ABL5 Blood Gas Analyser& Blood Gas Analysis

Assoc. Prof. Faridah Abdul RashidDept. of Chemical PathologySchool of Medical SciencesUniversiti Sains Malaysia

faridah@kb.usm.my

18 February 2005

• Blood Gas Analysers– pO2 Measurement

– pCO2 Measurement

– pH Measurement– Bicarbonate Values– Oxygen Saturation

Part 1

A Concise Review of Clinical Laboratory Science. Joel D. Hubbard. Williams & Wilkins Co. 1997. Chapter 1, page 25QY18.2 H875 1997

Blood Gas Analysers

• Blood gas analysers determine acid-base balance through the measurement of partial pressure of oxygen, carbon dioxide, and pH

• Analysers use electrodes as sensing devices, and bicarbonate and other parameters are calculated from the previously mentioned measurements using the Henderson-Hasselbalch equation

• Oxygen saturation is calculated as well• Analysers typically measure arterial blood gases (ABG)

ABL5 Blood Gas AnalyserRadiometer Copenhagen

• pO2 is measured amperometrically– Oxygen diffuses across an

electrode tip through an electrolyte solution to the cathode at a rate that is directly proportional to the amount of oxygen present

– The pO2 electrode is a current electrode

– The signal from the pO2 electrode is measured in pA

pO2 Measurement

• pCO2 is measured potentiometrically– Carbon dioxide diffuses

across a membrane through bicarbonate buffer, eventually forming hydrogen ions that are measured by an electrode

– The signal from the pCO2 electrode is measured in mV

pCO2 Measurement

• pH is measured potentiometrically by two electrodes: pH electrode and a reference electrode– Hydrogen ions diffuse into a

glass membrane, developing a potential that is proportional to the difference in the concentration of hydrogen ions between the sample and the buffer within the electrode.

pH Measurement

ABL5 Wet Section

The electrodes are kept at a constant temperature of 37 °C±0.2 °C

Electrodes (left to right):pH Reference, pH measuring, pCO2, and pO2

Electrode Signals

• Electrode signals are amplified and then converted to the accurate pH, pCO2, and pO2 values for the unknown sample

Acid-Base Relationships

• Acid-base disorders are considered in terms of the Henderson-Hasselbalch equation, which states acid-base relationships:

pH = pK + log([A-]/[HA])

where A- is the proton acceptor (base) and HA is the proton donor (acid)

Bicarbonate Values

• Bicarbonate values are calculated using the Henderson-Hasselbalch equation:

[HCO3-] = 0.031 PCO2 x 10(pH-6.1)

[HCO3-] in mmol/L

PCO2 in mm Hg

Oxygen Saturation

• Oxygen saturation is the percentage of total hemoglobin that has bound oxygen and is calculated from pO2 and pH measurements

• Principle (http://www.lhsc.on.ca/resptherapy/policies/a-10artp.htm)

• Policy• Specimen requirements

Part 2

Principle

• Arterial blood is obtained anaerobically by inserting a short-beveled, sharp needle into an artery

• A device, such as a syringe, is attached either directly or via a short piece of tubing plus an adapter (winged infusion set) to the needle

Principle

• When repeated measurements of arterial blood gases are required, a needle cannula or catheter may be inserted and fixed in the artery and left there for several days

• Indwelling arterial cannulae and catheters should be removed as soon as possible

Principle

• An arterialized blood specimen may also be obtained from properly warmed capillary puncture sites

• All sampling systems should be leakproof; and after blood collection, free of air bubbles

Policy

• The Respiratory Therapist will perform arterial punctures for the collection of arterial blood upon a physician’s order

• The sample site is confined to the radial and femoral arteries

Policy

• Femoral punctures are only to be performed in emergencies when the patient has low blood pressure that does not produce a palpatable radial pulse, or upon a physician’s order

• Femoral punctures are contraindicated in patients with femoral grafts and cellulitis

Policy

• All patients will be tested for collateral circulation prior to an arterial puncture

• Blood samples with appropriately completed labels and requisitions will be left at the unit desk for transport to the laboratory in a sealed package

