1. Chapter 18 Analysis and Monitoring of Gas Exchange

2. Learning Objectives
Describe the difference between monitoring and analysis.
Describe the two types of electrochemical oxygen analyzers.
Describe calibration and problem-solving techniques for oxygen analyzers.
State how to obtain, process, and analyze arterial and capillary blood gas samples.
List the quality control procedures applied to blood gas analysis.

3. Learning Objectives (cont.)
List the potential advantages of point-of-care testing.
Describe how to obtain and interpret transcutaneous oxygen and carbon dioxide monitoring.
Describe the basic principles used by an oximeter to monitor oxygen saturation.
State when and how to perform pulse oximetry.
Identify true statements related to interpretation of pulse oximetry results.
Describe how to perform capnometry and interpret capnograms.

4. Analysis vs. Monitoring
Laboratory analysis: discrete measurements of fluids or tissue that must be removed from body
Such measurements are made with an analyzer
Monitoring is an ongoing process by which clinicians obtain & evaluate dynamic physiological processes in a timely manner, usually at bedside
This is done with monitor

5. Invasive vs. Noninvasive Procedures
Invasive procedures require insertion of sensor or collection device into body
Noninvasive monitoring is means of gathering data externally
Generally, invasive procedures provide more accurate data but carry greater risk
After gradient between invasive & noninvasive method is established, trends in change of noninvasive method can be useful in making clinical decisions

6. Measuring FIO2
Most bedside systems to measure FIO2 use electrochemical principles
2 most common:
Polargraphic (Clark) electrode- needs battery
Galvanic fuel cell- no battery needed
Response times for Clark electrodes = about 10 to 30 seconds
Response times for galvanic fuel cells = 60 seconds

7. Clark Polarographic Analyzer

8. paramagnetic analyzer polarographic analyzer
Working as a new therapist in the ICU you come across an O2 analyzer that uses battery to only power its alarm system. This analyzer should be classified as a:
paramagnetic analyzer
polarographic analyzer
galvanic fuel cell analyzer
zirconum cell analyzer
Answer: C

9. Troubleshooting O2 Analyzers
Calibration according to manufacturer’s recommendation must be done before using device
Failure to calibrate or inconsistent readings are signs of malfunctioning
Best way to avoid problems is through preventative maintenance
If analyzer fails to calibrate, problem could be related to:
Low batteries, sensor depletion, electronic failure

10. condensation of water vapor
All of these may be reasons for a Galvanic fuel cell to malfunction, except:
batteries
sensor depletion
electric failure
condensation of water vapor
Answer: A

11. Sampling & Analyzing Blood Gases
Analyzing arterial blood samples is important part of diagnosing & treating patients with respiratory failure
Radial artery is most often used because:
Near surface & easy to stabilize
Collateral circulation usually exists (confirmed with the Allen test)
No large veins are near
Radial puncture is relatively pain free
Arterial cannulation can be done if frequent sampling is needed

12. Arterial Sampling
Fig 18-2

13. Arterial Sampling (cont.)
Modified Allen’s test:
Done prior to radial artery puncture ONLY
Normal test indicating collateral circulation - hand flush pink within 5-10 seconds
Cannot be performed on critically ill patients who are uncooperative or unconscious

14. Modified Allen Test
Fig 18-3

15. should not perform an ABG on patient.
The RT receives a doctor order to perform an ABG on a 32 year-old patient. Prior to performing the ABG, the RT does a modified Allen’s test on the patient’s right hand that takes more than 10 seconds for patient’s hand to flush pink. What should the RT do next?
should not perform an ABG on patient.
perform the ABG on the right brachial artery.
perform it on the right radial artery.
repeat the modified Allen’s test on the left hand.
Answer: D

16. Arterial Sampling (cont.)
Sample volume of 0.5 to 1 mL of blood is adequate
Actual volume depends on:
Anticoagulant used
Requirements of specific analyzer used
Whether or not other tests will be performed on obtained sample

17. Arterial Sampling (cont.)
Important transmission-based and safety precautions:
Never recap used needle without safety device; never handle using both hands, & never point it toward any part of body
Never bend, break, or remove used needles from syringes by hand
Always dispose of used syringes, needles, & other sharp items in appropriate puncture-resistant sharps containers

18. Indications for Arterial Sampling
Sudden, unexplained dyspnea
Acute shortness of breath/tachypnea
Abnormal breath sounds
Cyanosis
Heavy use of accessory muscles
Changes in ventilator settings
CPR
Diffuse infiltrates in chest radiograph
New infiltrates in CXR
Sudden cardiac arrhythmias
Acute hypotenesion

19. Blood Sample Analysis
Clinicians can avoid most pre-analytical errors by ensuring that sample is:
Obtained anaerobically
Properly anticoagulated
Analyzed within 10 to 30 minutes

20. Interpretation of ABGs
Interpret oxygenation status
PaO2 (see below)
SaO2 (normal = 95100%)
CaO2 (normal = 1820 vol%)
PaO2 6079 mm Hg = mild hypoxemia
PaO2 4059 mm Hg = moderate hypoxemia
PaO2 <40 mm Hg = severe hypoxemia

