chapter 18 analysis and monitoring of gas exchange

Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc.

Chapter 18

Analysis and Monitoring of Gas Exchange


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.

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Copyright © 2013, 2009, 2003, 1999, 1995, 1990, 1982, 1977, 1973, 1969 by Mosby, an imprint of Elsevier Inc. 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.


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


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


Measuring FIO2

Most bedside systems to measure FIO2 use electrochemical principles

2 most common:1. Polargraphic (Clark) electrode- needs battery2. Galvanic fuel cell- no battery needed

Response times for Clark electrodes = about 10 to 30 seconds

Response times for galvanic fuel cells = 60 seconds


Clark Polarographic Analyzer

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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:

A.paramagnetic analyzer

B.polarographic analyzer

C.galvanic fuel cell analyzer

D.zirconum cell analyzer


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

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All of these may be reasons for a Galvanic fuel cell to malfunction, except:

A.batteries

B.sensor depletion

C.electric failure

D.condensation of water vapor


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

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Arterial Sampling

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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

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Modified Allen Test

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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?

A.should not perform an ABG on patient.

B.perform the ABG on the right brachial artery.

C.perform it on the right radial artery.

D.repeat the modified Allen’s test on the left hand.



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

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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

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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


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

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Interpretation of ABGs

Interpret oxygenation status PaO2 (see below)

SaO2 (normal = 95100%)

CaO2 (normal = 1820 vol%)

• PaO2 6079 mm Hg = mild hypoxemia

• PaO2 4059 mm Hg = moderate hypoxemia

• PaO2 <40 mm Hg = severe hypoxemia


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?

A.it may increase the PaCO2 of the sampleB.it only affects the gas pressures after 1 hr of drawn

C.it may decrease the PaO2 of the sample

D.air bubbles have no impact on the PaO2/PaCO2 values


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

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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

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Brachial Artery Catheter

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3-way Stopcock

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If a indwelling vascular catheter is placed in the brachial artery, what is the sample assessing?

A.pulmonary gas exchange

B.gas exchange at the tissues

C.right ventricular preload

D.right ventricular afterload


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


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

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Instrumentation

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

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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

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Instrumentation (cont.)

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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)


Quality Assurance

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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

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2-point Calibration

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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?

A.external quality control

B.remedial action

C.calibration verification

D.performance validation


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

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GEM Premier 4000

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Blood Gas Monitoring

Provides continuous or interval measurements without removing blood from patient

4 systems in current clinical use:1. Transcutaneous blood gas monitor

2. Intra-arterial (in-vivo) blood gas monitor

3. Extra-arterial (ex-vivo) blood gas monitor

4. Tissue oxygen monitor

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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


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

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Transcutaneous Monitoring (cont.)

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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?

A.an air leak

B.electrode placement

C.decreased skin perfusion

D.electrode overheating


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

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Blood Gas Catheter

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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

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Ex vivo Blood Gas Monitor

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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

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Factors that affect transcutaneous blood gas monitoring accuracy includes all of the following, except:

A.variation in skin characteristics

B.temperature

C.electrode placement

D.hematocrit


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


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

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Tissue Oxygen Probe

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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

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Hemoximeter

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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


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



2 types of sensors:1. Transmission sensor

• Sensor has two sides One side has red & infrared LEDs Other side has photodetector

• Sensor is placed on finger, toe or earlobe

2. Reflectance sensor• Sensor only on one side (containing both LED light

sources & photodetector)

• Sensor is placed on skin surface, usually forehead

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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%

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Venous Oximetry (cont.)

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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

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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:

A.venous oximeter

B.pulse oximeter

C.co-oximeter

D.tissue oximeter


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


Capnometry (cont.)

Capnometer functions on basis that CO2

absorbs infrared light proportion to amount of

CO2 present

2 techniques:1. Mainstream technique places an analysis

chamber in patients breathing circuit

2. Sidestream technique pumps small volume of gas from circuit into nearby analyzer


Capnometry (cont.)


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


Capnometry (cont.)

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Capnometry (cont.)

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chapter 18 analysis and monitoring of gas exchange

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