Analysis and Monitoring of Gas Exchange

Analysis and Monitoring of Gas Exchange

Mark S. Siobal

Ultimately, gas exchange takes place inside each of the body’s cells, where complex metabolic pathways use oxygen (O2) to create energy, while producing carbon dioxide (CO2) as a waste product. Although it is possible to analyze gas exchange at the cellular level, clinical focus normally is on gas exchange between the lungs and blood or between the blood and tissues. Gas exchange between the lungs and blood is usually analyzed by measuring O2 and CO2 levels in the arterial blood. Clinicians also can measure CO2 levels in the expired air to monitor ventilation. The most common approach to analyzing gas exchange between the blood and tissues is to measure O2 levels in the mixed venous (pulmonary artery [PA]) blood. This chapter focuses on these important concepts and the parameters that reflect gas exchange.

Invasive Versus Noninvasive Procedures

Invasive procedures require insertion of a sensor or collection device into the body, whereas noninvasive monitoring is a means of gathering data externally.1 Because laboratory analysis of gas exchange requires blood samples, it is usually considered invasive. Monitoring can be either invasive or noninvasive. Generally, invasive procedures tend to provide more accurate data than noninvasive methods, but they carry greater risk.

When both approaches are available, the need for measurement accuracy should dictate which is chosen. However, clinicians can sometimes combine the two approaches—using the invasive approach to establish accurate baseline information, while applying the noninvasive method for ongoing monitoring of a stable patient. After the gradient between the invasive and noninvasive method is established, trends in the change of the noninvasive method can be useful in making clinical decisions.

Measuring Fractional Inspired Oxygen

Analysis of gas exchange begins with knowledge of the system inputs—the inspired O2 and CO2 concentrations. Healthy individuals breathe air that contains a fixed O2 concentration (21%) and negligible amounts of CO2. Patients who are ill often have hypoxemia and are given supplemental O2. O2 analyzers are used to measure the fractional inspired O2 concentration (FiO2).


Although many methods exist for measuring O2 concentrations, most bedside systems apply electrochemical principles. There are two common types of electrochemical O2 analyzers: (1) the polarographic (Clark) electrode and (2) the galvanic fuel cell. Under ideal conditions of temperature, pressure, and relative humidity, both types are accurate to within ± 2% of the actual concentration.1

The Clark electrode is similar to electrodes used in blood gas analyzers and transcutaneous monitors (see later section on Transcutaneous Blood Gas Monitoring). This system typically consists of a platinum cathode and a silver–silver chloride anode (Figure 18-1). O2 molecules diffuse through the sensor membrane into the electrolyte, where a polarizing voltage causes electron flow between the anode and cathode. While silver is oxidized at the anode, the flow of electrons reduces O2 (and water) to hydroxyl ions (OH) at the cathode. The more O2 molecules that are reduced, the greater is the electron flow across the poles (current). The resulting change in current is proportional to the PO2, with its value displayed on a galvanometer, calibrated in percent O2. Response times for Clark electrode O2 analyzers range from 10 to 30 seconds.

Most galvanic fuel cells use a gold anode and a lead cathode. In contrast to the Clark electrode, current flow across these poles is generated by the chemical reaction itself. Unless accessories such as alarms are included, a galvanic cell needs no external power; this means that galvanic cells respond more slowly than Clark electrodes, sometimes taking 60 seconds.

The Clark electrode and galvanic cell are suitable for basic FiO2 monitoring. When greater accuracy or faster response times are needed (e.g., when performing indirect calorimetry), a paramagnetic, zirconium cell, Raman scattering, or mass spectroscopy analyzer should be selected.

Problem Solving and Troubleshooting

Because O2 analyzers include replaceable components that deteriorate over time (batteries, electrodes, membranes, electrolytes), the best way to avoid problems is through preventive maintenance, which should include both scheduled parts replacement and routine operational testing by biomedical engineering personnel. As with any preventive maintenance program, it is essential that detailed records be kept on each piece of equipment.

