Anesthesia A Comprehensive Review PDF

Preview

Summary

ANESTHESIA EQUIPMENT AND PHYSICS:Standard monitors: oxygenation, ventilation, circulation (BP/HR), temperature.Pacemakers: have a 3-5 letter code: “PSR”= PACER 1. First letter: identifies chambers “Paced” a. V= ventricle; A=atria; D=dual 2. Second letter: identifies chamber where endogenous current ...

Description

ANESTHESIA EQUIPMENT AND PHYSICS: Standard monitors: oxygenation, ventilation, circulation (BP/HR), temperature. Pacemakers: have a 3-5 letter code: “PSR”= PACER 1. First letter: identifies chambers “Paced” a. V= ventricle; A=atria; D=dual 2. Second letter: identifies chamber where endogenous current is “Sensed” a. A, V, D, O=none sensed 3. Third letter: describes “Response” to sensing: a. O=none; I=inhibited; T=triggered; D= dual (I+T) 4. Fourth letter: describes programmability or “rate modulation” a. O=none; R=rate modulation (faster heart rate with exercise) 5. Fifth letter= describes “multisite pacing” a. A, V, D, O OR contamination by volatile anesthetic: greatest source is around face-mask. Volatile Anesthetic Uptake: The amount of volatile anesthetic taken up by the patient in the first minute, is equal to that amount taken up between the squares of any two consecutive minutes. 1. So if 50ml was taken up in the first minute, then between the 16th (4x4) and 25th (5x5) minutes, 50ml would be taken up as well! 2. Also 1ml of anesthetic liquid= 200ml anesthetic vapor Vaporization of a liquid: requires the transfer of heat from the objects in contact with liquid (metal cylinder, surrounding atmosphere). So at high gas flows, atmospheric water will condense as frost on the outside of the compressed gas cylinder. Rotameters: tapered vertical tubes, smallest in diameter at bottom (Thorpe tube). Gas enters at the bottom and elevates a bobbin or float, which comes to rest when gravity on the float is balanced by the fall in pressure across the float. The rate of gas flow through the tube depends on: 1. Pressure drop along the length of the tube 2. Resistance to gas flow 3. Physical properties of the gas (density and viscosity). 4. Rotameters cannot be used interchangeably between different gases. Helium: lower density, and the critical velocity is greater than that for nitrogen, so work of breathing is less! Nasal cannula: The FiO2 delivered is approximately 0.04 for each L/min increase in O2 flow, up to a maximum of about 0.45 FiO2 (oxygen flow rate of 6L/min). Increase in respiratory rate and larger tidal volumes will lower the FIo2. (NOSE=4 letters=.04) In a closed scavenging system: the reservoir bag expands during expiration, and contracts during inspiration. During inspiration, the pressure relief valve closes and direct gas to the patients breathing circuit. If this valve is incompetent (open), there will be a direct communication between the patients breathing circuit and the scavenging system, and would

