EQUIPMENT



•  O2 & Air: Pressure gauge reading reflects the volume of gas in the E-cylinder


Boyle’s: O2 gauge = 300 psi → liters O2 left = 660 L × (300 psi/2200 psi) = 90 L


“Fudge” [psi × 0.3]: O2 gauge = 300 psi → liters O2 left = (300 psi × 0.3) = 90 L


•  N2O: If liquid N2O remains, pressure = 750 psi → weigh tank to assess N2O volume


Liters of N2O in tank = (N2O tank weight in grams/44 g) × 0.5 L
Only N2O gas in tank (∼25% N2O left) → pressure falls <750 psi → calc per Boyle’s


ANESTHESIA MACHINE (*SEE APPENDIX B FOR CHECKOUT RECOMMENDATIONS)







BREATHING CIRCUIT: CONNECTS ANESTHESIA MACHINE TO PATIENT


Circle system—most commonly used; prevents rebreathing of exhaled CO2 (see Fig. 3-1)


Reservoir bag:


•  Reserve gas volume


Oxygen analyzer:


•  Measures inspired/expired O2.


Adjustable pressure limiting valve (APL or pop-off valve):


•  Can be adjusted to facilitate manual bag compression to assist ventilation of pt’s lungs.


•  Allow venting of excess gas to waste scavenging system.


Bag/ventilator switch:


•  Exclude/include reservoir bag & APL from system.


Inspiratory one-way valve:


•  Open during inspiration & closed during expiration.


•  Prevents expiratory gas from mixing with fresh gas in inspiratory limb.


Expiratory one-way valve:


•  Open during expiration & closed during inspiration


•  Gas is then either vented through APL valve or passes to CO2 absorber CO2 absorbent: Removes CO2 from breathing circuit (chemical neutralization)


•  Most common absorbent = soda lime (Ca, Na, K-OH, & H2O)


CO2 + H2O → H2CO3
H2CO3 + 2NaOH → Na2CO3 + 2H2O + heat
Na2CO3 + Ca(OH)2 → CaCO3 + 2NaOH


Spirometer:


•  Measures exhaled tidal volume & respiratory rate


Circuit pressure gauge:


•  Measures circuit airway pressure in cm H2O




Figure 3-1. A circle system.




Closed-Circuit Anesthesia


•  Use of FGFs exactly equal to the uptake of oxygen and anesthetic agents


•  Requires (1) very low FGF, (2) total rebreathing of exhaled gases after absorption of carbon dioxide by CO2 absorber, (3) closed APL or ventilator relief valve


•  Advantages: ↑ heat & humidification of gases; less pollution & agent use; cost savings


•  Disadvantages: Inability to rapidly change agent concentration; may provide hypoxic/hypercarbic mix; anesthetic overdose from excessive agent concentration


Airway Pressures


•  Airway pressure = airway resistance + alveolar pressure (i.e., chest & lung compliance)


•  PIP = highest pressure in circuit during inspiration


•  Plateau pressure = pressure during inspiratory pause (only measuring compliance)



Management of Increased Airway Pressures



Open Breathing Systems (Historical, not Typically Used in Modern Medicine)


•  Insufflation: Blowing of anesthetic gas across pt’s face


•  Open-drop anesthesia: Volatile anesthetic dripped onto gauze-covered mask on pt’s face


Mapleson Breathing Circuits A to E: Five Systems Described in 1950s


•  Differ in fresh gas inflow tubing, mask, reservoir bag & tubing, & expiratory valve locations


•  Characterized by (1) no valves directing gases to & from pt, (2) no CO2 neutralization


•  Mapleson A circuit = most efficient for spontAneous ventilation (see Fig. 3-2)


•  (FGF = minute ventilation, which is sufficient to prevent CO2 rebreathing)


•  Mapleson D circuit = most efficient for controlleD ventilation


•  (FGF forces alveolar gas away from pt & toward pressure release valve)


Bain Circuit: Modification of Mapleson D


•  Fresh gas supply runs coaxially inside corrugated expiratory tubing


•  Advantages: Compact, portable, easy scavenging, exhaled gases warm inhaled gases


•  Disadvantages: Risk of kinking/disconnect of coaxial tubing (i.e., fresh gas inlet)


PATIENT MONITORS


Pulse Oximetry


•  Noninvasive, continuous means of assessing arterial O2 saturation


•  Two light-emitting diodes @ 660 nm (absorbed by Hb) & 940 nm (O2Hb), & photoreceptor


•  SaO2 = O2Hb% & Hb% are calculated from ratio of light absorbed at photoreceptor


•  Accuracy is unaffected by fetal hemoglobin, sickle hemoglobin, and polycythemia


Figure 3-2. Mapleson A and D breathing apparatuses.




Capnography


•  Continuous visual display of exhaled CO2 waveform; relies on two assumptions:


• All CO2 is a product of tissue metabolism


• PaCO2 (arterial) is 5–10 mm Hg > PACO2 (alveolar) ≈ ETCO2


•  Provides the following information:


• Adequacy of ventilation & perfusion


• Presence of airway obstruction @ equipment malfunction (based on waveform)


• Positioning of double-lumen tubes (via separate capnometers to each lumen)


•  Causes of abrupt ETCO2 decreases:


• Esophageal intubation


• Kinked, obstructed, or disconnected airway or gas sampling line


• Low cardiac output (e.g., pulmonary embolism, cardiac arrest)


•  Causes of abrupt ETCO2 increases:


• Hypoventilation


• Hyperthermia (including malignant)


• Rebreathing (e.g., incompetent unidirectional valve, exhausted absorbent)



Figure 3-3. Typical capnogram.



ELECTRICAL SAFETY IN THE OR


Electrosurgery


•  Surgical diathermy: High frequency alternating current to cut/cauterize blood vessels


• Electrosurgical units (ESUs) generate high-frequency current; tip of small electrode → through pt → out large electrode (dispersion pad)


•  Malfunction of dispersion pad: Inadequate contact/conducting gel/disconnect


• Current will exit pt through alternate path (ECG pads, OR table) & may burn pt


•  Bipolar electrodes limit current propagation to a few millimeters


•  ESU may interfere with pacemaker & ECG recordings


Risk of Electrocution


•  Contact with two conductive materials at different voltage potentials may complete circuit & result in electric shock


•  Leakage current is present in all electrical equipment


• Fibrillation threshold at skin is 100 milliamps (above leakage current magnitude)


• Current as low as 100 microamps applied directly to heart may be fatal



Ungrounded Power & Protection from Electric Shock


•  Isolation transformer: Isolates OR power supply from ground potential


• If live wire contacts grounded pt, isolation transformer prevents current flow to pt


•  Line isolation monitors (LIMs): Monitors how well power supply is isolated from ground


• Alarm sounds if unacceptable current flow to ground becomes possible


• Alarm does not interrupt power unless ground leakage circuit breaker activated


• Isolated power circuits do not protect against microshock


• Note: New building codes no longer required ORs to have ungrounded power


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Aug 28, 2016 | Posted by in ANESTHESIA | Comments Off on EQUIPMENT

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