease in the systolic blood pressure, and an artificial increase in the diastolic blood pressure.

An underdamped system will cause an artificial increase in systolic blood pressure and an artificial decrease in diastolic blood pressure.

Damping can be caused by a number of factors affecting different parts of the system, including:

The tubing/cannula: The presence of air bubbles, increased blood viscosity or formation of blood clots.
The diaphragm/tubing: Increased malleability/compliance
The tubing: Presence of kinks, narrowing or too many ports of injection.

The answer here is a damped system will have a low systolic pressure, a high diastolic pressure with a normal mean pressure.