Even with the best preventive maintenance, O2 analyzers sometimes malfunction. The clinician would know that an analyzer is not working if it fails to calibrate or gives an inconsistent reading during use. The most common causes of analyzer malfunction are low batteries (Clark electrode systems), sensor depletion, and electronic failure. Because a low battery condition is so common with Clark electrode systems, the first step in troubleshooting is to replace the batteries. If the analyzer still does not calibrate on fresh batteries, the problem is probably a depleted sensor. With most analyzers, a depleted sensor must be replaced (some Clark electrodes can be recharged). If an analyzer still fails to calibrate after battery and sensor replacement, the most likely problem is an internal failure of its electrical system. In this case, the device should be taken out of service and repaired.

Inaccurate readings also can occur with electrochemical analyzers, resulting from either condensed water vapor or pressure fluctuations. Galvanic cells are particularly sensitive to condensation. To avoid this problem during continuous use in humidified circuits, the clinician should place the analyzer sensor proximal to any humidification device.

Fuel cell and Clark electrode readings also are affected by ambient pressure changes. Under conditions of low pressure (high altitude), these devices read lower than the actual O2 concentration. Conversely, higher pressures, such as pressures that occur during positive pressure ventilation, cause these devices to read higher than the actual FiO2. These observations are consistent with the fact that both devices measure the PO2 but report a percent concentration scale.

Sampling and Analyzing Blood Gases

In the clinical setting, it is common for the collection of blood specimens (sampling) to be performed separately from their analysis. Each procedure involves different knowledge and skill. For these reasons, these topics are covered separately.


Clinicians have been using blood samples to assess gas exchange parameters for more than 50 years.2 The definition of respiratory failure still is based largely on blood gas measurements. Depending on the need, blood gas samples can be obtained by percutaneous puncture of a peripheral artery, from an indwelling catheter (arterial, central venous, or PA), or by capillary sampling.

Arterial Puncture and Interpretation

Results obtained from sampling arterial blood gas (ABG) are the cornerstone in the diagnosis and management of oxygenation and acid-base disturbances. ABGs are considered the “gold standard” of gas exchange analysis, against which all other methods are compared.

Arterial puncture involves drawing blood from a peripheral artery (radial, brachial, femoral, or dorsalis pedis) through a single percutaneous needle puncture (Figure 18-2). The radial artery is the preferred site for arterial blood sampling for the following reasons:

Other sites (brachial, femoral, and dorsalis pedis) are riskier and should be used only by clinicians specifically trained in their use. Likewise, arterial puncture in infants (through either the radial or the temporal artery) requires advanced training. Arterial cannulation sites with indwelling catheters include radial, brachial, femoral, dorsalis pedis, umbilical (in neonates), and axillary arteries. The focus here is on radial artery puncture.

To guide practitioners in providing quality care, the American Association for Respiratory Care (AARC) has published Clinical Practice Guideline: Sampling for Arterial Blood Gas Analysis.3 Complementary recommendations have been published by the National Committee for Clinical Laboratory Standards.4 Modified excerpts from the AARC guideline appear in Clinical Practice Guideline 18-1.

18-1   Sampling for Arterial Blood Gas Analysis

AARC Clinical Practice Guidelines (Excerpts)*


• Abnormal results of a modified Allen test (lack of collateral circulation) may be indicative of inadequate blood supply to the hand and suggest the need to select another puncture site.

• Arterial puncture should not be performed through a lesion or distal to a surgical shunt. For example, arterial puncture should not be performed on a patient undergoing dialysis. If there is evidence of infection or peripheral vascular disease involving the selected limb, an alternative site should be selected.

• Because of the need for monitoring the femoral puncture site for an extended period, femoral punctures should not be performed outside the hospital.

• Coagulopathy or medium-dose to high-dose anticoagulation therapy, such as heparin or warfarin (Coumadin), streptokinase, and tissue plasminogen activator (but not aspirin), may be a relative contraindication.

*For the complete guideline, see American Association for Respiratory Care: Clinical practice guideline: sampling for arterial blood gas analysis. Respir Care 37:891, 1992.


