IDEAL POSITION OF TIP OF RIGHT IJV CATHETER

• Different methods have been suggested in the literature
• Catheter tips positioned approximately 3 cm below the right tracheobronchial angle will lie in close proximity to the atriocaval junction but will remain extracardiac in location
• Pere P.W. studied correlation between the length of catheter inserted and patient's height and observed that catheters inserted through right IJV from midcervical point or lower puncture to Height/10cm ended in SVC, while those inserted more than Height/10 + 1cm, 47% ended in right atrium
• Some studies argued that the catheter tip should lie above the pericardial reflection to prevent serious and potentially lethal complications like cardiac tamponade, malignant arrhythmias, placement in coronary sinus and tricuspid valve damage. The upper limit of the pericardial reflection cannot be seen on a plain chest X-ray, but it is generally accepted to be below the carina.
• Most clinicians aim to place the catheter tip at the level of the carina with whichever formula they follow. This position carries the risk of migration, thrombosis, and malfunction.
• A recent paper has demonstrated that a catheter with its tip positioned peripheral to the atriocaval junction was more likely to undergo internal repositioning and venous thrombosis.
• The lower placement is preferred for left-sided IJV cannulations, because aiming the catheter tip placement above the pericardial reflection in this scenario is more likely to lead to oblique placement and abutment against the wall, which is a risk factor for perforation. It seems, therefore, that a free-floating catheter tip in a wider portion of the IJV is more important than placing the catheter above the pericardial reflection.
• This becomes even more relevant in the light of the observations by Schuster and colleagues¹³that the pericardium can ascend alongside the medial wall of the SVC by up to 5 cm (mean 3 cm). Thus, catheter tip placement even 3 cm above the SVC–RA junction might not obviate the risk of tamponade in all patients.
• Recently many studies have questioned the placement of the catheter tip in the middle SVC. They argue , emphasizing the importance of having a free-floating tip (not abutting the vessel wall) rather than its placement above the pericardial reflection. This can be achieved by placing the catheter tip in lower SVC and upper RA (target zone, within 2 cm above and 1 cm below the SVC–RA junction). In this position, the SVC is wider, meaning that the catheter tip is likely to float freely with minimal chance of abutment. Furthermore, a catheter placed in this position is amenable to confirmation by TOE.
• Ahn and colleagues have suggested a radiological landmark-based technique for ‘parking’ the catheter within the target zone. They adjudged the depth of CVC insertion by summing [1] the distance between the sternoclavicular joint and the carina (measured offline from the chest radiograph using an internal measuring tool available on the picture archiving and communication system), [2] the distance between the point of insertion and the sternoclavicular joint, and [3] 1.5 cm. The eventual location of the CVC tip was confirmed by TOE. The authors concluded that the catheter tip was positioned more accurately and the determined depth of CVC insertion correlated better with the actual distance from the skin insertion point to the RA–SVC junction with the radiological landmark-based technique compared with Peres’ formula of central venous catheterization via the right IJV
• N.B: Also note that from the right internal jugular and left subclavian approaches, the veins respectively take straight and gently curving trajectories to the superior vena cava. However, the right subclavian vein takes a near-right angle turn into the superior vena cava, and the left internal jugular approach incorporates two turns, one into the brachiocephalic vein and a second into the superior vena cava. These turns create potential for the venous side walls to be punctured by a dilator failing to negotiate a curve appropriately.
  
  
  
• Ref: Quest to determine the ideal position of the central venous catheter tip D. K. Tempe1,* and S. Hasija, British Journal of Anaesthesia 118 (2): 148–50 (2017)
  
  
  
  
  
  
  
  

