appxav1.pdf

Cristalli, C., Manzoni, A. “Basics of Blood Gas Instrumentation.”

The Biomedical Engineering Handbook: Second Edition.

Ed. Joseph D. Bronzino

Boca Raton: CRC Press LLC, 2000

A PPENDIX

Basics of Blood Gas

Instrumentation

A.1

pH and PCO

Electrochemical Sensors

A.2

Electrochemical Sensors of Dissolved Oxygen

A.3

Equivalent Electronic Circuit for a Potentiometric

Electrochemical Cell

Cristina Cristalli

A.4

General Aspects of a Mearuing Device

A.5

Factors which Inﬂuence the Measurements

Instrumentation Laboratory

A.6

New Generations of pH and Blood Gas Sensors

Angelo Manzoni

A.7

Continuous Monitoring of Blood Gases

Instrumentation Laboratory

A.8

Optical Sensors in Blood Gas Analyzers

Modern anesthesiology and intensive care, neonatal care, and emergency medical facilities depend heavily

upon the frequent measurements of a selected group of critical care analytes in blood. Tests recognized

as being essential for patient management include pH and blood gas (O

partial pressures) and

electrolytes (sodium, potassium, and calcium) measurements. Clinical care clinicians use information

from blood gas reports in a variety of ways for patient interpretation and to establish plans for patient

therapy. The measurement of pH and PCO

and CO

, for example, represents any disturbances in acid–base

balance, and the calculation of the bicarbonate concentration from the Henderson–Hasselbalch equation

gives the most complete and consistent model of acid–base physiology for use in infants, children, and

adults with a variety of disorders. Several efforts, with different levels of success, have been made in order

to determine the primary variables to be measured directly from whole blood in critical and emergency

situations. In fact, in a different way from the clinical chemistry routine, some analytes must be deter-

mined quickly

at the patient’s bedside or emergency site without modifying the sample. Instru-

mentation for these measurements should be as effective as the

in vitro

in vivo

monitoring and should be capable

of making the same determinations as the clinical laboratory

Electrochemical sensors have been demonstrated to be the most appropriate way to achieve the

objective of the measurement of blood components without any sample treatment. In 1905, the ﬁrst pH

glass electrochemical determination was demonstrated, and the ﬁrst practical pH glass electrode for

anaerobic blood pH determination was the capillary electrode of McInnes and Belcher introduced in

early 1930. Blood pH was not considered interesting from a clinical point of view until 1952, when a

polio epidemic was seen in Copenhagen and patients required artiﬁcial ventilation. Astrup adopted the

measurement of pH and CO

to teach students how to change the

patient’s ventilation towards a more normal level. In August 1954, Stow described his concepts and

construction of a CO

content for calculation of the PCO

electrode capable of use in measuring blood PCO

, sensitive to CO

but insensitive

to acids and alkali in ﬂuids of known PCO

. In 1953, Clark presented the ﬁrst polyethylene-covered

oxygen electrode. This marked an historic turning point in blood gas analysis and all of respiratory

physiology. In 1959, the ﬁrst example of a three-electrode blood gas apparatus containing a Beck-

electrode, and a McInnes/Belcher pH electrode

was described. After a few years (early 1960s), Thomas Ross, the founder of Instrumentation Laboratory,

introduced the ﬁrst commercial three-electrode system in the market (IL 113). Further development

brought an interesting level of automation to blood gas analysis, satisfying user demands at the beginning

of the 1970s when Radiometer introduced the ﬁrst automated blood gas analyzer in 1973, ABL 1. Corning

Diagnostics (now Bayer), Nova Biomedical, AVL, and Eschweiler are companies that have used the blood

gas analyzer before the introduction of the portable blood gas analyzer by Mallinckrodt Sensor Systems

(now IL), iSTAT (now Abbot/HP) and Diametrics. The portable blood gas analyzer was developed in

order to bring the analyzer closer to the patients, the so-called Near Patient or Point of Care testing.

Sensor technology has notably improved in the development and manufacturing of sensors for the

direct determination of components of diagnostic interest such as sodium, potassium, calcium, lithium,

magnesium, and chloride for the electrolytes, and glucose, lactate, urea, and creatinine for the substrates.

This makes it possible to not only determine the blood pH and gases, but to do this together to other

analytes so that a complete diagnostic proﬁle of the patient can be obtained.

In general, electrochemical sensors transform the information arising from the concentration of

chemical species in solution or the interaction or reaction of a substance with its substrate to an electrical

signal of an equivalent and measurable quantity, such as a potential, current, or charge transfer. Biomed-

ical sensors work as an interface between the biological system and the electrical one, without modifying

the characteristics of the two systems, guaranteeing the reliability of the resulting information. This

appendix will go over the basic concepts of blood gas instruments describing the principles for the

measurements of the signals coming from different sensors and the main criteria for data acquisition

and reduction. It will conclude with an overview of the new trends in sensor technologies.

man/Clark oxygen electrode, a Stow/Severinghaus CO

A.1

pH and PCO 2 Electrochemical Sensors

Electrochemical potentiometric systems constitute the main category of sensors for the determination

of ionic and molecular species in blood, such as pH, PCO

, and electrolytes. pH and sodium

potentiometric sensors are based on different glass membrane compositions obtained through the mix-

ture of metal–alkali oxides. Potassium, calcium, chloride, lithium, and magnesium potentiometric sensors

are based on polymeric matrix membranes containing the electroactive material that makes the mem-

brane selective for the determination of the respective ionic species in the blood. The potentiometric

sensors obtained by incorporating these membranes in their structures are called Ion Selective Electrodes

(ISE). They make possible the deﬁnition of an electrochemical system in which a potential difference is

seen at the sensor (ISE) when it is in contact with the sample to be measured (electrolytic solution). The

resulting electrical signal is dependent on the concentration, or more correctly on the ionic activity, of

the particular ionic species in the sample.

