the analytical unit and the control unit: The analytical unit

consists of an ISE system, a photometric measuring system,

and a CPU. The control unit consists of a monitor (CRT), a

keyboard, and a printer. Features of the Hitachi 911 include;

STAT results available quickly, ready to use 24 hours per

day, 360 tests/hr throughput (photometric), 720 tests/

hr throughput with ISE (Ka+

, Na+

, Cl–

), 46 programmable

tests, automatic calibration, and refrigerated storage for 64

reagent containers.

Assay Types

¾ Monochromatic

¾ Bichromatic

Enzymology 547

¾ Endpoint

¾ Kinetic

¾ Simultaneous endpoint and kinetic

¾ Endpoint with sample blanking

¾ Kinetic with sample blanking

¾ Simultaneous double endpoint and double kinetic.

Sampling System

¾ Photometric 360 tests/h

¾ Photometric and ISE 760 tests/h

¾ Sample volume per test: 3–50 mL

¾ Sampling rate: Once every 10 seconds for photometric

chemistries, once every 20 seconds for ISE

¾ Bar code reader formats: Coda bar, interleaf 2 of 5, code

39, code 128.

ISE System

¾ Sample volume:15 mL

¾ Photometric and ISE 760 tests/h.

Reagent System

Reagent dispense volume: 25–300 mL per reagent (in 1 mL

increments).

Reagent storage capacity: 2 compartments (12 C or less)

32 position each for reagent. Each reagent compartment

has an additional position #33 for hitergent total reaction

volume/test: 250–500 mL.

Quality Control Criteria given with Clinical Chemistry

Chapter apply to Enzymology section also.

FIG. 20.1: Roche Hitachi 911 chemistry analyzer

21

Blood Gases and Electrolytes

C H A P T E R

BLOOD GASES

Introduction

Reasons for obtaining blood gases:

1. Assessment of adequacy of oxygenation

2. Assessment of adequacy of ventilation

3. Assessment of acid-base status by measuring the

respiratory and non-respiratory components.

Reasons for using arterial blood rather than venous blood

to measure blood gases:

1. Arterial blood is a good way to sample a mixture of

blood that has come from various parts of the body.

a. Venous blood in an extremity gives information

mostly about that extremity. The metabolism in

the extremity can differ from the metabolism in

the body as a whole. This difference is accentuated.

i. In shock, when the extremity is cold or under

perfused

ii. With local exercise of extremity, as opening

and closing the fist

iii. In local infection of the extremity.

b. Blood from a central venous catheter usually is

an incomplete mix of venous blood from various parts of the body. For a sample of completely

mixed blood, a sample would have to be obtained

from the right ventricle or pulmonary artery, and

even then information is not obtained about how

well the lungs are oxygenating the blood.

2. Arterial blood gives the added information of how well

the lungs are oxygenating the blood.

a. If it is known that arterial O2 concentration is

normal (indicating that the lungs are functioning

normally), but the mixed venous O2 concentration is low, it can be inferred that the heart and

circulation are failing.

b. Oxygen measurements of central venous catheter

blood can tell if the tissues are getting oxygenated, but they do not separate the contribution of

the heart from the lungs. If central venous catheter blood has a low O2 concentration, it means

either that:

 i. The lungs have not oxygenated the arterial

blood well, so that venous blood has a low concentration, or

 ii. The heart is not circulating the blood well. In

this case, the tissues of the body must take more

than the usual amount of O2 from each cardiac

cycle because the blood is flowing slowly. This

produces a low venous O2 concentration.

Note: The site of arterial puncture must satisfy three

requirements:

1. Available collateral blood flow

2. Superficial or easily accessible

3. Periarterial tissues (should be nonsensitive).

The radial artery satisfies the criteria tested above,

although the brachial and femoral are also arteries of

choice.

Procedure for Obtaining Arterial Blood Sample

1. Place the patient either in a sitting or supine position.

2. Elevate the wrist with a small pillow and ask the

patient to extend fingers downward (this will flex

the wrist and move the radial artery closer to the

surface).

3. Palpate the artery and rotate the patient’s hand back

and forth until a good strong pulse is felt.

Blood Gases and Electrolytes 549

4. Swab the area liberally with an antiseptic agent such

as betadine.

5. Optional: Anesthetize the area with a small amount

of 1% xylocaine (approximately ¼ mL or less). This

allows a second attempt without undue pain if the

first attempt is a failure.

6. Using a 20- or 21-gauge needle, make the puncture

and then attach the preheparinized 12 mL syringe

once the artery has been entered.

7. Pull the plunger on the syringe (being careful not to

accidentally pull the needle out of the artery) and

collect a 3 to 5 mL sample.

8. Withdraw needle and place a 4” × 4” absorbent

bandage over the puncture site and maintain pressure with two fingers for a minimum of 2 minutes.

