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Modern blood gas analyzers are electronic marvels

compared to the methods used for this purpose 20 years

ago. On attaching the sample syringe to the cuvette, they

automatically draw the sample into a heated sampling

chamber with miniaturized electrodes that quickly and

accurately (if properly calibrated) measure pH, PCO2 and

PO2 values. Based on these three measured values, these

Blood Gases and Electrolytes 557

units automatically calculate HCO3, total CO2, percent

oxygen saturation and O2 content, which is based on entry

of the patient’s measured hemoglobin values.

A companion to such units, known as a co-oximeter,

directly measures percent oxygen saturation and

hemoglobin, then accurately calculates oxygen content

and carboxyhemoglobin, a value that reflects the degree

of carbon monoxide in the blood in smoke inhalation

victims.

In addition to arterial sampling, critical care specialists

often order blood gas panels in blood drawn through a

central venous line since PO2 and O2 content values of this

blood, when compared against arterial PO2 and O2 content,

enable an estimate of cardiac output, another valuable

service performed by blood gas testing. Such samples are

often collected and run from patients undergoing cardiac

catheterization and the results must be returned while the

patient is still on the table.

What are blood gases?

These are two broad components to the blood gas panel:

respiratory and metabolic. The values reported are as follows:

¾ pH—This is a logarithmic expression of hydrogen ion

concentration—the acidity of alkalinity of the blood.

The normal human arterial pH is 7.4. Any pH below

this is acid, and any pH above it is alkaline. There is a

narrow range of pH values (7.35 to 7.45) that the human

body and its complicated system of enzyme-supported

system operates within. pH values below 7.0 and above

7.6 are incompatible with life.

¾ HCO3—This value is derived through the blood gas

analyzer’s manipulation of the Henderson-Hasselbalch

Equation. An uncompensated decrease in the HCO3

value causes a decline in pH. An increased HCO3 results

in alkalinization of the blood. Either condition can be

life-threatening. Decreased HCO3 is often the result

of kidney or other major organ failure or uncontrolled

diabetes. Increased HCO3 is more rare and is usually

the result of inappropriate administration of certain

drugs such as some kinds of diuretics or an excess of

NaHCO3.

¾ PCO2—This value is measured directly by the CO2

electrode. An increased PCO2 is often the result of acute,

chronic or impending respiratory failure, whereas

a decreased PCO2 is the result of hyperventilation

stimulated by a metabolic acidosis or hysteria and

severe anxiety reactions. The normal arterial PCO2 is 40

mm Hg.

¾ PO2—The partial pressure of oxygen in the blood is

measured directly by a polarographic O2 electrode. The

normal acceptable range is roughly between 85 and

100. An increased PO2 is usually the result of excessive

oxygen administration that needs to be adjusted

downwards on such results. A decreased PO2 is often the

result of any number of respiratory or cardiopulmonary

problems.

AVL COMPACT 2 BLOOD GAS ANALYZER (FIG. 21.1)

The AVL compact series is a marvel of design and function.

As with all AVL blood gas analyzers, this little workhorse is

designed to make operation as simple as possible allowing for

faster, safer, more reliable results in hectic emergency situations. The AVL compact series focuses on the most important

requirements of critical care analysis and is suitable for

intensive care situations, neonatology and lung function

testing. Patient data entry is a breeze via the built-in keyboard

and LCD display. Furthermore, a logical menu guides the user

through all functions making operation simple, reliable and

safe. The simplicity and small footprint of the AVL compact

series mask its advanced analytical performance and data

processing capabilities. Measured parameters in the AVL

compact series include PH, PCO2, PO2, and PBaro. Features

of the AVL compact series include automatic calibration and

cleaning cycles and 20-second analysis time, 32 samples/h.

Other features of the AVL compact series are LCD display

with integrated numeric keypad, automatic sample handling

system, and on-board quality control features able to store up

to 34 QC results for three levels.

FIG. 21.1: AVL compact 2 blood gas analyzer

558 Concise Book of Medical Laboratory Technology: Methods and Interpretations AVL Compact 2 Blood Gas Analyzer

Specifications:

Dimensions: 14 × 13 × 14 (36 × 33 × 36 cm)

Weight: 29 lbs (13 kg).

ELECTROLYTE ANALYSIS BY FLAMEPHOTOMETER

In this instrument, the solution under test is passed under

carefully controlled conditions as a very fine spray in the

air supply to a burner. In the flame, the solution evaporates

and the salt dissociates to give neutral atoms. Some of these

though only a very small proportion, move into a higher

energy state (their electrons move to the outer orbits). When

the electrons of these atoms fall back to their original orbits,

they release energy in the form of light. Which is used in

flame photometry of this type and is called emission flame

photometry. Light of characteristic wavelengths is emitted

and passes through a suitable filter, grating or prism on to

a photocell, and the amount of current thus produced is

measured. This varies with the concentration of sodium;

for example, in the solution being tested. Using solutions

of known sodium concentrations, a calibration curve can

be made and this is used for reading the sodium content of

the fluids examined.

Many gases have been used for the flame. These include

acetylene, propane, butane and coal gas. Both the gas

pressure and air pressure have to be carefully regulated so

as to maintain a constant steady flame, which should be

blue in color and have no yellow streaks. The spray is formed

by passing compressed air through an atomizer, into which

the liquid which is being tested is drawn either by suction

or gravity, and then enters the burner in its air supply. The

pressure used is usually about 10 to 15 lb per square inch.

