آموزشی, بلاد گز, دستگاه ها پزشکی

بلادگز

بلاد گز

بلادگز یا ABG (یا Arterial Blood Gas) به مفهــــــوم سیستم اتوماتیــــک آنالایزر گاز خون می باشــــد که برای
اندازه گیری گازهای خون مورد استفاده قرار می گیرد. دستگاه بلادگز (ABG )به صورت خودکار عمل کرده و پس
از دادن نمونه پارامترهای مورد نظر ( گازهای موجود در خون ) را روی کاغذ مخصوص چاپ می کند.

 

بلادگز

Interpretation of arterial blood gas (ABG)

Abstract
Disorders of acid–base balance can lead to severe complications in many disease states, and occasionally the abnormality may be so severe as to become a life-threatening risk factor.
The process of analysis and monitoring of arterial blood gas (ABG) is an essential part of diagnosing and managing the oxygenation status and acid–base balance of the high-risk patients, as well as in the care of critically ill patients in the Intensive Care Unit.
Since both areas manifest sudden and life-threatening changes in all the systems concerned, a thorough understanding of acid–base balance is mandatory for any physician, and the anesthesiologist is no exception.
However, the understanding of ABGs and their interpretation can sometimes be very confusing and also an arduous task. Many methods do exist in literature to guide the interpretation of the ABGs.
The discussion in this article does not include all those methods, such as analysis of base excess or Stewart’s strong ion difference, but a logical and systematic approach is presented to enable us to make a much easier interpretation through them.
The proper application of the concepts of acid–base balance will help the healthcare provider not only to follow the progress of a patient, but also to evaluate the effectiveness of care being provided.
Introduction
Arterial blood gas (ABG) analysis is an essential part of diagnosing and managing a patient’s oxygenation status and acid–base balance.
The usefulness of this diagnostic tool is dependent on being able to correctly interpret the results. Disorders of acid–base balance can create complications in many disease states, and occasionally the abnormality may be so severe so as to become a life-threatening risk factor.
A thorough understanding of acid–base balance is mandatory for any physician, and intensivist, and the anesthesiologist is no exception.
The three widely used approaches to acid–base physiology are the HCO3 (in the context of pCO2), standard base excess (SBE), and strong ion difference (SID).
It has been more than 20 years since the Stewart’s concept of SID was introduced, which is defined as the absolute difference between completely dissociated anions and cations.
According to the principle of electrical neutrality, this difference is balanced by the weak acids and CO2.
The SID is defined in terms of weak acids and CO2 subsequently has been re-designated as effective SID (SIDe) which is identical to “buffer base.”
Similarly, Stewart’s original term for total weak acid concentration (ATOT) is now defined as the dissociated (A) plus undissociated (AH) weak acid forms. This is familiarly known as anion gap (AG), when normal concentration is actually caused by A.
Thus all the three methods yield virtually identical results when they are used to quantify acid–base status of a given blood sample.

 

Arterial Blood Gas

Arterial Blood Gas

Why is it Necessary to Order an ABG Analysis?

