Arterial Blood Gas Sampling [ABG Machine Use]

Summary / Explanation

Introduction:

Point-of-care arterial blood gas (ABG) analysis is a crucial blood test that helps in diagnosing and treating patients effectively, resulting in better clinical outcomes. ABG machines are primarily utilized in different clinical environments for assessing the exchange of gases (oxygen and carbon dioxide) as well as the acid-base level in the bloodstream. This is particularly vital in critical care and emergency settings due to its ability to provide rapid and accurate insights that are crucial for managing life-threatening conditions such as respiratory failure and shock.[1]

Blood sampling:

Blood sampling for gas analysis is typically conducted using either arterial blood gas (ABG) or venous blood gas (VBG) tests, each requiring distinct techniques and precautions:

• ABG Sampling: This is most commonly performed at the radial artery. To ensure adequate collateral circulation, the modified Allen’s test is conducted before puncture. The needle is inserted at the site where the radial pulse is most palpable, utilizing the arterial pressure for passive blood collection.
• VBG Sampling: Venous samples can be drawn from either central or peripheral sites. Given that venous pressure is often lower, manual drawing of blood into the syringe may be necessary to obtain an adequate sample.

For both methods, maintaining aseptic technique is crucial to prevent contamination and infection. After the puncture, the site should be properly dressed, and pressure applied to minimize bleeding. Blood is collected in a heparinized syringe to prevent clotting. Post-collection, the syringe should be carefully disconnected from the needle, which is then safely discarded. The syringe tip is covered, and the sample is accurately labeled with essential details for analysis, including the patient’s inspired oxygen level. These steps help ensure the integrity and reliability of the blood gas analysis.[2]

Indications for blood gas analysis:

Blood gas analysis is used mainly to assess the adequacy of ventilatory and oxygenation status, evaluating acid-base status in patients with a wide range of conditions such as uremia and diabetic ketoacidosis. It can also be used for evaluating the adequacy of circulatory response by measuring central venous oxygen saturation in patients with conditions such as sepsis, septic shock, and after major surgery. It measures main serum electrolytes and lactate. Additionally, it helps in assessing oxyhemoglobin saturation, total hemoglobin, and dyshemoglobin saturation.[3]

Contraindications to blood gas analysis:

Contraindications to performing blood gas analysis effectively include using a malfunctioning blood gas analyzer or one that has not been validated through quality control and proficiency testing. Additionally, specimens that are improperly anticoagulated, contain visible air bubbles or have been stored improperly or for too long are unsuitable for analysis. Other issues that can prevent accurate testing include incomplete requisitions, lacking essential patient and test information, and specimens that are inadequately labeled. These factors can all compromise the reliability and accuracy of blood gas analysis results.[4]

Possible hazards or complications of blood gas analysis:

Local complications from blood sampling can range from minor issues like pain and hematoma to more severe problems such as hemorrhage, vessel trauma, occlusion, vasospasm, thromboemboli, air emboli, and pseudoaneurysm.

There is a risk of infection for specimen handlers from blood-borne pathogens, which mandates the need for strict adherence to safety protocols. Inaccurate analysis or reporting of results could lead to inappropriate medical treatments, compounding patient risks. Additionally, there is a concern for cross-contamination between different samples or misidentification of patients, which could result in incorrect treatment decisions.[5]

Test parameters:

• Directly measured parameters:

Blood gas analyzers are directly measuring several key variables. These include pH which is determined using the Sanz electrode, partial pressures of carbon dioxide (pCO2) and oxygen (pO2) are measured using the Severinghaus and Clark electrodes respectively. Electrolytes such as ionized calcium (Ca2+), sodium (Na+), potassium (K+), and chloride (Cl-) are measured using ion-selective electrodes. Metabolites, including glucose, lactate, and creatinine, are analyzed using immobilized enzymes or specialized electrodes. Additionally, co-oximetry parameters assess various hemoglobin forms, the proportion of oxyhemoglobin relative to the total hemoglobin, indicating the blood’s oxygen-carrying capacity, total hemoglobin (CtHb), and fractions like oxyhemoglobin (FO2Hb), deoxyhemoglobin (FHHb), carboxyhemoglobin (FCOHb), and methemoglobin (FMetHb).

