Hypoxia and Equipment Failure

1. Hypoxia and Equipment Failure April 25, 2011

2. Case Stem: • A 70 year old man is to undergo cystoscopy and transurethral resection of a bladder tumor under general anesthesia through an LMA. He gave a history of mild asthma and used an albuterol inhaler when necessary. Breathing room air (FiO2 = 0.21), his pulse oximeter saturation reading (SpO2) was 94%.

3. What is Hypoxia and Hypoxemia?

4. Hypoxia • Reduction of oxygen supply to tissue below physiologic levels. • Decreased oxygen tension (PO2) inside the body at tissue level or outside the body (hypoxic gas mixture)

5. Hypoxemia • Deficient oxygenation of blood. • Decreased oxygen tension in the arterial blood (PaO2)

6. Is PaO2 related to age?

7. Yes. • Age-dependent decrease in PaO2. • Marshall and Whyche equation • Mean PaO2 (mmHg) = 102-0.33(age in years) Sorbini et al. found PaO2 decreased from about 95 mmHg at 20 years of age to 73 at 75 years (about 4-5 mmHg per decade)

8. Was this patient hypoxemic?

9. No. • Hypoxemia is considered to exist when the PaO2 is less than 60 mmHg which is equivalent to a hemoglobin O2 saturation of 90% • Using the Marshall Whyche equation • 102-0.33(70) = 79 mmHg

10. What is a pulse oximeter and what is a hemoximeter?

11. Pulse Oximeter • Noninvasive device that provides an estimate (SpO2) of the arterial hemoglobin saturation with oxygen. • Uses patient body part as in vivo cuvette through which 2 different wavelengths of light are transmitted.

12. Hemoximeter • Used to analyze an arterial blood sample. • Laboratory cooximeter that uses six or more different wavelengths of light to measure total hemoglobin, oxygenated hemoglobin, deoxygenated hemoglobin, methemoglobin, carboxyhemoglobin and other aberrations.

13. How does a pulse oximeter work?

14. Pulse oximeter • Light-emitting diodes transmit red light at wavelengths 660 nm and infrared light at 960 nm through the probe site. • Light is sensed by a single photodetector • Ratio of absorbances (660/990 nm) is related to hemoglobin O2 saturation (Spectophotometry) • Plethysmography – detection of pulsatile flow (as blood pulses, absorbance increases)

15. What affects the accuracy of a two-wavelength pulse ox?

16. Pulse oximeter • Patient movement (shivering, peripheral nerve stimulation, “twitching”) • Presence of intense ambient light • Electrocautery use • Administration of IV dyes with absorbance peaks at 66o nm (methylene blue) • Dyshemoglobinemias • Nail polish • Poor pulsatile flow at probe site (hypotension, Raynaud’s) • Venous pulsations (tricuspid regurg)

17. How do methemoglobin and carboxyhemoglobin affect SpO2 readings?

18. Methemoglobin • Iron in heme moiety is oxidized (dapsone, benzocaine, nitric oxide, prilocaine) to Fe3+ state rather than Fe2+ state. • Cannot carry O2 • Shows similar absorbances at 660 and 940 nm (SpO2 tends toward 85%) • Overestimates the fractional saturation and underestimates the functional saturation

19. Carboxyhemoglobin • CO + Hb has similar absorbance to HbO2 at 660 nm, but very low absorbance at 94o nm. • SpO2 overestimates fractional saturation and underestimates functional saturation. • SpO2 will appear in the 90s • Hemoximeter required to determine true O2 sat

20. What is a capnometer and capnography?

21. Capnography • Most use infrared spectroscopy to measure PCO2 • A built in barometer measures barometric pressure so that CO2 can be displayed as a percentage. • “Gold standard” for establishing presence of ventilation.

22. What is meant by end-tidal CO2 concentration?

23. End-tidal CO2 • Tension of CO2 in the exhaled gas at end of exhalation. • Represents the CO2 tension in the alveolar gas (PACO2) • Does not account for dead space ventilation • Presence of CO2 depends on • Production of CO2 by the tissues • CO and pulmonary blood flow to carry CO2 • Ventilation

24. Capnogram • Phase I – Expiratory baseline • Phase II – Expiratory upstroke • Phase III – Expiratory plateau • Horizontal in healthy lungs • Upward Slope with obstructive airway disease • Maximum expired CO2 is considered the end-tidal • α angle – slope between II and III (increase in acute bronchospasm) Phase IV – Inspiratorydownstroke

25. Capnograms Aberrations

26. Elevated baseline CO2 • Capnometer not properly calibrated to zero • Delivery of CO2 to breathing system through fresh gas inflow • Incompetent unidirectional valves • Failure of CO2 absorber (channeling, exhaustion, bypass)