Box 18-1 lists the equipment needed to perform an arterial puncture. Commercial vendors provide kits containing most of the equipment listed. If provided, the needle capping device serves two purposes. First, it isolates the sample from air exposure (to ensure accurate results). Second, it helps prevent inadvertent needlestick injuries. There are many different capping device designs; devices that allow single-handed recapping are preferred. If a capping safety device is not provided, the clinician should use the single-handed “scoop” method to cap the needle before removing it and plugging the syringe.


Box 18-2 outlines the basic procedure for radial artery puncture of adults. Before radial artery puncture is performed, a modified Allen test (Figure 18-3) is recommended. The test is normal (indicating adequate collateral circulation) if the palm, fingers, and thumb flush pink within 5 to 10 seconds after pressure on the ulnar artery is released. A normal test result indicates the presence of collateral circulation but may not predict the development of complications after radial artery puncture or cannulation.

Box 18-2

Procedure for Radial Artery Puncture

• Check the medical record to (1) confirm the order and indications and (2) determine the patient’s primary diagnosis, history (especially bleeding disorders or blood-borne infections), current status, respiratory care orders (especially oxygen therapy or mechanical ventilation), and anticoagulant or thrombolytic therapy.

• Confirm steady-state conditions (20-30 minutes after changes).

• Obtain and assemble necessary equipment and supplies.

• Wash hands and don barrier protection (e.g., gloves, eyewear).

• Identify the patient using current patient safety standards.

• Explain the procedure to the patient.

• Position the patient, extending the patient’s wrist to approximately 30 degrees.

• Perform a modified Allen test, and confirm collateral circulation.

• Clean site thoroughly with 70% isopropyl alcohol or an equivalent antiseptic.

• Inject a local anesthetic subcutaneously and periarterially (wait 2 minutes for effect).*

• Use a preheparinized blood gas kit syringe, or heparinize a syringe and expel the excess (fill dead space only).

• Palpate and secure the artery with one hand.

• Insert the needle, bevel up, through the skin at a 45-degree angle until blood pulsates into the syringe.

• Allow 1 ml of blood to fill syringe (the need to aspirate indicates a venous puncture).

• Apply firm pressure to puncture site with sterile gauze until the bleeding stops.

• Expel any air bubbles from the sample, and cap or plug the syringe.

• Mix the sample by rolling and inverting the syringe.

• Place the sample in a transport container (ice slush) if specimen is not to be analyzed within 10-30 minutes.

• Dispose of waste materials and sharps properly.

• Document the procedure and patient status in the chart and on the specimen label.

• Check the site after 20 minutes for hematoma and adequacy of distal circulation.


From Malley WJ: Clinical blood gases: application and noninvasive alternatives, Philadelphia, 1990, Saunders; Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 2005, Mosby.

The modified Allen test has been a widely used clinical method to assess adequacy of ulnar artery collateral blood flow despite the lack of evidence that it can predict ischemic complications in the setting of complete radial artery occlusion.5 The criteria for an abnormal test result are not agreed on, which renders the significance of an abnormal test unclear. The test result may be inaccurate in predicting postcannulation hand ischemia, has poor interrater reliability, and is known to yield a high incidence of false normal and abnormal results. The modified Allen test cannot be performed on most critically ill patients who are either uncooperative or unconscious. In addition, prior radial artery cannulation, severe circulatory insufficiency, wrist or hand burns, or jaundice makes interpreting the results difficult. Performance of a modified Allen test before radial artery puncture or cannulation should not be considered a “standard of care,” but the need for its use and appropriate application should be well recognized.6

In patients who have undergone previous radial artery cannulation, the modified Allen test can provide documentation of possible arterial thrombosis and should be used to direct catheter placement. In that circumstance, it is imprudent to ignore totally the utility of the modified Allen test, especially if another arterial site is available for cannulation.7

In most cases, a sample volume of 0.5 to 1 ml of blood is adequate. The actual sample volume needed depends on the following: (1) the anticoagulant used, (2) the requirements of the specific analyzer used, and (3) whether other tests are to be performed on the sample.

