•  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