27. Prolonged expiratory plauteau and expiratory upstroke • Mechanical obstruction to exhalation • COPD • Bronchospasm

28. Dips in expiratory plateau • Spontaneous ventilation efforts

29. Cardiogenic oscillations • Ventilator pressure relief valve pertubations

30. Elevated expiratory plateau • Incorrect calibration • Increased CO2 production / delivery • Laparoscopic CO2 gas insufflation • Decreased CO2 removal • Hypoventilation • Leak

31. Decreased expiratory plateau • Incorrect calibration • Air leak into gas sampling system • Hyperventilation • Decreased CO2 production (hypothermia) • Increased arterial-alveolar CO2 gradient (VQ mismatch / pulmonary embolus)

32. Prolonged inspiratorydownstroke and raised baseline • Incompetent or missing inspiratory unidirectional valve • Inspiratory obstruction to gas flow (kinked tube)

33. What is the A-a difference in CO2?

34. A-a gradient • Measure of alveolar dead space ventilation • 2.5cc / Kg = volume of anatomic dead space • (PaCO2 – PETCO2) / PaCO2 = Ratio of dead space to tidal volum • Alveolar dead space increased by ventilation in excess of perfusion or decrease in perfusion (shunt has minimal effect) • PaCO2-PACO2 – nl 3-5 mmHg

35. Important safety features on anesthesia machine

36. Safety Features • Pin index (cylinder) and diameter index (pipeline) safety systems • “Fail-safe” valve – pressure sensitive device that interrupts flow of all hypoxic gases on the machine to their flow control valves if the supply pressure of O2 in the high pressure system falls below a threshold (between 12-20 psig) • O2 supply failure alarm – pressure below 30 psig • O2 flow control knob – fluted and on the right • Key-fill systems for vaporizers • Pop-off (pressure relief) valve

37. Safety Features • Gas flow proportioning – ensure minimum O2 of 25% when N2O is used • Vaporizer interlock system

38. What are sites for gas leakage?

39. Gas Leakage • Breathing system • Partially deflated tracheal tube cuff • Disconnection of sidestream gas analyzer • Humidifiers • Bag • Low-pressure machine components • Cracked rotameter flow tubes • Incorrectly mounted vaporizers • Vaporizer leak around agent filling device • Fracture in gas piping

40. Machine Check for Leaks • Drager • Circle breathing system tubing removed • Insp and exp limb connected by tubing • Resevoir bag removed and replaced with test terminal with sphygmomanometer bulb • Pressurize with bulb to 50 cm H2O – pressure should not decrease by 20 in 30 sec • Test with vaporizers on • Datex-Ohmeda • One-way outlet check valve at the common gas outlet • Connect bulb and squeeze, should not refill in 30 sec

41. Preop Equipment Check

42. Step 1 – Emergency Ventilation Equipment • Step 2 – Check O2 Cylinder supply • Step 3 – Central pipeline supply • Step 4 – Low-pressure system check (flow control valves and vaporizer status) • Step 5 – Leak check of low-pressure system • Step 6 – Turn on machine master switch and other electrical equipment • Step 7 – Test flowmeters

43. Step 8 – Adjust / Check scavenging system (test pop-off) • Step 9 – Calibrate O2 monitor • Step 10 – Check initial status of breathing system (circuit, CO2 absorbent) • Step 11 – Leak check of breathing system • Step 12 – Test ventilation system (connect resevoir bag to Y-piece) • Step 13 – Check, calibrate, and set alarm limits • Step 14 – Check final status of machine

44. What emergency equipment should be readily available?

45. Emergency Equipment • Back-up ventilation equipment • Emergency airway equipment • Cricothyroid kit / Difficult airway cart • Working flashlight • Backup battery • O2 tank and regulator • Malignant hyperthermia cart • “Code” cart • Fire extinguisher

46. What are goals of premedication?

47. Premedication • Anxiolysis • Minimization of gastric volume and acidity • Antibiotic prophylaxis • Antisialagogue effect

48. Standard ASA Monitors

49. Standard ASA Monitors • Standard I – Qualified anesthesia personnel shall be present in the room throughout the conduct of all general, regional, and monitored anesthetic care. • Standard II – During all anesthetics, the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated.

50. Standard ASA Monitors • Oxygen analyzer with low O2 alarm • Quantitative method of blood oxygenation (pulse ox) • Ventilation evaluation (chest rise, auscultation) • Correct positioning of airway devices • End-tidal CO2 with airway devices • Ventilator disconnection alarm • ECG, BP, HR (every 5 min for the latter 2) • Body temperature (if perturbations are expected)