Policy

• The Respiratory Therapist will ensure that samples for blood gas analysis contain the proper date, F1O2 and their initials

• The Respiratory Therapist will ensure that samples for blood gas analysis is appropriately “iced” before it is sent to the laboratory

Policy

• The Therapist will ensure that precautions against accidental exposure to blood is avoided by wearing gloves when performing a puncture and not recapping the needle after use

• The needle will be inserted into a cork, or a rubber stopper provided with blood gas kit, or self-capping syringe

Policy

• All blood samples must be drawn into appropriate tubes and labeled

• A maximum of 3 attempts will be made to obtain a sample if unsuccessful. After two atttempts, the Therapist will consult with another certified Therapist or the physician to request their attempt for the sample

Policy

• An arterial puncture will not be performed upon patients who:

a) Have circulatory disordersb) Have PTT > 72 seconds or INR > 2.2

Normal PT 9.0 – 12.0 secondsNormal PTT 26 – 37 secondsNormal INR 0.9 – 1.1

Policy

c) Are receiving Warfarin/CoumadinAlways refer to PT/PTT’s

d) Are receiving I.V. Heparine) Are receiving thrombolytic therapy, i.e.,

Activase rt-pa or Streptokinasef) Do not have collateral circulation through

the ulnar artery, verified by Modified Allen’s test (radial puncture only)

Policy

g) Have platelet counts 20,000 and may be done when the platelet count is 50,000, upon consultation with physician

h) Are under six years of age

Specimen Requirements

• Specimen requirements for blood gas analysis include the following.

(1) Arterial blood is collected in a glass or plastic syringe. Capillary specimens can also be used

(2) Lyophilized or liquid heparin is the preferred anticoagulant

Specimen Requirements

(3) No air bubbles should exist in the sample because they lower the pCO2 value

(4) The specimen must be placed in iced water and transported to the laboratory in 15 minutes at 4°C and tested immediately. Otherwise, pH values decrease, and pCO2 values increase

(5) Blood clots are unacceptable

Part 3

Laboratory procedures:

• ABG sample collection procedures and analysis

• Principles of pH, PCO2, and PO2 measurements

Own reading

ABL5 pH/Blood gas analyser

(Emergency request form)

Reporting ABG values

mmHg

mmol/L

incorrect units

correct units

Measured parameters

Calculated parameters

Collection of blood for blood gas analysis

• Obtaining the blood sample is usually performed by the intensive care nurse

• Usually a sample of arterial blood is used for the blood gas analysis

• Heparinised syringe is used for blood gas analysis

Technique for drawing a blood sample

• The radial (or brachial) artery is often chosen because of its accessibility

• The wrist is extended by positioning it over a rolled towel

• After the skin has been sterilised, the artery is stabilised with 2 fingers of one hand while the arterial puncture is made with the other hand using a heparinised syringe

Technique for drawing a blood sample

• After 5 ml of blood has been drawn into the syringe, air is removed and the syringe is capped

• The syringe is placed inside an air tight plastic bag

• The plastic bag containing the syringe is placed in an ice-water slurry and taken immediately to the Chemical Pathology Routine Lab for blood gas analysis

ABG samples

ABG syringe in plastic bag

Factors affecting arterial blood gas analysis

Collection stage:

• Venous blood sample

• Wrong syringe (non-heparinised)

• Wrong artery is chosen

• The wrist is not extended and positioned properly

• Non-stabilised artery while the arterial puncture is made

Factors affecting arterial blood gas analysis

Transportation stage:

• Air is not removed from the syringe

• The syringe is not capped

• The syringe is not placed inside an air tight plastic bag

• The plastic bag containing the syringe is placed on ice but not in an ice-water slurry

Remove air bubbles from ABG syringe

Rub ABG syringe in palms

Inject blood sample into ABG analyser

Measured parameters in ABG

pH electrode system

Ion-selective electrodes

Reference and pH electrodes

Ion-selective electrodes

PCO2 and PO2 electrodes

Ion-selective electrodes