Figure A.1 s chematically illustrates the main components of a potentiometric pH sensor and its

reference electrode. In order to be able to measure the electrical signal present at the sensor/sample

interface, the Ion Selective Electrode (indicator electrode) should be connected to one of the two input

terminals of the potential measuring device, the Voltmeter, together with a second electrode, the reference

electrode, connected to the second input terminal of the Voltmeter. The structure and characteristic of

the reference electrode that is required to complete the potentiometric measuring system guarantees that

the change of the measured electrical signal depends only on the concentration of the ionic species in

solution interacting with the ISE membrane surface. The relationship between the ionic concentration

of the species in solution (a) and the measured electrical signal (E) is represented by the Nernst equation:

, PO

E = E° + K log a

where E° is the standard potential of the electrochemical system including the reference electrode potential

and K is the slope or the sensitivity of the electrochemical system expressed in mV/log a.

FIGURE A.1

pH sensor and reference electrode connected to the measuring device.

FIGURE A.2

PCO

sensor structure.

In the actual blood gas analytical instrumentation, the determination of CO

partial pressure (PCO

)

is performed with a potentiometric sensor, the so-called Stowe-Severinghaus sensor.

The experimental setup represented in Fig. A.2 s hows the schematic diagram of the electrochemical

system for the measurement of a molecular species in solution such as CO

that enters the blood remains in plasma; the balance enters the red cell. A small

amount of the plasma CO

Only 5% of the CO

forms carbonic acid and the balance remains as dissolved CO

(dCO

) which

exerts a pressure (Henry’s law) measured as PCO

, in equilibrium in the sample, diffuses

through a gas permeable membrane, and changes the pH of the thin layer of bicarbonate-based electrolyte.

. The dCO

H +

–

(A.1)

dCO 2

→

CO 2

H 2 OH 2 CO 3

↔↔

HCO 3

The pH variation of the bicarbonate solution, measured by a pH sensor, is directly proportional to

the logarithm partial pressure of the CO

dissolved in the sample.

The logarithmic output of these sensors creates problems of inaccuracy because small uncertainties

in the measurement of the potential can cause signiﬁcant variations in the determination of the concen-

tration of the species to measure. Precision and accuracy in the measurement can be reached with

FIGURE A.3

Schematic view of the structure of an amperometric oxygen sensor.

appropriate calibration procedures. The calibration procedures should be designed in order to minimize

the inﬂuence of the reference electrode and the interference of other ionic species present in the sample

on the measured potential. pH sensors are calibrated with buffer solutions having pHs referred to as

NIST primary standards: phosphate buffers at pH = 7.392 and 6.839 at 37°C, with an ionic strength of

0.1 mole/Kg. Gas sensors (PCO

) are generally calibrated with humidiﬁed gas mixtures taking

into account the barometric pressure and temperature of each gas as well as the vapor pressure of water.

and PO

A.2

Electrochemical Sensors of Dissolved Oxygen

) mea-

surement is made by an electrochemical amperometric sensor introduced by Clark in the beginning of

the 1950s. Most of the total oxygen content in blood is bound to hemoglobin. The remaining is dissolved

in plasma, and PO

In the majority of commercially available blood gas analyzers, the oxygen partial pressure (PO

(the measured quantity) is directly related to the concentration of O

dissolved in

plasma. Figure A.3 s hows a schematic diagram of a typical amperometric oxygen sensor.

The amperometric sensor for the determination of the partial pressure of molecular oxygen in an

aqueous solution depends on the chemical reduction of oxygen at a metallic or other conducting surface

(working electrode) when a potential difference between the reference electrode and the working electrode

is applied by an external voltage source. This constitutes the difference between amperometric and

potentiometric sensors; an external energy source drives an electrochemical reaction of the analyte

resulting in a current that is proportional to the analyte partial pressure in the amperometric case, while

potentiometric measurements such as pH and PCO

take place without any applied voltage and no

current ﬂow in the measuring circuit.

The oxygen sensor measures the current originated during the reduction of oxygen according to the

reaction:

4e –

4H +

(A.2)

O 2

↔

H 2 O

to a platinum electrode to which a proper voltage to support the reaction (ca. –0.7 V) vs. an Ag/AgCl

reference/anode electrode) has been applied.

The platinum sensor is covered by an oxygen permeable membrane that protects it from a possible

poisoning caused by the presence of different substances present in the sample, such as the proteins, and

controls the oxygen diffusion at the platinum surface.

Silver/silver chloride (Ag/AgCl) is the most common reference electrode used in the amperometric

oxygen sensor. In fact, the chloride ions contained in the electrolyte reacts with the silver anode to form

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