The utilization of an ABG analysis becomes necessary in view of the following advantages:
  • Aids in establishing diagnosis.
  • Guides treatment plan.
  • Aids in ventilator management.
  • Improvement in acid/base management; allows for optimal function of medications.
  • Acid/base status may alter electrolyte levels critical to a patient’s status.
Accurate results for an ABG depend on the proper manner of collecting, handling, and analyzing the specimen. Clinically important errors may occur at any of the above steps, but ABG measurements are particularly vulnerable to preanalytic errors.
The most common problems that are encountered include nonarterial samples, air bubbles in the sample, inadequate or excessive anticoagulant in the sample, and delayed analysis of a noncooled sample.
Preanalytical errors are caused at the following stages:
During preparation prior to sampling
  • Missing or wrong patient/sample identification;
  • Use of the incorrect type or amount of anticoagulant
    – dilution due to use of liquid heparin;
    – insufficient amount of heparin;
    – binding of electrolytes to heparin;
  • Inadequate stabilization of the respiratory condition of the patient; and
  • Inadequate removal of flush solution in arterial lines prior to blood collection.
During sampling/handling
  • Mixture of venous and arterial blood during puncturing;
  • Air bubbles in the sample. Any air bubble in the sample must be expelled as soon as possible after withdrawing the sample and before mixing with heparin or before any cooling of the sample has been done. An air bubble whose relative volume is up to 1% of the blood in the syringe is a potential source of significant error and may seriusly affect the pO2 value.
  • Insufficient mixing with heparin.
During storage/transport
  • Incorrect storage
  • Hemolysis of blood cells
General Storage Recommendation
  • Do not cool the sample.
  • Analyze within 30 min. For samples with high paO2 e.g., shunt or with high leukocyte or platelet count also analyze within 5 min.
  • When analysis is expected to be delayed for more than 30 minutes, use of glass syringes and ice slurry is recommended.
During preparation prior to sample transfer
  • Visually inspect the sample for clots.
  • Inadequate mixing of sample before analysis.
Insufficient mixing can cause coagulation of the sample. It is recommended to mix the blood sample thoroughly by inverting the syringe 10 times and rolling it between the palms as shown in Figure 1. This prevents stacking (such as coins or plates) of red blood cells.

 

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Figure 1

Correct method of mixing of the arterial sample with the anticoagulant in two dimensions to prevent stacking of red blood cells.

During anticoagulation
Modern blood gas syringes and capillary tubes are coated with various types of heparin to prevent coagulation in the sampler and inside the blood gas analyzer:
  • Liquid nonbalanced heparin
  • Dry nonbalanced heparin
  • Dry electrolyte-balanced heparin (Na+, K+, Ca2+)
  • Dry Ca2+-balanced heparin
Other anticoagulants, e.g., citrate and EDTA are both slightly acidic which increase the risk of pH being falsely lowered.
Liquid heparin
The use of liquid heparin as the anticoagulant causes a dilution of the sample, i.e., dilutes the plasma, but not the contents of the blood cells.
As a consequence, parameters such as pCO2 and electrolytes are affected. Only 0.05 mL of heparin is required to anticoagulate 1 mL of blood.
Dead space volume of a standard 5 mL syringe with 1 inch 22 gauge needle is 0.2 mL; filling the syringe dead space with heparin provides sufficient volume to anticoagulate a 4-mL blood sample.
If smaller sample volumes are obtained or more liquid heparin is left in the syringe, then the dilution effect will be even greater. The dilution effect also depends on the hematocrit value.
Plasma electrolytes decrease linearly with the dilution of the plasma along with pCO2, cGlucose, and ctHb values. pH and pO2 values are relatively unaffected by dilution.
 paO2 is only as little as 2% of the O2 physically dissolved in the plasma, and so the oximetry parameters given in fractions (or %) will remain unaffected.
Syringes for blood gas analysis can have a wide range of heparin amounts. The units are typically given as IU/mL (international units of heparin per milliliter) blood drawn into the syringe.
In order to obtain a sufficient final concentration of heparin in the sample, blood volume recommended on the syringe must be drawn.
Example: a syringe stated to contain 50 IU/mL when filled with 1.5 mL of blood means that the syringe contains a total 75 IU of dry heparin. If the user draws 2 mL of blood, then the resulting heparin concentration will be too low and the sample may coagulate.
If the user draws only 1 mL, then the resulting heparin concentration will be higher than that aimed for, which may lead to producing falsely low electrolyte results.
Heparin binds to positive ions such as Ca2+, K+, and Na+. Electrolytes bound to heparin cannot be measured by ion-selective electrodes, and the final effect will be measurement offalsely low values.
The binding effect and the resulting inaccuracy of results are especially significant for corrected Ca2+. The use of electrolyte-balanced heparin significantly reduces the binding effect and the resulting inaccuracy.
The following steps for rapid interpretation of ABG are recommended:
Check for the consistency of ABG
While making an interpretation of an ABG always check for the consistency of the report by the modified Henderson equation.
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