result in delivery of part of the tidal volume to the scavenging circuit. Adequate positive pressure may not be achieved and hypoventilation may result. This would also cause the bag to inflate during inspiratory phase. Nail polish: because blue nail polish has a peak absorbance similar to that of adult deoxygenated hemoglobin (near 940mm), it has the greatest effect on decrease in the % saturation reading of pulse oximeter. Methylene blue: has greatest effect on SaO2 (65-85% sat!). Indigo carmine also has effect. METHYLENE BLUE AND BLUE NAIL POLISH! Microshock: refers to eclectic shock directly to or near the heart Macroshock: passages of current from one part of the body to another. Microshock: occurs when at least 50-100 microampere is directly inadvertantly applied to the heart with resultant ventricular fibrillation! Classically delivered through pacing wires, but can also occur through monitoring equipment with a short circuit (CVP/PA catheter lines, pacemaker). EKG leads have a very LOW risk of supporting a micro-shock. Ventricular fibrillation: the minimum microshock to elicit V-fib is **** 50-100mA *** Line isolation monitor: alarms when grounding occurs in the OR, or when the maximum current that a short circuit could cause exceeds **2-5mA**. It is purely a monitor and does not interrupt electrical current, so it will NOT prevent microshock or macroshock. Line isolation monitor (LIM): monitor loss of current of isolated electrical devices (devices using isolation transformers instead of grounding). When isolated electrical devices develop a single short circuit, the LIM will alarm! This is still ok, it is not a shock hazard yet! BUT, if one of the devices develops a SECOND short, it becomes a shock hazard!! So if LIM alarms, it is reasonable to unplug the last device that was plugged in and have it checked for a short as well! Grounding pads: to avoid burns with electro-cautery, pads are used so that current passes through electrode, through tissue and out the grounding pad. Pads should have a large surface area, placed away from heart, and have very high frequencies. Without a grounding pad, it is possible that the current can leave through the EKG lead metal discs and cause burns!! Also EKG leads can cause burns when under MRI (copper and graphite EKG cables will prevent this!) Esophageal detector device (EDD): a bulb that is first compressed and then attached to the ET tube after the tube is inserted into the patient. The pressure generated is about 40cmH2O; so if the ETT is accidentally placed into the esophagus, the negative pressure will collapse the esophagus, and the bulb will not inflate. If the ETT is in the trachea, then the air from the lung will enable the bulb to inflate. Misleading results have bee noted in morbidly obese, pregnancy, status asthmaticus, and with copious secretions. Capnograph: the difference between the end tidal Co2 (lower) and the PaCO2 (higher) is about 5-10mmHg, and is due to alveolar dead space ventilation. Anything that increases dead space will increase this difference (example: Pulmonary Embolism) Capnograph: if inspiratory baseline is greater than zero, there is rebreathing. Differential includes: incompetent expiratory valve, exhausted CO2 absorbent, or gas channeling through

CO2 absorbent. Though it may also be elevated with an incompetent inspiratory valve (which would show a slanted downstroke). Expiratory upstroke: can become slanted and less steep during partial airway obstruction (kinked ET tube, COPD, bronchospasm) and will also cause prolonged plateau. Inspiratory downstroke: slanted and less steep with an inspiratory valve incompetency, slow ventilation, and partial rebreathing of CO2 Rebreathing CO2: will show an increasing ETCO2, and capnogram will show increasing CO2 levels that do not return to baseline. This can be due to channeling, malfunction of either inspiratory or expiratory unidirectional valves. A small inspiratory limb leak would result in only minimal lost tidal volume and an ability of inspiratory flow to continue to expiratory limb with NO rebreathing of CO2. Double canister absorber in series leads to a better removal of CO2 and less resistance. Also, at high gas flow rates (>5L/min), CO2 reabsorption is NOT an issue! Total dead space: includes gas in airway/advanced airway that does not participate in gas exchange at the level of the alveoli. This includes gas in airways, ETT, and the Y-piece. Proximal to the Y piece, fresh gas flow is continually cycled and is NOT part of the volume rebreathed. Therefore, increasing the length of the circle system TUBING, does NOT increase dead space!! Vaporizer: if it is tipped, part of the anesthetic liquid may get into the bypass chamber, and it is recommended to flush the vaporizer at high flows with the vaporize set at a low concentration until output shows no excessive agent (usually about 30 minutes). VAPORIZER OUTPUT= FxS/B-S (Gas flow ml/min x Saturated VP mmhg) / (Barometric pressure – SVP) Gas density is directly proportional to atmospheric pressure: As altitude increases, atmospheric pressure decreases, and density of gas decreases. At low gas flow rates, flow is laminar and atmospheric pressure has little effect on function of rotameters because laminar flow is not influenced by density remember! Only length, viscocity! At high gas flows, turbulent pattern is influenced by density and so at high altitudes, gas flow through rotameter would be greater than expected at high flows, due to decreased resistance! High gas flow/turbulent: resistance= density/pie^5 Low gas flow/laminar: resistance= 8 x length x viscocity/ pie-r^4 Increase in altitude decrease Barometric pressure decrease density  decrease resistance to turbulent flow  increase gas flow  increase vaporizer output (but only at very high gas flows, when flow is turbulent!!) Higher altitude Lower Barometric pressure  decrease PAO2 (alveolar gas equation) Administering anesthetic at higher atmospheric pressure (lower altitude): increases barometric pressure  decrease vaporizer output but also decreases uptake and accelerates rate of rise of FA/FI  speeds induction. “just think of more pressure forcing the volatile gases into the lungs/brain”

