Gas Exchange Analysis and Monitoring

Analysis and Monitoring of Gas Exchange

• Analysis involves discrete measurements of body fluids or tissues, requiring removal from the body.
• Measurements are performed using an analyzer.
• Monitoring is a continuous process, evaluating physiological processes at the bedside.
• Monitoring uses bedside monitors.

Analysis vs. Monitoring

• Laboratory analysis provides discrete measurements of fluids or tissues, requiring removal from the body.
• Monitoring is a continuous process, evaluating dynamic physiological processes in a timely manner, usually at the bedside.

Invasive vs. Noninvasive Procedures

• Invasive procedures require insertion of a sensor or collection device into the body.
• Noninvasive methods gather data externally.
• Invasive procedures provide more accurate data but carry a higher risk compared to noninvasive methods.
• Noninvasive trends are useful in making clinical decisions after establishing a gradient between invasive and noninvasive methods.

Measuring FIO2

• Most bedside systems use electrochemical principles to measure FIO2.
• Two common methods are polargraphic (Clark) electrodes, requiring batteries, and galvanic fuel cells, not needing batteries.
• Clark electrodes have response times between 10-30 seconds.
• Galvanic fuel cells have response times of 60 seconds.

Troubleshooting O2 Analyzers

• Calibration must be performed according to the manufacturer's instructions before use.
• Failure to calibrate or inconsistent readings indicate malfunction.
• Preventative maintenance is the best way to avoid problems.
• Analyzer calibration failures could be due to low batteries, sensor depletion, or electronic failure.

Sampling & Analyzing Blood Gases

• Analyzing arterial blood samples is crucial in diagnosing and treating respiratory failure.
• Radial artery is the most common site due to its superficial location, stabilizable collateral circulation (confirmed with the Allen test), and minimal pain during puncture.
• Arterial cannulation is an option when frequent sampling is needed.

Arterial Sampling

• The Modified Allen's test is performed before radial artery puncture to confirm adequate collateral circulation.
• The test involves occluding both the radial and ulnar arteries, then releasing the ulnar artery, observing the hand for return of blood flow within 5-10 seconds.
• The test cannot be performed on critically ill or uncooperative/unconscious patients.
• The appropriate blood sample volume is 0.5 to 1 mL, depending on anticoagulants used and further testing.
• Proper needle handling, syringe disposal, and sharps container usage are crucial for safety and infection control.

Indications for Arterial Sampling

• Unexplained dyspnea, acute dyspnea/tachypnea, abnormal breath sounds, cyanosis, heavy use of accessory muscles, ventilator setting changes, CPR, diffuse/new chest infiltrates in CXR, sudden cardiac arrhythmias, and acute hypotension.

Blood Sample Analysis

• Clinicians can avoid pre-analytical errors by obtaining samples anaerobically.
• Samples must be properly anticoagulated and analyzed within 10-30 minutes.

Interpretation of ABGs

• PaO2 (partial pressure of oxygen) is significant in evaluating oxygenation status.
• SaO2 (oxygen saturation) is typically normal at 95-100%.
• CaO2 (arterial oxygen content) ranges between 18-20 volume percent when normal.
• PaO2 values ranging from 60 to 79 mm Hg indicate mild hypoxemia; 40-59 mm Hg, moderate hypoxemia; and below 40 mm Hg, severe hypoxemia.

Indwelling Catheters

• Indwelling catheters provide ready access for blood sampling and continuous monitoring of vascular pressures.
• Infection and thrombosis risks are higher compared to intermittent punctures.
• Common catheterization routes include peripheral arteries (radial, brachial, and pedal), femoral artery, central vein, and pulmonary artery.
• Three-way stopcocks provide sampling access for most intravascular lines.
• Pulmonary artery catheters include separate blood sampling and IV infusion ports with a balloon at the tip.

Brachial Artery Catheter Image

• (Diagram with labeled parts).

3-way Stopcock Image

• (Diagram with labeled parts).

Capillary Blood Gases

• Capillary samples accurately reflect arterial pH and PCO₂ levels.
• Capillary PO₂ is not suitable for assessing arterial oxygenation and needs to be evaluated using pulse oximetry.
• Common errors during capillary sampling include inadequate warming and squeezing of the puncture site, resulting in venous/lymphatic sample contamination.

Analyzing

• Analyses of blood samples measure pH, PCO₂, and PO₂ levels.
• Supporting secondary values are calculated, such as plasma bicarbonate, base excess, and hemoglobin saturation.
• Some systems combine blood gas and hemoximetry (total hemoglobin) measurement.