9. Meanwhile, any air-bubbles in the blood sample

should be expelled as quickly as possible; the syringe

should be capped and gently rotated to mix heparin

with blood.

10. If the sample is not going to be analyzed for 15–20

minutes, place it in an icewater container until it can

be analyzed.

Clinical Alert

1. Arterial gases will not indicate to what degree the

patient is suffering from an abnormality. For this

reason, the vital signs and mental function of the

patient must be used as guides to determine adequacy

of tissue oxygenation.

2. Arterial puncture site must have pressure applied and

be watched carefully for bleeding.

3. Blood for gases (and electrolytes) must be drawn

without trauma and be protected from room air at all

times. Be aware that air bubbles in the syringe will also

change gas values.

Blood Gas Symbols

Large capital letters are used as primary symbols for

blood.

C = Concentration of gas in blood

S = Percent saturation of hemoglobin with CO2 or O2

Q = Volume of blood

QT = Volume of blood per unit time

P = Gas pressure or partial pressure of a gas in a gas

mixture or in blood.

To indicate whether blood is capillary, venous, arterial,

lower case letters are used as subscripts:

v = Venous blood

a = Arterial blood

c = Capillary blood

s = Shunted blood.

Combination of Symbols

PO2 = Oxygen tension or partial pressure of oxygen.

PvO2 = Venous oxygen tension or partial pressure of

oxygen in venous blood.

PaO2 = Arterial oxygen tension or partial pressure of

oxygen in arterial blood.

PCO2 = Partial pressure of carbon dioxide.

PaCO2 = Partial pressure of carbon dioxide in arterial

blood.

PvCO2 = Partial pressure of carbon dioxide in venous

blood

SO2 = Oxygen saturation

SaO2 = Percent saturation of oxygen in arterial blood

SvO2 = Percent saturation of oxygen in venous blood

TCO2 = Total carbon dioxide content

Blood Gases, Arterial (ABG) Blood

Normal Values

Must be corrected for body temperature.

SI units

pH

Adults 7.35–7.45 7.35–7.45

Panic values < 7.2 and > 7.6 < 7.2 and > 7.6

Children

Birth–2 months 7.32–7.49 7.32–7.49

2 months–2 years 7.34–7.46 7.34–7.46

Over 2 years 7.35–7.45 7.35–7.45

PaCO2 35–45 mm Hg 4.7–6.0 kPa

Panic values < 20 mm Hg < 2.7 kPa

PaO2 75–100 mm Hg 10.0–13.3 kPa

Panic values < 40 mm Hg < 5.3 kPa

HCO3 22–26 mEq/L 22–26 mmol/L

Panic values < 10 mEq/L < 10 mmol/L

O2 saturation 96–100% 0.96–1.00

Panic values < 60% < 0.60

Oxyhemoglobin dissociation

Curve No shift

Blood Gases, Capillary Blood

Normal Values

Must be corrected for body temperature.

550 Concise Book of Medical Laboratory Technology: Methods and Interpretations SI units

pH

Adults 7.35–7.45 7.35–7.45

Panic values < 7.2 or > 7.6 < 7.2 or > 7.6

Children (arterialized capillary sample)

Birth–2 months 7.32–7.49 7.32–7.49

2 months–2 years 7.34–7.46 7.34–7.46

Over 2 years 7.35–7.45 7.35–7.45

PCO2 26.4–41.2 mm Hg 3.5–5.4 kPa

Panic values < 20 mm Hg < 2.7 kPa

> 70 mm Hg

PO2 75–100 mm Hg 10.0–13.3 kPa

Panic values < 40 mm Hg < 5.3 kPa

HCO3 22–26 mEq/L 22–26 mmol/L

Panic values < 10 mEq/L < 10 mmol/L

O2 saturation 96–100% 0.96–1.00

Panic value < 60% < 0.60

Oxyhemoglobin

Dissociation

Curve No shift

Blood Gases, Capillary Blood

Normal Values

Must be corrected for body temperature.

SI Units

pH

Adults 7.35-7.45 7.35-7.45

Panic Values <7.2 or >7.6 <7.2 or >7.6

Children (arterialized capillary sample)

Birth-2 months 7.32-7.49 7.32-7.49

2 months-2 years 7.34-7.46 7.34-7.46

Over 2 years 7.35-7.45 7.35-7.45

PCO2 26.4-41.2 mm Hg 3.5-5.4 kPa

Panic values <20 mm Hg <2.7 kPa

>70 mm Hg

PO2 75-100 mm Hg 10.0-13.3 kPa

Panic values <40 mm Hg <5.3 kPa

HCO3 22-26 mEq/L 22-26 mmol/L

Panic values <10 mEq/L <10 mmol/L

O2 Saturation 96-100% 0.96-1.00

Panic value <60% <0.60

Blood Gases, Venous Blood

Normal Values

Must be corrected for body temperature.

SI units

pH 7.32–7.43 7.32–7.43