The following rules for careful handling of the needle help avoid transmission of blood-borne diseases:

Problem Solving and Troubleshooting

There are two major problem areas associated with arterial puncture. The first problem area involves difficulties in getting a good sample. The second problem area involves preanalytic error.

Getting a Good Sample

Problems with getting a good sample include an inaccessible artery, absent pulse, deficient sample return, and alteration of test results caused by the patient’s response. If the selected artery cannot be located, another site should be considered. Likewise, if an adequate pulse cannot be palpated at the chosen site, another site should be selected, or an acceptable noninvasive approach should be considered as an alternative (e.g., pulse oximetry).

If the clinician gets only a small spurt of blood, the needle has probably passed through the artery. In this situation, the needle is slowly withdrawn until a pulsatile flow fills the syringe. The tip of the needle is never redirected without it first being withdrawn to the subcutaneous tissue. If the needle must be withdrawn completely and the clinician does not have an adequate sample, the procedure is repeated with a fresh blood gas kit.

Small sample volumes or the need to apply syringe suction also may indicate that venous blood has been obtained. However, when drawing arterial blood from hypotensive patients or when using small needles (<23-gauge), the clinician may need to pull gently on the syringe barrel. Excessive suction can alter the blood gas results. If the clinician suspects that pain or anxiety during the procedure may have altered the results (most typically causing hyperventilation), he or she should consider using a local anesthetic for subsequent sampling attempts.

Preanalytic Error

Preanalytic errors are problems occurring before sample analysis that can alter the accuracy of the blood gas results. Table 18-1 summarizes the most common errors associated with arterial blood sampling, including recommendations on how to recognize and avoid these problems.8,9 Clinicians can avoid most preanalytic errors by ensuring that the sample is obtained anaerobically, is properly anticoagulated (with immediate expulsion of air bubbles), and is analyzed within 10 to 30 minutes.

The traditional method used to avoid preanalytic errors caused by blood cell metabolism is to chill the sample quickly by placing it in an ice slush. Chilling is needed if the sample is not to be analyzed within 10 to 30 minutes.3 Chilled samples should be discarded if they are not analyzed within 60 minutes. PaO2 of samples drawn from subjects with elevated white blood cell counts may decrease rapidly, and immediate chilling is recommended. Chilled samples can result in potassium transport between blood cells and plasma and can result in erroneous elevation in potassium measured from a blood gas sample. Use of a glass syringe or a plastic syringe with low diffusibility minimizes the risk of room air gases contaminating the sample. Pneumatic tube transport of samples containing small air bubbles can have a noticeable effect on increasing PaO2.9

Interpretation of Arterial Blood Gases

Given that gas exchange is a dynamic process, looking at results from a single blood sample is akin to looking at a single frame in a feature-length movie. If the scene is changing rapidly, the single frame can be misleading. Conversely, if the scene is relatively stable, a single frame can provide useful information. Blood gas results must be interpreted in light of the patient status at the time the sample was obtained.

Any major change in either patient condition or therapy disrupts the patient’s steady state. However, over time, a steady state normally returns. The time needed to restore a steady state varies with the patient’s pulmonary status. Patients with healthy lungs achieve a steady state in only 5 minutes after changes, whereas patients with chronic obstructive pulmonary disease (COPD) may require 20 to 30 minutes. If a patient’s FiO2 is changed, the measured PaO2 would accurately reflect the patient’s gas exchange status within 5 minutes in healthy individuals but may require 20 to 30 minutes in patients with COPD.

To document the patient’s status, the following need to be recorded: (1) date, time, and site of sampling; (2) results of the modified Allen test, when performed; (3) patient’s body temperature, position, activity level, and respiratory rate; and (4) FiO2 concentration or flow and all applicable ventilatory support settings. Noting such information may prove useful in interpretation of the results.

