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1.1 INTRODUCTION:

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a procedure to measure

the partial pressure of oxygen (O2) and carbon dioxide (CO2) gases and the pH (hydrogen ion

concentration) in arterial blood.Blood gas analyzers are used to measure the pH, partial pressureof oxygen (pO2) & partial pressure of carbon dioxide (pCO2) of the body fluids with special

reference to the human blood. Measurements of these parameters are essential for determining the

acid-base balance in the body. Sudden change in the pH & pCO2 can lead to ventricular

hypotension, cardiac arrhythmias or even death. Hence, this indicates the maintenance of the

physiological neutrality of the human blood and consequently the crucial role the Blood gas

analyzer plays in determining the pressures.

1.2 PURPOSE:

Blood gas analysis is used to diagnose and evaluate respiratory diseases and conditions that

influence how effectively the lungs deliver oxygen to and eliminate carbon dioxide from the blood.

The acid-base component of the test is used to diagnose and evaluate metabolic conditions that

cause abnormal blood pH.

Because high concentrations of inhaled oxygen can be toxic and can damage lungs and eyes,

repeated blood gas analysis is especially useful for monitoring patients on oxygen, for

example, premature infants with lung disease, so that the lowest possible inhaled oxygen

concentration can be used to maintain the blood oxygen pressure at a level that supports the

patient. In incubated patients under artificial ventilation, monitoring the levels of arterial carbon

dioxide and oxygen allow assessment of respiratory adequacy so that the rate or depth of

ventilation, the ventilator dead space, or airway pressure can be changed to preserve the patient's

optimal physiologic balance.

The measurement of arterial blood pH and carbon dioxide pressure with subsequent calculation of

the concentration of bicarbonate (HCO3-), especially in combination with analysis of serum

electrolytes, aids in the diagnosis of many diseases. For example, diabetes mellitus is often

associated with a condition known as diabetic acidosis. Insulin deficiency often results in the

excessive production of ketoacids and lactic acid that lower extracellular fluid and blood pH.

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Unabated acid-base disorders are life threatening. Acidosis is associated with severe consequences,

including shock and cardiac arrest, and alkalosis with mental confusion and coma.

2.1 ACID-BASE BALANCE:

The normal pH of the extracellular fluid lies in the range of 7.35-7.45, indicating that the

body fluid is slightly alkaline. When the pH exceeds 7.45, the body is considered to be in a state of

alkalosis. A body pH below 7.35 indicates acidosis. Both acidosis and alkalosis are disease

conditions widely encountered in clinical medicine. Any tendency of the pH of blood to deviate

towards these conditions is dealt with by the following three physiological mechanisms; (i)

buffering by chemical means, (ii) respiration, (iii) excretion, into the urine by the kidneys.

The blood and tissue fluids contain chemical buffers, which react with added acids and bases and

minimize the resultant change in hydrogen ions. They respond to changes in carbon dioxide

concentration in seconds. The respiratory system can adjust sudden changes in carbon dioxide

tension to normal levels in just a few minutes .Carbon dioxide can be removed effectively by

increased breathing, and hence hydrogen concentration can be maintained. The kidney requires

many hours to readjust hydrogen ion concentration by excreting highly acidic or alkaline urine to

enable body conditions to return towards normal.

In order to maintain O2, pCO2 and pH within normal limits, throughout the wide range of body

activity, the rate and depth of respiration vary automatically with changes in the metabolism.

Control of alveolar ventilation takes place by means of chemical as well as nervous mechanisms.

The three important chemical factors regulating alveolar ventilation are the arterial concentrations

of CO2, H+ and O2.Carbon dioxide tension in the blood stream and cerebrospinal fluid is the

major chemical factor regulating alveolar ventilation. The following table lists out the normal

range for pH, pCO2, pO2, total CO2, base excess and bicarbonate(all measurements made at

37C.)