IMPEDENCE AND RESISTANCE

• Impedance and resistance are terms used to describe the opposition to electrical current flow.
• Resistance is used to describe opposition to flow in DC circuits, whereas impedance is used for AC circuits.
• Unlike direct currents, alternating currents exhibit reactance (capacitive and inductive) because they have frequency and are phase associated.
• In AC circuits, impedance (Z) consists of a real part (ohmic resistance) and an imaginary part (reactance of an inductor or capacitor). It is measured in ohms, and is the total opposition to current flow
• Capacitors placed in DC circuitscreate an increasing resistance to current flow. This is because the charge at the negative plate of the capacitor accumulates, until maximum capacitance is reached and current flow ceases.
• Capacitors in AC circuitsallow current to flow, as the alternating direction of current flow prohibits a significant build up of charge on one of the plates.
• Reactanceis the resistance to AC that a capacitor or inductor exhibits and is inversely proportional to frequency.
• This principle is used in filters to screen out DC currents and low frequency AC.
• Inductors are coils of conducting wire wound around a ferrous or air core.
• An increasing current flowing through an inductor generates a magnetic field around it. This magnetic field in turn creates an electromagnetic force, which opposes the current flow, known as back-emf.This effect is known as inductance, and its SI unit is the henry (H).
• In a circuit where the rate of current change is 1 A/s, an inductance of one henry would generate one volt across the inductor.
• Henry (H) = Voltage (V) × Time (s) / Amperes (A)
• When the power is switched off, collapse of the magnetic field induces flow of electrons in the inductor and circuit, prolonging the flow of current for a short period.
• Inductors placed in DC circuits will initially encounter transient resistance while the magnetic field is established. Once a steady state is reached, the reactance is negligible.
• Conversely, inductors in AC circuits encounter increasing reactance proportional to the frequency. This is because the creation and subsequent reversal of magnetic field development produces a constant back-emf resisting current flow. Therefore, high-frequency AC will cause a high reactance in the inductor.
• Inductors are used to filter out high-frequency alternating currents, or to smooth out the effect of power surges in monitoring equipment
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LASERS IN MEDICINE

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

A laser comprises a laser tube constructed from an active lasing medium that can be a gas, solid or a liquid, with a mirror at each end of the tube

Lasers produce an intense parallel beam of coherent monochromatic light (one specific wavelength of light) by the stimulated emission of photons from excited atoms

Injecting energy from an external source (pumping) causes the lasing medium to become excited. Gas lasers are excited using an electric current applied to either end of the laser tube, while solid state and liquid lasers are excited using a high intensity light source. The mirrors cause photons to bounce back and forth within the laser medium, triggering further emission of photons by the process of stimulated emission. One mirror is partially reflective which allows some photons to escape in the form of the laser beam. The beam is then focused as required

Electrons of atoms within a lasing medium normally reside in a stable low-energy level known as the ground state. Pumping excites electrons, raising them to higher energy levels. Because higher energy states are unstable in comparison to the ground state, there is a tendency for electrons to release excess energy and return to lower energy levels. This process is known as decay.

As an electron decays from the metastable to ground state, it emits a photon of energy. If this photon strikes an excited electron in the metastable state, it incites it to emit another photon, which will have the same wavelength, waveform and direction as the incident photon. They are said to be in phase and coherent.

Red and near-infrared lasers have the deepest penetration.

Carbon dioxide lasers emit infrared light and have limited penetration, but are precise and can be used for cutting and vaporising.

Argon lasers predominantly produce blue–green light at 488 and 514 nm, and are commonly used in ophthalmology.

Neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers have the deepest penetration and can cut and coagulate. They are used to resect gastrointestinal and bronchial tumours, and gynaecological lesions.

LASERs are classified 1–4, 1 being least dangerous.

Most medical lasers are class 3B and class 4. They pose a high risk to staff and patients.

Lasers can ignite flammable material such as endotracheal tubes and surgical drapes. They can also cause airway and body cavity fires in the presence of high concentrations of flammable gases.

The risk of airway fires can be reduced by using the lowest inspired oxygen concentration possible that achieves suitable oxygen saturations. In addition, using laser-safe endotracheal tubes with the cuffs filled with saline and dye helps to further reduce the risk of fires. The water in the cuff acts as a heat sink to reduce the likelihood of perforating and igniting the cuff with the laser. The dye provides a visual indication in the event of cuff perforation.

The Wheatstone bridge

• The Wheatstone bridge is an electrical circuit that uses an arrangement of four resistors to measure an unknown electrical resistance.
• The typical Wheatstone bridge contains a power source, a galvanometer (G), two resistors of known resistance (R1, R2), a variable resistor (R4) and an unknown resistance, which is the one to be measured (R3). The connection across CD containing the galvanometer is known as the bridge.
• This circuit is sensitive to changes in the ratio of resistances across pairs of resistors.
• When the voltages at C and D are equal, the ratios of resistances equal each other (R1/R2 = R3/R4), no current will flow through the galvanometer and the bridge is balanced.
• By altering the resistance of the variable resistor R4 until the ratio of resistance across the limb ADB equals that of ACB, the bridge can be balanced, and no current flows across CD. By knowing the resistance required at R4 to
  balance the bridge, R3 can be calculated by using the equation R1/R2 = R3/R4
• A strain gauge is either a foil arrangement or a conductive metallic strip. In the arterial transducer, strain gauges are mounted on a diaphragm.
• The arterial pulsation is transmitted via a continuous column of fluid to the diaphragm, which causes it to stretch. The attached strain gauge will also stretch and its resistance increases. Conversely, when the diaphragm relaxes the resistance in the strain gauge falls. R3 is the strain gauge attached to the diaphragm, and the variable resistor R4 has been adjusted to match the resistance of R3 in the resting position, so that the bridge is balanced. Movement of the diaphragm would alter the resistance of R3, which unbalances the bridge and results a potential difference across CD. The resulting potential difference is quite small, so it is common to use a differential amplifier in place of the galvanometer to increase the sensitivity of the circuit in detecting the signal.