In the first step of interpretation of the results, the clinician must ensure he or she is looking at the results of the correct patient. The name and patient identification number from the blood gas report must match the patient. Interpretation of the results can be divided into two basic steps: (1) interpretation of the oxygenation status and (2) interpretation of the acid-base status.

The oxygenation status is determined by examination of the PaO2, arterial O2 saturation (SaO2), and arterial O2 content (CaO2) The PaO2 represents the partial pressure of O2 in the plasma of the arterial blood and is the result of gas exchange between the lung and blood. The PaO2 is reduced in various settings but most often when lung disease is present. PaO2 of less than 40 mm Hg is called severe hypoxemia, PaO2 of 40 to 59 mm Hg is called moderate hypoxemia, and PaO2 of 60 mm Hg to the predicted normal is called mild hypoxemia.

SaO2 represents the degree to which the hemoglobin (Hb) is saturated with O2 (see Chapter 11). Normally, the Hb saturation with O2 is 95% to 100% with healthy lungs. When the lungs cannot transfer O2 into the blood at normal levels, the SaO2 decreases in most cases in proportion to the degree of lung disease present. Blood gas analyzers report a calculated SaO2. Measurement of SaO2 by hemoximetry and Hb content is required for accurate determination of CaO2.

CaO2 represents the content of O2 in 100 ml of arterial blood and is a function of the amount of Hb present and the degree to which it is saturated. A normal CaO2 is 18 to 20 ml of O2 per 100 ml of arterial blood. A reduced CaO2 is often the result of low PaO2 and SaO2, reduced Hb level, or both.

The acid-base status of the patient is determined by evaluating the pH, PaCO2, and plasma HCO3. The steps for interpreting the acid-base status of the ABG results are described in Chapter 13.

Indwelling Catheters (Arterial and Central Venous Pressure and Pulmonary Artery Lines)

Indwelling catheters provide ready access for blood sampling and allow continuous monitoring of vascular pressures, without the traumatic risks associated with repetitive percutaneous punctures. However, infection and thrombosis are more likely with indwelling catheters than they are with intermittent punctures.

The most common routes for indwelling vascular lines are a peripheral artery (usually radial, brachial, or less commonly dorsalis pedis and axillary) or femoral artery, a central vein (usually the vena cava), and the PA. In neonates, the umbilical artery is cannulated for arterial blood sampling. Table 18-2 summarizes the usefulness of these various sites in providing relevant clinical information. Chapter 46 provides details on the use of these systems for hemodynamic pressure and flow monitoring.


Figure 18-4 shows the basic setup used for an indwelling vascular line, in this case, a brachial artery catheter. The catheter connects to a disposable continuous-flush device (Delta-flow; Utah Medical Products, Midvale, UT). This device keeps the line open by providing a continuous low rate of flow (2 to 4 ml/hr) of intravenous saline solution through the system.

Heparinized saline flush solution has been commonly used with indwelling vascular catheters. However, it has been shown that heparin does not significantly improve arterial catheter function, extend the duration of use, or decrease the number of manipulations required. Additionally, results of coagulation studies can be affected by heparinized flush solution, and unnecessary exposure to heparin may increase the risk of heparin-induced thrombocytopenia.10 Because arterial pressures are much higher than venous pressures, the intravenous bag supplying these systems must be pressurized, usually by using a hand bulb pump. A strain-gauge pressure transducer connected to the flush device provides an electrical signal to an amplifier or monitor, which displays the corresponding pressure waveform.


Access for sampling blood from most intravascular lines is provided by a three-way stopcock (Figure 18-5). Equipment and supplies are the same as specified for arterial puncture, with the addition of a second “waste” syringe. Box 18-4 outlines the proper procedure for taking an arterial blood sample from a three-way stopcock system.

Box 18-4   Procedure for Sampling Arterial Blood from an Indwelling Catheter

• Check the medical record to affirm order (as per arterial puncture).

• Confirm steady-state conditions (20-30 minutes after changes).

• Obtain and assemble needed equipment and supplies.

• Wash hands and don barrier protection (e.g., gloves, eyewear).