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Table 1:

Parameter Arterial Capillary

Blood

Venous Plasma

Ph 7.37 to 7.44 7.35 to 7.45

pCO2 Men 34 to 35 mmHg 36 to 50 mmHg

Women 31 to 42 mmHg 34 to 50 mmHg

pO2 Resting adult 80 to 90 mmHg 25 to 40 mmHg

Bicarbonate Men 23 to 29 mmol/l 25 to 30 mmol/l

Women 20 to 29 mmol/l 23 to 28 mmol/l

Total CO2(plasma) Men 24 to 30 mmol/l 26 to 31 mmol/l

Women 21 to 30 mmol/l 24 to 29 mmol/l

Base excess Men -2.4 to +2.3 mmol/l 0.0 to +5.0 mmol/l

Women -3.3 to +1.2 mmol/l -1.0 to +3.5 mmol/l

2.2 BLOOD pH MEASUREMENT:

The acidity or alkalinity of a solution depends on its concentration of hydrogen ions.

Increasing the concentration of hydrogen ions makes a solution makes it more acidic, decreasing

the concentration of hydrogen ions makes it more alkaline. The amount of hydrogen ions generally

encountered in solutions of interest is extremely small and, hence , the figure is usually represented

in the more convenient system of pH notation. pH is the measure of hydrogen ion concentration ,

expressed logarithmically. Specifically, it is the negative exponent log of the hydrogen ion

concentration.

pH= -log(H+)

Electrochemical pH determination utilizes the difference in potential occurring between solutions

of different pH separated by a special glass membrane. If the pH of one of the solutions is kept

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constant, so that the potential varies in accordance with the pH of the other solution, then the

system can be used to determine the pH. The device used to effect this measurement is the glass

electrode.

Glass Electrode: The potential (E) of the glass electrode may be written by means of the Nernst

Equation;

E=((-2.3RT)/zF)*(log10[Ci/Co])

E = equilibrium potential (mV);

z = charge on the ion;

(2.3RT)/F = constant (60mV at 37C);

Ci = intracellular concentration ;

Co = extracellular concentration

The Nernst equation is important because it shows what the equilibrium potential would be for one

ion.

For instance; The resting membrane potential is normally ~70mV. So during an action potential Na

channels open their gates briefly and Na rush inside the cell. Na is ionized and carries a positive

charge. So when Na rushes into the cell it makes the inside of the cell more positive. If you were to

break off the gate and allow Na to move freely back and forth, the Nernst equation shows us that

the equilibrium point for Na is ~+65mV.

2.2.1 pH Measurement:

For making pH measurements, the solution is taken in a beaker. A pair of electrodes: one

glass or indicating electrode and the other reference or calomel electrode, are immersed in the

solution. The pH meter is usually equipped with controls for calibration and temperature

compensation.

A measuring silver/silver chloride electrode is encased in a bulb of special pH-sensitive glass and

contains a buffer solution that maintains a constant pH (Figure 2). This glass electrode is placed in

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the blood sample and a potential difference is generated across the glass, which is proportional to

the difference in hydrogen ion concentration. The potential is measured between a reference

electrode (in contact with the blood via a semi-permeable membrane) and the measuring electrode.

Both electrodes must be kept at 37C, clean and calibrated with buffer solutions of known pH.

For very precise work the pH meter should be calibrated before each measurement. For normal use

calibration should be performed at the beginning of each day. The reason for this is that the glass

electrode does not give a reproducible e.m.f. over longer periods of time. Calibration should be

performed with at least two standard buffer solutions that span the range of pH values to be

measured. For general purposes buffers at pH 4 and pH 10 are acceptable. The pH meter has one

control (calibrate) to set the meter reading equal to the value of the first standard buffer and a

second control (slope) which is used to adjust the meter reading to the value of the second buffer.