• Derived parameters:

Derived Parameters are calculated based on the measured values, these include bicarbonate (HCO3-) which is calculated using the Henderson–Hasselbalch equation to assess the metabolic component of acid-base balance, base excess (BE) which is derived using the Van Slyke equation indicating the amount of excess or deficit of base in the blood, and anion gap which is calculated to help identify the cause of metabolic acidosis by measuring the difference between the primary measured cations (Na+ and K+) and the anions (Cl- and HCO3-).[6]

Common Preanalytical Problems:

Preanalytical errors are the most frequent source of inaccuracies in laboratory testing and are critical in blood gas analysis due to its time-sensitive nature. Common preanalytical issues include:

• Time sensitivity:

The stability of blood gas samples is highly time-sensitive due to ongoing metabolism in blood cells that can change the concentrations of gases and other analytes. To ensure accuracy, the International Federation of Clinical Chemistry and Laboratory Medicine advises that blood gas samples needing measurements of pO2 and pCO2 should be analyzed within 15 minutes of collection. If the measurements of pO2 and pCO2 are not critical, the samples should be analyzed within 30 minutes. Typically, samples arriving at the laboratory more than 30 minutes after collection are not accepted for analysis to prevent compromised results from degraded sample integrity.[7]

• Temperature considerations:

The storage temperature of blood gas samples significantly impacts their quality. Cooling the samples is generally beneficial as it slows down glycolysis, thereby minimizing changes in the sample's composition after collection. However, there is a caveat; the permeability of plastics, commonly used in blood gas syringes, tends to increase at lower temperatures. This increased permeability can lead to erroneous results by allowing gases to diffuse through the syringe material, compromising the integrity of the sample.[8]

• Type of syringe:

The choice between glass and plastic syringes affects the accuracy of pO2 measurements. Glass syringes are superior for pO2, but both types perform similarly for pH and pCO2. High-density polypropylene syringes designed specifically for blood gas analysis may offer better diffusion characteristics than general-purpose syringes.[9]

• Anticoagulant and mixing:

The use of appropriate anticoagulants is crucial. Heparin is commonly used because it effectively prevents coagulation without significantly affecting the measurements of common electrolytes, as it can bind to positive ions. Blood samples must be thoroughly mixed immediately after collection to ensure uniform dispersion of the anticoagulant and to prevent sedimentation of cells.[10]

• Air bubbles:

Air bubbles in the sample can falsely elevate pO2 readings. Some syringes are designed with vented caps to simplify the removal of air bubbles, thereby enhancing the accuracy of the results.[11]

The Analyzer:

• Sample input port:

This port is designed to accommodate male luer-style syringes as well as glass or flexible capillary tubes. In situations where the sample size is small, most analyzers allow manual advancement of the sample into the electrode area. Typically, the machine pump draws between 70 and 140 microliters of the sample into the sample holding area for processing. It is crucial to avoid introducing air into the sample pathway during this process and ensure a good seal between the port and the sample. Common causes of air introduction include tears in the flexible port assembly, broken capillary glass, clotted blood, or holes in the flexible tubing.

• Calibration gas and reagent bank:

All blood gas analyzers are equipped to conduct periodic self-calibration checks to verify proper electrode response and accuracy. The calibration process will be detailed later in this review.

• Wash and rinse bank:

This component ensures clear and clean fluid pathways within the analyzer. A wash is conducted after each sample to prevent cross-contamination between tests, while also maintaining the correct pH balance and moisture level in the electrode manifold to prolong electrode life. Many analyzers incorporate the intentional introduction of air slugs during the rinse process, which creates turbulence and scrubbing action along the fluidic tubing walls to enhance washing effectiveness. When troubleshooting this section, observe the air/fluid pattern and associated noise during the cycle. With experience, one can identify indications of air leaks or occlusions within the fluidic pathways.[12]

The Analytical Phase:

During the analytical phase of testing, several critical aspects need to be carefully monitored to ensure the accuracy of blood gas analysis, with corrective actions taken as necessary. Before analyzing patient samples, it is essential to confirm that the quality control and calibration procedures have been completed and that the instrumentation is functioning properly. Specimens have to be properly labeled, stored, and analyzed within an acceptable time frame. Any air bubbles or clots in the specimen have to be evacuated before mixing and sealing the syringe. This prevents interference with the measurement process. Additionally, it's crucial to ensure that the sample is drawn uninterrupted into the analyzer, fully covering all electrodes. These steps are integral to ensuring the analytical phase runs smoothly and produces accurate and reliable data.[13]

Limitations of Blood Gas Analysis:

Blood gas analysis has several limitations and factors that can impact the validation of results.