• Identify the patient using current patient safety standards.

• Explain the procedure to the patient.

• Attach the waste syringe to the stopcock port.

• Position the stopcock so that blood flows into the syringe and the IV bag port is closed.

• Aspirate at least 1-2 ml, or five to six times the tubing volume, of fluid or blood.

• Reposition the stopcock handle to close off all ports.

• Disconnect and properly discard waste syringe.

• Attach new heparinized syringe to the sampling port.

• Position the stopcock so that blood flows into the sample syringe and the IV bag port is closed.

• Fill syringe with 1 ml of blood.

• Reposition the stopcock handle to close off the sampling port and open the IV bag port.

• Disconnect the syringe, expel air bubbles from sample, and cap or plug the syringe.

• Flush the line and stopcock with the IV solution.

• Mix the sample by rolling and inverting the syringe.

• Confirm that the stopcock port is open to the IV bag solution and catheter.

• Confirm undampened pulse pressure waveform on the monitor graphic display.

• Place the sample in a transport container (ice slush) if specimen is not to be analyzed within 10-30 minutes.

• Dispose of waste materials properly.

• Document the procedure and patient status in the chart and on the specimen label.

IV, Intravenous.

The procedure is slightly different when obtaining mixed venous blood samples from PA catheters because PA catheters have separate sampling and intravenous infusion ports and a balloon at the tip is used to measure pulmonary capillary wedge pressure. The clinician must ensure that the balloon is deflated and withdraw the sample slowly (e.g., approximately 3 ml/min or 1 ml in 20 seconds). If the clinician fails to deflate the balloon or withdraws the sample too quickly, the venous blood may be “contaminated” with blood from the pulmonary capillaries. The result is always a falsely high O2 level. In addition, close attention must be paid to the infusion rate through the catheter. Rapid flow of IV fluid can dilute the blood sample and affect O2 content measurements.

Capillary Blood Gases

Capillary blood gas sampling is used as an alternative to direct arterial access in infants and small children. Properly obtained capillary blood from a well-perfused patient can accurately reflect and provide clinically useful estimates of arterial pH and PCO2 levels.6 However, capillary PO2 is of no value in estimating arterial oxygenation, and O2 saturation by pulse oximetry must also be evaluated when a capillary blood gas sample is obtained. Respiratory therapists (RTs) must exercise extreme caution when using capillary blood gases to guide clinical decisions. Direct arterial access is still the preferred approach for assessing gas exchange in infants and small children with severe acute respiratory failure.

Capillary blood values are meaningful only if the sample site is properly warmed. Warming the skin (to approximately 42° C) causes dilation of the underlying blood vessels, which increases capillary flow well above tissue needs. Blood gas values resemble the values in the arterial circulation; this is why a sample obtained from a warmed capillary site is often referred to as arterialized blood. It has been shown that capillary blood samples from the earlobe reflect arterial PCO2 and PO2 better than samples drawn from a finger stick.11 The posterior medial or lateral curvature of the heel is the recommended site for capillary puncture specimens in infants less than 1 month old to avoid nerve and bone damage.

To guide practitioners in providing quality care, the AARC has published Clinical Practice Guideline: Capillary Blood Gas Sampling for Neonatal and Pediatric Patients.12 Modified excerpts from the AARC guideline appear in Clinical Practice Guideline 18-2.

18-2   Capillary Blood Gas Sampling for Neonatal and Pediatric Patients

AARC Clinical Practice Guideline (Excerpts)*


Capillary punctures should not be performed at or through the following:

Capillary punctures should not be performed:

Relative contraindications include:

*For the complete guideline, see American Association for Respiratory Care: Clinical practice guideline: capillary blood gas sampling for neonatal and pediatric patients. Respir Care 46:506, 2001.


Box 18-5 outlines the basic procedure for capillary blood sampling. The most common site for sampling is the heel, specifically the lateral aspect of the plantar surface.

Jun 12, 2016 | Posted by in RESPIRATORY | Comments Off on Analysis and Monitoring of Gas Exchange
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