21. it may increase the PaCO2 of the sample
The RT is called to run an ABG drawn by the unit resident on a patient in a non-rebreather mask. The RT notices visible bubbles at the top of the blood sample. How this may impact the PaO2 and PaCO2 results?
it may increase the PaCO2 of the sample
it only affects the gas pressures after 1 hr of drawn
it may decrease the PaO2 of the sample
air bubbles have no impact on the PaO2/PaCO2 values
Answer: C

22. Indwelling Catheters Provides ready access for blood sampling
Allow continuous monitoring of vascular pressures
Infection & thrombosis are more likely than intermittent punctures
Normal routes are peripheral arteries (radial, brachial, pedal), femoral artery, central vein, & pulmonary artery

23. Indwelling Catheter (cont.)
Access for sampling blood from most intravascular lines is provided by a three-way stopcock
Pulmonary artery catheter has separate blood sampling and IV infusion ports and balloon at tip

24. Brachial Artery Catheter

25. 3-way Stopcock

26. pulmonary gas exchange gas exchange at the tissues
If a indwelling vascular catheter is placed in the brachial artery, what is the sample assessing?
pulmonary gas exchange
gas exchange at the tissues
right ventricular preload
right ventricular afterload
Answer: A

27. Capillary Blood Gases
Good capillary sample can accurately reflect & provide clinically useful estimates of arterial pH & PCO2 levels
Capillary PO2 is of no value in estimating arterial oxygenation. SaO2 must be evaluated by pulse oximetry.
Most common technical errors in capillary sampling are inadequate warming & squeezing of puncture site
Squeezing puncture site may result in venous & lymphatic contamination of sample

28. Analyzing Measures pH, PCO2, & PO2 levels in blood sample
Several secondary values are calculated:
Plasma
Bicarbonate (HCO3-)
Base excess (BE) or deficit
Hemoglobin saturation (HbO2%).
Some may combine blood gas & hemoximetry (total hemoglobin) measurements

29. Instrumentation
Can be based on several different types of sensor technology to measure pH, PCO2, & PO2:
Electrochemical electrodes
Optical fluorescence
Photoluminescence

30. Instrumentation (cont.)
Electrodes:
PaO2: Clark polarographic electrode
PaCO2: Severinghaus electrode
pH: pH electrode actually consists of 2 electrodes or half-cells:
Measuring electrode
Reference electrode

31. Instrumentation (cont.)

32. Quality Assurance
Accurate ABG results depend on rigorous quality control efforts
Components of quality control are:
Record keeping (policies & procedures)
Performance validation (testing new instrument)
Preventative maintenance & function checks
Automated calibration & verification
Internal statistical quality control
External quality control (proficiency testing)
Remedial action (to correct errors)

33. Quality Assurance

34. Quality Control Components
Recordkeeping
Performance Validation
Preventive maintenance and function checks
Automated calibration
Calibration verification by control media
Internal statistical quality control
External quality control
Remedial actions

35. 2-point Calibration

36. external quality control remedial action calibration verification
Changes on procedures, staff training and retraining, closer supervision and increase in maintenance checks are an example of which of the following quality control elements?
external quality control
remedial action
calibration verification
performance validation
Answer: B

37. Point-of-Care Testing
Performing blood gas analysis from laboratory to patient’s bedside
Reduces turnaround time, thus should improve care & lower costs
Used increasingly in hospitals & physician offices
Used for blood chemistry & hematology parameters

38. GEM Premier 4000

39. Blood Gas Monitoring
Provides continuous or interval measurements without removing blood from patient
4 systems in current clinical use:
Transcutaneous blood gas monitor
Intra-arterial (in-vivo) blood gas monitor
Extra-arterial (ex-vivo) blood gas monitor
Tissue oxygen monitor

40. Transcutaneous Monitoring
Provides continuous, noninvasive estimates of PO2 and PCO2 using skin sensor
Sensor warms underlying skin to increase arterial blood flow
2 most important factors influencing accuracy of transcutaneous measurements: age & perfusion status
Low perfusion & increasing age reduce agreement between PtcO2 & PaO2
Agreement between PtcCO2 & PaCO2 is better because CO2 is more diffusible through skin

41. Transcutaneous Monitoring (cont.)
Most common sites for electrode placement for infants & children are abdomen, chest & lower back
Should compare monitor readings with those obtained with concurrent ABG
Validation with ABG should be repeated anytime patient’s status undergoes major change

42. Transcutaneous Monitoring (cont.)

43. decreased skin perfusion electrode overheating
Current data in an infant reveals a PTCO2 of 98 mm Hg and a PTCO2 of 22 mm Hg. A stat ABG is ordered showing a PaO2 of 56 mm Hg and a PaCO2 of 49 mm Hg? What could be the cause of this discrepancy?
an air leak
electrode placement
decreased skin perfusion
electrode overheating
Answer: A