METHODS TO MEASURE THE CONCENTRATION OF A GAS

• The Rayleigh refractometer utilises the refractive index of a gas to calculate its concentration.
• Thermal conductivity is used in katharometers. In these devices, the cooling of a wire causes a change in resistance proportional to gas concentration.
• Solubility is employed in devices such as rubber strips when an increase in length accompanies gas absorption.
• Light emission features in the Raman light scattering measurement device.
• Both infrared and ultraviolet absorption are used in gas concentration measurement.

ECG EMG EEG ; THE BIOLOGICAL SIGNALS

• The movement of ions across cell membranes during the depolarisation and repolarisation of myocytes and neurones generates electric potentials.
• Silver metal electrodes covered with a layer of silver chloride gel within an adhesive sponge pad can be used to measure these potentials at the skin.
• Ion movement near the electrode–skin interface induces movement of chloride ions within the gel layer. The ion concentration gradient promotes electron production at the electrode.
• A lead wire and voltmeter attached to the electrode allows measurement of the potential relative to a reference point. The reference point is usually a second skin electrode.
• Signals are then amplified, processed and displayed.
• Skeletal and cardiac muscles have higher amplitudes than cerebral neurones.This is because the amplitude of biological potentials is proportional to the number of simultaneously depolarising cells.
• The frequency of potentials is related to the fluctuating ion activity across cell membranes.
• Skeletal myocytes which undergo tetany have high frequencies of
  up to 1 kHz.
• Conversely, cardiac myocytes have lower frequencies due to their refractory periods 

HUMIDITY AND ANESTHESIA

• Absolute humidity is the mass of water vapour present in a given volume of gas at
  defined temperature and pressure (expressed as g of H2O/m3 of gas).
• Relative humidity is the mass of water vapour present in a given volume of gas,
  expressed as a percentage of the mass of water vapour required to saturate the same volume of gas at identical temperature and pressure.
• The amount of water vapour required to saturate a known volume of gas increases with temperature, i.e. a gas saturated at 20°C contains less water than the same volume, saturated at 37°C
• Relative humidity (RH), can be calculated from the ratio of the mass of water vapour present (mP), to the mass required for satruation (mS) as
  RH = mP/mS
• If droplets are present, supersaturation has occurred and relative humidity exceeds 100%.
• From the gas laws, mass of a gas in a mixture is proportional to the partial pressure it exerts, thus: RH = water vapour pressure/ saturated water vapour pressure
• Instruments used to measure humidity are called hygrometers. Examples include:
  Regnault’s hygrometer
  Hair hygrometer
  Wet and dry bulb thermometers
  Humidity transducers

PRESSURE AND ITS MEASUREMENT BY MANOMETER

💎Pressure = Force/Area
💎SI unit is Pascal
💎1Pa = 1N/m-2
💎It is the simplest method of pressure measurement
💎lt does not need calibration
💎So it can be used to calibrate other devices
💎The pressure is balanced against a column of liquid of known density - usually water for low pressures and mercury for higher pressures
💎The pressure is equal to the depth multiplied by the liquid density multiplied by the acceleration due to gravity; hence the commonly used units cm of H2O and mm of Hg
💎Mercury is 13 times more dense than water
💎The vertical height gives the pressure value