The higher the vapor pressure, the higher the concentration of deliver anesthetic. Since halothane has a higher vapor pressure than sevoflurane, if the sevo vaporizer was filled with halothane, there would be a higher than expected vaporizer output! Splitting ratio of gases: depends on temperature, vapor pressure, and anesthetic agent Increase temperature  increase vapor pressure  increase vaporizer output Halothane and isoflurane: 1:47 (vapor pressure 240) Sevoflurane and enflurane: 1:27 (vapor pressure 160) 1. So if sevoflurane was placed in an isoflurane vaporizer, then (27/47)=.6. a. if 1% isoflurane is set, then only .6% of sevo would actually be delivered! Since the vapor pressure is lower, there is less vaporizer output! Nitrous oxide: when NO enters the vaporizing chamber, a portion of the NO dissolves in the liquid agent, so the vaporizer output transiently decreases. When NO is withdrawn as a carrier gas, the NO that has dissolved in the volatile solution begins to come out of the solution, and transiently increases the vaporizer gas output. Ultrasound: used in regional anesthesia in frequency of 2.5-10MHz. 1. Wavelength is inversely proportional to frequency. wavelength= velocity/frequency 2. As you increase the frequency, you get higher resolution (smaller number) and lower penetration (smaller number) Peak airway pressure: 1. Increasing inspiratory flow rate: would produce higher peak pressures. 2. Changing I:E ratio: from 1:3 to 1:2 would allow more time for the tidal volume to be administered, and would result in lower peak airway pressures because it can be delivered slowly! Central hospital oxygen supply: pressure and volume delivered is 50psi (required for oxygen flow-meter to run) and 50L/min (for oxygen flush in case of emergency. This is all part of the INTERMEDIATE PRESSURE CIRCUIT. Anesthesia machines: 1. High pressure circuit: from oxygen cylinder to the oxygen pressure regulator (first stage) decreases O2 pressure from 2000psi to 45psi. 2. Intermediate pressure circuit: consistent of pipeline pressure of about 50-55 psi and goes to second stage regulator that lowers pressure to 14-16psi. a. Includes oxygen flush valve which bypasses the low pressure circuit. 3. Low pressure circuit: consists of the flow tubes, vaporizer manifold, vaporizers, and vaporizer check valve to common gas outlet. 4. First stage pressure regulators: reduce these pressures to 45psi 5. Second stage regulators: reduce the pressure to 14-16psi. EKG LEAD PLACEMENT: V1: 4th intercostal space to right of sternum V2: 4th intercostal space to left of sternum V3: between V2 and V4 V4: 5th intercostal space, mid-clavicular line V5: 5th intercostal space, anterior axillary line V6: 5th intercostal space, mid-axillary line