A third control allows the temperature to be set. Standard buffer sachets, which can be obtained

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from a variety of suppliers, usually state how the buffer value changes with temperature. For more

precise measurements, a three buffer solution calibration is preferred. As pH 7 is essentially, a

"zero point" calibration (akin to zeroing or tarring a scale or balance), calibrating at pH 7 first,

calibrating at the pH closest to the point of interest ( e.g. either 4 or 10) second and checking the

third point will provide a more linear accuracy to what is essentially a non-linear problem. Some

meters will allow a three point calibration and that is the preferred scheme for the most accurate

work. Higher quality meters will have a provision to account for temperature coefficient

correction, and high-end pH probes have temperature probes built in. The calibration process

correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH

scale. After each single measurement, the probe is rinsed with distilled water or de-ionized

water to remove any traces of the solution being measured, blotted with a scientific wipe to absorb

any remaining water which could dilute the sample and thus alter the reading, and then quickly

immersed in another solution.

2.2.2 Electrodes for Blood pH Measurement:

Several types of electrodes can be utilized for the measurement of blood pH. Most common

type of electrode is the syringe electrode, which is preferred for the convenience of taking small

samples of blood anaerobically.

Micro capillary glass electrodes are preferred when it is required to monitor pH continuously. For

example; during surgery. These types of electrodes are especially useful when a very small volume

of the sample is to be analyzed. A micro-electrode for clinical applications requires only 20-25uL

of capillary blood for the determination of pH.

Glass electrode assemblies as normally supplied by manufacturers require that the glass electrode,

as a small bulb, together with either the standard calomel electrode or a potassium chloride-agar

bridge, shall dip into the liquid under examination. Under these conditions a reduction in the

volume of liquid below 5-10 ml. means that the containing vessel becomes so small that the risk of

scratching the thin glass bulb becomes considerable. Rather smaller volumes can safely be used in

the Morton (1930) electrode since only the glass electrode dips into the liquid; contact with the

calomel electrode being through a stop cock at the base of the electrode vessel.

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The pH electrode consists of 2 half cells: the glass electrode and a reference electrode (e.g; calomel

electrode). This unit develops an electrical potential across the glass which is dependent on the

difference in a H+across the glass membrane. This effectively allows measurement of the pH of

the test solution because the pH in the solution on the other side of the membrane is constant.

Other potentials develop in the pH electrode (e.g; liquid junction potential, asymmetry potential &

diffusion potentials) and these are usually not quantified in a particular electrode. The problem is

overcome by standardization and calibration. Standardization refers to the process of requiring that

these potentials are the same when measuring the sample solution and when measuring the

calibrating solutions. In particular, the liquid junction potential must remain unchanged. The

calibrating solutions are chemical standard buffer solutions with a known pH. Many of the

components of the electrode (eg the calomel reference cell) are very temperature sensitive. The

temperature of the measurement must be precisely controlled: usually at 37C.

2.3 MEASUREMENT OF BLOOD pCO2 :

The carbon dioxide partial pressure (pCO2 ) is an indicator of CO2 production and

elimination: for a constant metabolic rate, the pCO2 is determined entirely by its elimination

through ventilation. A high pCO2 (respiratory acidosis, alternatively hypercapnia) indicates under

ventilation (or, more rarely, a hypermetabolic disorder), a low pCO2 (respiratory alkalosis,alternatively hypocapnia) hyper- (or) over ventilation. The pCO2 , along with the pH, can be used

to distinguish among metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory

alkalosis. Hypoventilation exists when the ratio of carbon dioxide production to alveolar

ventilation increases above normal values. Hyperventilation exists when the same ratio decreases.

The following table indicates the various areas of variation of pCO2 within the human body;

Table 2:

LocationpCO2

(Torr)

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http://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Carbon_dioxide


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Outside air - dry air at sea level 0.3

Alveolar air 35

Arteriole blood 40

Venous blood 50

Cells 50

The blood pCO2 is the partial pressure of carbon dioxide of blood taken anaerobically. It is

expressed in mmHg and is related to the percentage CO2 as follows:

pCO2 = Barometric Pressure Water vapor Pressure*(%CO2)/100

At 37C, the water vapor pressure is 47mmHg, so at 750 mm barometric pressure, 5.7% CO2corresponds to a pCO2 of 40mm.

pCO2 is measured by direct potentiometer. In the calculation of results for pCO2, concentration is

related to potential through the Nernst equation. Results are measured at 37C when using

cartridges that require thermal control and corrected to 37C when using cartridges that do not

require thermal control.