The atomizer is a very important part of the apparatus.

A steady fine spray of droplets of uniform size should be

produced if there is to be a constant emission of light.

The light produced is first passed through a lens to focus

it and then through suitable filters, a diffraction grating or

prism before it falls on to the photocell. For determining

sodium, an orange or yellow (589 nm) filter can be used.

Potassium emits light at 404.4 and 766.5 nm. For the

former, a violet; and for the latter a deep red filter should

be used. Lithium (671 nm) needs a near red filter for its

measurement.

The most satisfactory dilution to use should be

established experimentally. For sodium, this may be 1

in 100, 1 in 200, even 1 in 500; for potassium, it is usually

lower, often 1 in 50, but it is possible by varying the

sensitivity to use the same dilution for both. Both sodium

and potassium can interfere with each other, therefore, in

standard solution both the elements are added.

Equipment: Systronics provides digital read-out flame

photometer (Mediflame 127/128/129) that provides two

parameters at a time.

Flame Photometer 129

(Courtesy: Systronics)

Microprocessor-based Automation

Systronics flame photometer 129 is a microprocessorbased unit designed for medical applications (Fig. 21.2).

The microprocessor provides automation in operation,

measurements, and end-result presentation. The unit can

do the estimation of sodium (Na), potassium (K), calcium

(Ca) and lithium (Li) in single aspiration of a sample.

FIG. 21.2: Flame photometer

FIG. 21.3: Measurement results can be printed

 immediately or later on

Blood Gases and Electrolytes 559

For user’s convenience, the unit offers three measuring

modes: (i) serum, (ii) urine, and (iii) bio-fluid, general. The

last mode is helpful for analyzing biological samples other

than serum and urine.

Frequently used measurement set-ups can be stored

once (in a battery-back-up memory) and recalled

whenever required with a stroke of a button. This

eliminates the typical chores of instructions required to

be given to a microprocessor-based instrument before it

starts the operation. Facility for restandardization with a

single standard is available to minimize the effect of any

unforeseen drift without going for the full recalibration.

A 4-line 20-character LCD readout provides easy user

interface and presentation of results. A Centronics printer

port is provided for Epson compatible dot matrix/inkjet and

HP compatible laser printers. Printer is optional (Fig. 21.3).

Salient Features

¾ Specifically designed for medical applications

¾ Up to four elements measured with single aspiration

¾ Automatic filter selection (narrow band interference)

¾ Curve fittings for nonlinear range (up to 5 standards)

¾ Calibration stored in memory

¾ Restandardization cuts on full recalibration

¾ Record kept of date and time of analysis

¾ Saved set-ups cut operation steps

¾ Measurement results can be recalled later on for display

and printout (330 max)

¾ Measurement results can be printed (individual, full

Batch, of the day, all in memory)

¾ 4-line, 20-character alphanumeric LCD readout

¾ Centronics printer port for epson compatible dot

matrix/inkjet and HP laser printers

¾ RS-232 interface (optional)

¾ Compressor with built-in air filter and air regulator.

Easy Menu Driven Operation

Operating mode

1. Serum

2. Urine

3. Biofluids (GEN)

Specifically designed for medical use.

Set flame

Ignite flame,

Aspirate DW,

Press enter

Step-by-step guidance for operation.

Set flame

Ignite Flame,

Aspirate DW,

Press enter

Adjust fuel for

best flame stability

< 0578.7)

Digital monitoring of flame stability.

Restandardization

Enter Std. Value

Na = 140

A restandardization facility cuts on full recalibration

Measurement recall

Enter date: 23/02

Enter batch no.: 1

Enter sample no.: 01

Measurement results can be recalled later on for display.

Set-up

1. Directory

2. Call set-up

3. Edit set-up

Saved set-ups drastically cut operation steps.

Technical Specifications

Range of operation

Element Serum Urine BIP-Fluids

Na 100–200 mEq/L 0–250 mEq/L All the four

1:100 dil 1:100 dil elements up

K 0–10 mEq/L 0–100 mEq/L 250 mEq/L

1:100 dil 1:100 dil with 1:100 dil

Li 0–2 mEq/L

1:10 dil

Ca 0–10 mEq/L

1:2 dil

i.  Curve fit software is provided for urine and biofluids.

ii.  Curve fit accuracy: ± 2% f.s.

iii. Suitable dilution for concentrations higher than given in the

above table.

¾ Filters (10 nm typical): Na and K supplied; Li and Ca

(optional)

¾ Reproducibility: ± 2% f.s.

¾ Minimum sample: Approximately 3 mL per element (at

Avarage. time of 4 seconds)

¾ Averaging: 2 to 15 seconds, selectable

560 Concise Book of Medical Laboratory Technology: Methods and Interpretations ¾ Aspiration time: (5 seconds + Average time) per element

+ 4 seconds.

¾ Operating air pressure: 0.45 kg/cm2

 (typical)

¾ Air compressor: With built-in air regulator and air filter

to deliver stable and moisture/oil free air supply

¾ Fuel Gas: LPG

¾ Power Supply: 230 Vac +/– 10%, 50 Hz.

System: The system consists of main unit with one each of

Na and K narrow band filters, a compressor with built-in

air regulator and air filter. Li and Ca filters and printer are

optional (Fig. 21.4).