Blood pressure cuff: the width should be approximately 40% of circumference of the patients arm. 1. If the width is too narrow: BP will be falsely elevated! 2. If cuff is too large: BP will be falsely Low! 3. If BP cuff is too loose: BP will be falsely elevated! (it has to squeeze more!) 4. Frequent BP measurement can result in edema of extremity distal to cuff. Oversized BP cuff: defined as one with a width that is more than 50% the circumference of the arm. Less pressure is needed to occlude the systolic pulse, so BP will be underestimated (falsely low). BP: every 20cm above/below heart, BP changes about 15mmHG (lower readings above heart, higher readings below heart). Leads II, V5: is great combination, this is what we use in our OR! Leads II, V4, V5: even better combination!! V4 and V5: most sensitive to indicated left ventricular ischemia (left coronary distribution). II: allows highest likelihood to examine p-wave (rhythm analysis) and may also pick up right coronary distribution ischemia. Rhythm: I…I Two lead EKG analysis (II, V5): left bundle branch block (block of both LAF and LPF) makes detection of myocardial ischemia almost impossible with two-lead analysis! (right bundle branch block, and uni-Left fascicular block are not as confounding). Invasive Arterial BP monitoring: 1. Underdamped signal: exaggerated readings are noted (widened pulse pressure). 2. Overdamped signal: readings are diminished (narrow pulse pressure).

3. *******MAP BP is accurate in both.****** Waveforms are exaggerated/underdamped more distally (radial vs aorta), with increased systolic pressures and pulse pressures. The MAP decreases as wave travels distally (very, very minimally!) otherwise we would get backwards flow! Vasodilation (either from vasodilating drips, volatile anesthetics, decreased sympathetic tone) will give an increased exaggeration of the systolic pressure relative to the MAP in more distal arterial sites (but not much of an increase in difference in MAPs between proximal and distal!). MAP (area under the curve) will obviously decrease at both sites. Natural frequency and dampening coefficient: are intrinsic properties of the monitoring system. They determine the fidelity (the truthfulness) of the resultant waveform. Increasing the natural frequency will increase fidelity (more accurate waveforms). Decreasing natural frequency will decrease fidelity (flattened/dampened waveforms). Things that will decrease the natural frequency are: 1. increasing the length of tubing (extension tubing!) 2. multiple connections/stop cocks (on the extension tubing!) 3. loose connections (stop cock too loose that’s what happened to us  flat line!) 4. increasing compliance of tubing (this is why tubing is so firm!) 5. clots, kinks, air bubbles (so  flush the a-line! And make sure its not kinked!!!) a. These will all decrease natural frequency, decrease fidelity  flat/overdamped waveforms! b. Zeroing the system is unrelated to dampening.

Dampening coefficient is a property of the fluid within the tubing which extinguishes motion (and therefore dampens the system). Dampened waveforms: are not reliable indicators of BP, especially systolic BP! They will have fewer reverberations following high pressure flush. Underdamped waveforms: will have multiple reverberations following high pressure flush, with exaggerated (whipped) systolic waveform. On-pump CABG: you absolutely need an invasive arterial line, because BP is undeterminable when patient is on pump! Arterial thrombosis following arterial lines: can reduce risk by using larger gauged catheters (smaller needles!), and leaving catheter in for shorter periods of time! Factors that will increase chance of thrombosis include using propylene catheters, high dose vasopressors, artery size, number of puncture attempts, female gender. Color-Coded Gases: Helium= brown Oxygen= green Air= yellow Nitrogen= black Nitrous= blue Carbon dioxide= Gray Universal negative pressure leak test: a bulb is attached to common gas outlet and compressed until fully collapsed. If it re-inflates, there is a leak! Check valves: permit only unidirectional flow of gases. Prevent retrograde flow of gases, or transfer of gases between cylinders. APL valve: adjustable pressure limiting valve= pop off valve Fail-safe valve: a pressure-sensor shutoff valve that prevents administration of hypoxic gases. So will discontinue flow of nitrous if oxygen pressure falls below 25psi. Prevention of hypoxic gas mixture: 1. DISS (wall to machine) PISS (cylinder to machine) 2. Oxygen supply failure alarm (alarms if pressure falls too low) 3. Fail-safe valve (prevents nitrous from entering circuit if oxygen pressure falls below 25psi) 4. Oxygen flowmeter most downstream (in case of leak) 5. Flowmeter proportioning system (ensures flow of nitrous cannot be so great as to cause a hypoxic mixture) 6. Oxygen monitor/alarm (as oxygen leaves common gas outlet to ensure hypoxic mixture is not present) Oxygen monitors: no specific monitor has shown improved outcomes over another 1. Clark electrode oxygen analyzer: two electrodes in a gel, and when oxygen tension rises, more current is generated. The amount of current correlates to a concentration of oxygen which is displayed on your screen. Eventually becomes consumed, need to be frequently calibrated, and should be checked often.