Instrumentation

• Instrumentation is based on several types of sensor technology for measuring pH, PCO₂, and PO₂.
• Examples include electrochemical electrodes, optical fluorescence, and photoluminescence.

Electrodes

• PaO₂: Clark polarographic electrode.
• PaCO₂: Severinghaus electrode.
• pH: pH electrode consisting of a measuring and reference electrodes.

Point-of-Care Testing

• Point-of-care testing performs blood gas analysis at the patient's bedside, reducing turnaround time and improving care/lowering costs.
• Blood chemistry and hematology parameters are also analyzed through this method.

GEM Premier 4000

• (Image/diagram of the device)

Oximetry

• Oximetry measures hemoglobin saturation using spectrophotometry.
• Substances absorb light differently, leading to distinct patterns.
• Hemoglobin forms (HbO₂, HbCO, metHb) absorb light differently.
• For example, HbO₂ absorbs less red light and more infrared light.

Oximetry (cont.)

• Types of oximetry used in clinical practice include hemoximetry (co-oximetry) requiring invasive arterial blood sampling.
• Pulse oximetry, a non-invasive monitoring technique.
• A fiberoptic catheter inserted into the vena cava or pulmonary artery allows for venous oximetry.
• Tissue oximetry is a non-invasive technique measuring hemoglobin saturation at the tissue level.

Tissue Oxygen Probe (diagram)

• A diagram of the device.
• Numbers/labels corresponding to device parts.

Hemoximetry

• Hemoximetry measures blood oxygen levels and hemoglobin saturations using a hemoximeter.
• Multiple lights pass through the sample to analyze different hemoglobin species (HbO₂, HbCO, or metHb).
• Highly accurate measurements with good quality assurance.

Pulse Oximetry

• Pulse oximetry combines photometry and plethysmography for noninvasive estimates of SaO2.
• Results are reported as SpO2 (pulse oximetry oxygen saturation).
• The method uses light absorption patterns to measure "pulsed" (arterial) blood oxygen saturation.
• Pulse oximetry results are not as accurate as hemoximetry.

Pulse Oximetry (cont)

• The accuracy of pulse oximetry is ±3% to 5% of hemoximetry.
• Finger probes are unreliable in shock.
• Pulse oximetry cannot distinguish HbCO from HbO₂ (falsely high readings in CO poisoning).
• Pulse oximetry does not measure CaO₂ or PCO₂, so for suspected O₂ transport issues, patients with hypoventilation should undergo ABG analysis.

Pulse Oximetry (cont.)

• Two types of pulse oximetry sensors are transmission and reflectance:
• Transmission sensors: two-sided sensors with LEDs and photodetectors, placed on the finger, toe, or earlobe.
• Reflectance sensors: one-sided sensors containing LEDs and a photodetector, commonly placed on the forehead.

Venous Oximetry

• Venous oximetry assesses the balance between oxygen delivery, utilization, and perfusion by indirectly measuring global tissue oxygenation.
• Normal values for mixed venous (pulmonary artery) oxygen saturation (SvO₂) range from 60% to 80%.

Venous Oximetry (cont.)

• (Diagram of venous oximetry).

Tissue Oximetry

• Tissue oximetry measures oxygen saturation (StO₂) at the tissue level, evaluating adequacy of circulation and oxygen delivery.
• Early detection of low StO₂ can identify tissue hypoperfusion, particularly in patients with traumatic injuries.

Capnometry

• Capnometry measures carbon dioxide (CO₂) in respiratory gases.
• A capnometer provides a graphic display (capnography) of CO₂ levels during breathing.
• This method is frequently used in patients undergoing general anesthesia or mechanical ventilation.

Capnometry (cont)

• CO₂ absorbs infrared light proportionally to its concentration measured via capnometry;
• Two techniques:
• Mainstream: patient's breathing circuit has an analysis chamber.
• Sidestream: small gas volumes from the circuit are pumped into a nearby analyzer.

Capnometry (cont.)

• A normal capnogram shows a CO2 pattern with zero at the start of exhalation, then a sharp rise (Phase II), and a plateau as alveolar gas is exhaled (Phase III).
• End-tidal PCO₂ (ETCO₂) usually averages 3-5 mm Hg below PaCO₂, estimating deadspace ventilation.

Capnometry (cont.)

• (Diagram of capnogram).

Capnometry (cont.)

• (Diagram of capnogram showing different patterns associated with lung diseases and/or disorders).

Explore the differences between analysis and monitoring in gas exchange. This quiz covers the processes of invasive and noninvasive procedures, along with their implications in clinical settings. Understand how these methodologies impact physiological evaluations and decisions.