NEBULIZERS

• Aerosols are small particles of liquids or solids suspended in a carrying gas
• Medical aerosols can be produced by a nebulizer
• The therapeutic efficacy of the aerosol is dependent on the liquid or solid’s ability to remain in suspension and the depth reached by the aerosol on inhalation, and is dependent on its stability. These are both determined by the particle size.
• For liquid medication to enter the alveolithe droplets must be smaller than the diameter of the terminal bronchioles and fall within the size range of 0.005 µm to 50 µm in diameter.
• For droplet sizes below 5 µm, gravity exerts a negligible effect.
• Particles or droplets in the range 5 to 10 µm tend to deposit in the upper airways,with material below 5 µm penetrating further into the lungs.
• Below 3 µm, the droplets enter the alveoli and become therapeutically beneficial.
• Droplets below 1 µm are ideal; but if significantly smaller than this, the particles will be exhaled without having a therapeutic effect.
• The temperature for an aerosol generated by a nebulizer must not exceed 37°C and the process must not alter the structure of the medication being carried.
• This is the essential difference between vaporizers that generate a vapour and nebulizers that produce liquid droplets.
• Jet or gas driven nebulizer (atomizers)
• A high flow of gas is driven over a capillary tube that is immersed into the fluid to be nebulized. The high pressure air driven through the small orifice, generates negative pressure as a result of the Venturi effect. These nebulizers are simple and low cost, but small variations in gas flow rate can result in inconsistent delivery of aerosol to the patient.
• Ultrasound driven nebulizer
• The ultrasound nebulizer incorporates a ceramic piezoelectric transducer that changes electrical energy into mechanical energy (pressure oscillations). The transducer sits at the bottom of the chamber and vibrates at a frequency of 1.5 MHz. The vibrations are transmitted through the water. The diaphragm is in contact with the solution to be nebulized and violently shakes the solution into particles. At low frequencies, larger particles are produced, but at higher frequencies, a fine mist is generated
• Ultrasonic nebulizers tend to produce a more consistent particle size than jet nebulizers and, as a result, produce a much greater deposition into the lungs.
• But long-term use of ultrasonic nebulization might inadvertently affect surface tension stability in the alveoli

ANTIPSEUDOMONAL AGENTS

A 70 year-old female is intubated 5 days after hospital admission for hypoxemic respiratory failure after a witnessed aspiration event. Prior to admission, the patient lived in a nursing home, and recently was treated for left leg cellulitis with a short course of intravenous antibiotics. Her medications include metoprolol, metformin, glyburide, atorvastatin, and baby aspirin. Three days after intubation, the patient is noted to have a temperature of 102.5 °F, a blood pressure of 70/50 mmHg, a white blood cell count of 20.0 × 109/L, with purulent secretions suctioned from the endotracheal tube. You decide to initiate antibiotic therapy. Which of the following is the best antibiotic regimen to initiate at this time?

A. Ceftriaxone and ertapenem

B. Imipenem, levofloxacin and vancomycin

C. Meropenem, cefepime, and piperacillin-tazobactam

D. Cefepime and daptomycin

E. Ceftriaxone and azithromycin

Answer: Yes its B!

Healthcare associated infections are almost routine in today’s critical care units, and the increasing rates of multi-drug resistant (MDR) organisms is taking a toll on our clinical and economic systems. Ventilator associated pneumonia (VAP) is a subtype of healthcare associated infection, and is defined by the diagnosis of clinical pneumonia 48–72 h after intubation. Duration of mechanical ventilation, antibiotic use history, geography, co-morbidities, and the epidemiology of the ICU population all determine the etiology of a nosocomial pneumonia. Aerobic gram negative bacilli are the most common pathogens causing VAP. These include Klebsiella, Escherichia coli, Pseudomonas, Acinetobacter, Stenotrophomonas, Enterobacter, Citrobacter, Proteus, and Serratia species. Pseudomonas is the most prevalent pathogen recovered in VAP. With the emergence of MDR organisms, Methicillin resistant Staphylococcus aureus (MRSA) is also an important etiology of VAP, as well as anaerobes such as Bacteroides species. Community acquired pathogens, including Streptococcus and Haemophilus species are less likely to cause VAP. The antibiotic regimen that should be initiated depends on the suspicion that a patient harbors MDR pathogens. Usually, if a patient is hospitalized for more than 5 days, the possibility of MDR pathogens is high, particularly if a patient has been on intravenous antibiotic therapy recently. The first line treatment would include an antipseudomonal cephalosporin or an antipseudomonal carbapenem or an antipseudomonal penicillin with Beta lactamase inhibitor, plus an antipseudomonal fluoroquinolone or aminoglycoside, plus an anti-MRSA agent . Azithromycin should be considered for atypical coverage if Legionella is high on the differential and in severely ill patients. If an MDR pathogen is not suspected, a third-generation cephalosporin or respiratory fluoroquinolone or non-antipseudomonal carbapenem should be considered. Daptomycin is not appropriate to use for pulmonary infections, as it is inactivated by surfactant.

See the pictures for examples of these drug categories