2. Paramagnetic device: has a faster response time, is self calibrating, has no consumable parts, and is more expensive. Boyles Law: temp, oxygen, pressure Oxygen: 625L, 2000psi Air: 625L, 1800psi Nitrous: 1590L, 750psi CO2: 1590L, 838psi NIOSH limits for Nitrous: is 25ppm in OR; and 50ppm in dental office (no volatile) NIOSH limits for volatile anesthetic: 2ppm NIOSH limits for volatile anesthetic contamination with Nitrous is 0.5ppm Mass spectrometer: functions by separating the components of a streak of charged particles into a spectrum based on mass:charge ratio. Then expressed as the fractional component of the original gas mixture. Helium is not detected on mass spectrometers: so when anesthetic gases are administered with helium 50%, all readings will be about twice their real value (because it counts helium as the anesthetic gas too!) Laryngospasm: positive pressure ventilation with 100% O2, and forward displacement of mandible. Then try SUX. Pharyngitis: more common in females (thinner mucosal covering of posterior vocal cords). Resolves in 2-3 days without treatment. “Faryngitis= Females” Volatile anesthetics: have NO bacteriocidal OR bacteriostatic effects. Low concentrations may inhibit viral replication. KILLING BACTERIA: 1. ***Shifts in humidity and temperature: in anesthesia breathing circuit and scavenging circuits are most important factors for killing bacteria.*** 2. High 02 concentrations and metallic ions also have lethal effect on bacteria. 3. Acid fast bacilli are most resistant (still there has never been transmission of TB from one patient to another due to anesthesia machine!) 4. Sodium hypochlorite (bleach) destroys HIV virus and is used to clean laryngoscopic blades of HIV pt. Nitrous: has a critical temperature that is above room temperature (temperature at which gas can no longer be converted to a liquid, regardless of pressure applied). So some of the nitrous still actually exists as a liquid at room temperature! So inside a nitrous tank is a mixture of gas and liquid, and the relative contents of each depend on the temperature! As gas leaves the tank, the liquid in the tank will vaporize to maintain the relative proportion of vapor to liquid in the tank (since this ratio doesn’t change, the pressure remains the same, until the last ¼ tank remains!!) This process requires heat, so the tank will cool. Anyway, the contents of nitrous oxide can be determined by weighing the tank and subtracting the empty-weight of the tank (tare weight, which is labeled on the tank). Compliance= Volume/Pressure “CVP” (volume y axis, pressure x axis)

Example: pulmonary circuit on a ventilator has a compliance of 6ml gas/cm h20 and a 500cc tidal volume, with a mean pressure of 30cc h20. What is actual tidal volume that reaches the ETT: 6ml= V/30  Volume= 180cc that is lost to circuit. So, 500cc tidal volume minus 180cc lost to circuit = 320cc actually reaches ETT Digoxin toxicity: numerous EKG changes, including the characteristic “Salvador Dali” mustache sign, PVC, AV nodal block, among others. The morphology of the QRS complex / ST segment is variously described as either “slurred”, “sagging” or “scooped” and resembling either a “reverse tick”, “hockey stick” or (my personal favourite) “Salvador Dali’s moustache”!

Central Venous pressure: Really does not correlate well to LVEDV! It is really just a monitor of right heart function. Pulmonary wedge pressure (LVEDP...