• Hemolysis of the sample:

This cannot be detected affecting the interpretation of important parameters such as serum K+ level (pseudohyperkalaemia).[14]

• Oxyhemoglobin measurement:

There can be discrepancies between fractional oxyhemoglobin (FO2Hb) and hemoglobin oxygen saturation (sO2), particularly in cases of elevated carboxyhemoglobin or methemoglobin.[15]

• Temperature correction:

Temperature correction of blood gas results can lead to confusion due to differing opinions on its utility. While some advocate for correction in specific cases, such as extreme temperatures during open-heart surgery or neonatal cooling, others argue that most situations do not warrant correction. Laboratories may choose to offer temperature-corrected values for certain patient populations, but it's important to always report the measured (37°C) values alongside any temperature-corrected values when requested.[9]

• Hemoglobin measurements:

Handheld analyzers typically calculate hemoglobin from measured hematocrit, while benchtop analyzers measure hemoglobin using co-oximetry. Variations in plasma composition can affect hemoglobin results derived from hematocrit. Hemodilution or altered osmolality can also impact hematocrit measurements.[16]

• Substances affecting analyzer membranes:

The presence of substances such as hyperlipidemia, methylene blue, or hydroxocobalamin may affect the analyzer membranes, leading to inaccurate readings.[11]

Analysis Validity:

For blood gas analysis results to be considered valid, several conditions must be met. First, the analytic procedure must adhere to both recommended guidelines and the manufacturer’s recommendations to ensure accuracy. Additionally, the analysis results must fall within the calibration range of the analyzer and the quality control product ranges. Finally, laboratory procedures and personnel must comply with established quality control standards and participate in proficiency testing programs to maintain the integrity and reliability of the analysis.[17]

Questionable Specimen Contamination:

If blood gas analysis yields questionable results that suggest specimen contamination, several steps should be taken to address the issue. Firstly, verify the labeling on the blood sample container to ensure that patient identifiers, details of acquisition, and FIO2 are correct. If the labeling is accurate and sufficient specimen remains, reanalyze the residual sample on a separate analyzer if possible. If the discrepancy remains unresolved, an additional sample should be obtained. Additionally, it is important to log the results and reasons for discarding any analyzed samples that are deemed unreliable.[18]

Quality Control:

Equipment quality control and control materials are essential for maintaining the accuracy and reliability of blood gas analysis equipment.

• Internal quality control:

This is performed to verify that the analyzer's outputs are reliable by analyzing materials that have known concentrations of analytes. If the results fall outside the acceptability limits, it should prompt equipment troubleshooting.

Internal quality control using commercial controls, is necessary to establish the mean and standard deviation for each constituent and determine the acceptable range for quality control results. The frequency of control runs and the number of levels should align with regulatory requirements and manufacturer's recommendations. Both electronic quality control, which monitors equipment performance, and periodic use of non-electronic controls are crucial. Keeping records that summarise all quality control data for a specified lot number is also important.[19]

• External quality control (proficiency testing):

Regulatory standards require proficiency testing for each regulated analyte. This involves analyzing samples (testing materials) provided by approved external bodies to ensure consistency and accuracy across different laboratories. Proficiency testing survey reports must be reviewed carefully to address any poor performance issues promptly.

If an existing instrument is replaced, duplicate analysis is necessary to compare the new instrument's performance with the old one.[20]

•  Calibration:

This ensures that the measurements are accurate by using known concentrations of analytes. There are two calibration systems: one for calibrating pO2 and pCO2 gases using calibration gas reagents and another for calibrating the metabolic and electrolyte plasma components of whole blood using liquid reagents.

Calibration verification should be performed prior to the initial use of the equipment and at six-month intervals. At least three levels of control material should be analyzed to verify the measuring range of the analyzer. The frequency of calibration verification can vary according to the directives of regulatory agencies.[21]