44. Intra-arterial (In Vivo)
Potential benefits of continuous blood gas analysis:
Real time monitoring
Reduction in therapeutic decision making time
Less blood loss
Lower infection risk
Improved accuracy
Elimination of specimen transport

45. Blood Gas Catheter

46. Extra-arterial (Ex Vivo)
Eliminates all problems associated with indwelling sensors
Provides quick results
Unable to provide real-time continuous data
Determine further justification of costs & patient benefits

47. Ex vivo Blood Gas Monitor

48. Tissue Oxygen (PtO2) Monitor
Tissue oxygen can be measured by probes inserted directly into organs, tissue, & body fluids
Clinical indications:
Monitor brain tissue oxygen as an early sign of ischemia
Assess brain blood flow autoregulation
Monitor adequacy of brain perfusion in patients with traumatic brain injury

49. variation in skin characteristics temperature electrode placement
Factors that affect transcutaneous blood gas monitoring accuracy includes all of the following, except:
variation in skin characteristics
temperature
electrode placement
hematocrit
Answer: D

50. Oximetry
Oximetry measures hemoglobin saturation using spectrophotometry
Oximetry works because each substance has its own unique pattern of light absorption
Each form of hemoglobin (e.g., HbO2, HbCO) has its own pattern of light absorption
For example, HbO2 absorbs less red light & more infrared light

51. Oximetry (cont.) Several types used in clinical practice:
Hemoximetry (co-oximetry) - laboratory analytical procedure requiring invasive sampling of arterial blood
Pulse oximetry - noninvasive monitoring technique performed at bedside
Venous oximetry – invasive monitoring through fiberoptic catheter placed in vena cava or pulmonary artery
Tissue oximetry – noninvasive method of measuring saturation of hemoglobin at tissue level

52. Tissue Oxygen Probe

53. Hemoximetry
Measures blood oxygen levels & hemoglobin saturations using a hemoximeter
Multiple lights pass through sample to measure multiple hemoglobin species such as HbO2, HbCO, & metHb
If good quality assurance measures are used, measurements are very accurate

54. Hemoximeter

55. Pulse Oximetry
Combines principles of pectrophotometry with photoplethysmography
Noninvasive portable monitoring device providing estimates of SaO2
Results are reported as SpO2
Pulse oximeter uses light absorption patterns to indicate saturation levels of “pulsed” blood (a.k.a., arterial blood)
Results are not as accurate as hemoximetry

56. Pulse Oximetry (cont.) Accurate to within ±3% to 5% of hemoximetry
Finger probes are not reliable in patients with shock
Cannot distinguish HbCO from HbO2; thus reads falsely high in CO poisoning
Does not measure CaO2 or PCO2; patients suspected of having O2 transport issues or hypoventilation should have an ABG

57. Pulse Oximetry (cont.) 2 types of sensors: Transmission sensor
Sensor has two sides
One side has red & infrared LEDs
Other side has photodetector
Sensor is placed on finger, toe or earlobe
Reflectance sensor
Sensor only on one side (containing both LED light sources & photodetector)
Sensor is placed on skin surface, usually forehead

58. Pulse Oximetry (cont.)

59. Pulse Oximetry (cont.)

60. Venous Oximetry
Assesses balance between oxygen delivery & utilization as indirect index of global tissue oxygenation & perfusion
Normal values for mixed venous (pulmonary artery oxygen saturation monitoring (SvO2) range between 60 to 80%

61. Venous Oximetry (cont.)

62. Tissue Oximetry
Oxygen saturation at tissue level (StO2) assesses adequacy of circulation & oxygen delivery
Early detection of low StO2 can be used as an early detection method of tissue hypoperfusion in patients with traumatic injuries

63. A patient rescued from a house fire is brought into the ER
A patient rescued from a house fire is brought into the ER. The patient is suspected of suffering from carbon monoxide (CO) poisoning, which device can help monitor CO levels:
venous oximeter
pulse oximeter
co-oximeter
tissue oximeter
Answer: C

64. Capnometry Measures CO2 in respiratory gases
Capnometer measures CO2 levels
Capnography is graphic display of CO2 levels as they change during breathing
Most often used in patients undergoing general anesthesia or mechanical ventilation

65. Capnometry (cont.)
Capnometer functions on basis that CO2 absorbs infrared light proportion to amount of CO2 present
2 techniques:
Mainstream technique places an analysis chamber in patients breathing circuit
Sidestream technique pumps small volume of gas from circuit into nearby analyzer

66. Capnometry (cont.)
Fig 18-22

67. Capnometry (cont.)
Normal capnogram shows PCO2 of zero at start of expiratory breath (Phase I)
Soon afterward, the PCO2 level rises sharply (Phase II) & plateaus as alveolar gas is exhaled (Phase III)
End-tidal PCO2 (PETCO2) is used to estimate deadspace ventilation & normally averages 3 to 5 mm Hg less than PaCO2

68. Capnometry (cont.)
Fig 18-23

69. Capnometry (cont.)