2.3.1 Clinical Significance:

pCO2 along with pH is used to assess acid-base balance. pCO2 (partial pressure of carbon

dioxide), the respiratory component of acid-base balance, is a measure of the tension or pressure of

carbon dioxide dissolved in the blood. PCO2 represents the balance between cellular production of

CO2 and ventilator removal of CO2 and a change in pCO2 indicates an alteration in this balance.

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Causes of primary respiratory acidosis (increase in pCO2) are airway obstruction, sedatives and

anesthetics, respiratory distress syndrome, and chronic obstructive pulmonary disease. Causes of

primary respiratory alkalosis (decreased pCO2) are hypoxia (resulting in hyperventilation) due to

chronic heart failure, edema and neurologic disorders, and mechanical hyperventilation.

The Severinghaus (or) carbon dioxide electrode is a modified pH electrode in contact with sodium

bicarbonate solution and separated from the blood specimen by a rubber or Teflon semi-permeable

membrane. Carbon dioxide, but not hydrogen ions, diffuses from the blood sample across the

membrane into the sodium bicarbonate solution, producing hydrogen ions and a change in pH.

Hydrogen ions are produced in proportion to the pCO2 and are measured by the pH-sensitive glass

electrode. As with the pH electrode, the Severinghaus electrode must be maintained at 37C, be

calibrated with gases of known pCO2 and the integrity of the membrane is essential. Because

diffusion of the CO2 into the electrolyte solution is required the response time is slow at 23

minutes.

Method comparisons will vary from site to site due to differences in sample handling, comparative

method calibration and other site specific variables.

Increased pCO2 is caused by:

Pulmonary edema

Obstructive lung disease

Decreased pCO2 is caused by:

Hyperventilation

Hypoxia

Anxiety

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Pregnancy

Pulmonary Embolism (In cases of massive pulmonary embolism, the infracted or non-

functioning areas of the lung assume greater significance and the pCO2 may increase.)

2.4 MEASUREMENT OF BLOOD pO2:

The partial pressure of oxygen in the blood or plasma indicates the extent of oxygen

exchange between the lungs and the blood, and normally, the ability of the blood to adequately

perfuse the body tissue with oxygen. The partial pressure of oxygen is usually measured with a

Polarographic electrode. There is a characteristic polarizing voltage at which any element in

solution is predominantly reduced and in the case of oxygen, it is 0.6 to 0.9 V. In this voltagerange, it is observed that the current flowing in the electrochemical cell is proportional to the

oxygen concentration in the solution. pO2 (Partial Pressure of Oxygen) reflects the amount of

oxygen gas dissolved in the blood. It primarily measures the effectiveness of the lungs in pulling

oxygen into the blood stream from the atmosphere.

Most of the Blood gas analyzers utilize an oxygen electrode for measuring oxygen partial pressure.

This type of electrode consists of a platinum cathode, a silver/silver chloride anode in an

electrolyte filling solution and a polypropylene membrane.

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The Polarographic (Clark) oxygen electrodemeasures the oxygen partial pressure in a blood or gas

sample. A platinum cathode and a silver/silver chloride anode are placed in a sodium chloride

electrolyte solution, and a voltage of 700 mv is applied (Figure 1). The following reactions occur.

At the cathode: O2 + 2H2O + 4e = 4OH

In the electrolyte: NaCl + OH = NaOH + Cl.

At the anode: Ag + Cl = AgCl + e.

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Electrons are taken up at the cathode and the current generated is proportional to oxygen tension. A

membrane separates the electrode from blood, preventing deposition of protein but allowing the

oxygen tension in the blood to equilibrate with the electrolyte solution. The electrode is kept at a

constant temperature of 37C and regular checks of the membrane are required to ensure it is not

perforated or coated in proteins. Sampling two gas mixtures of known oxygen tension allows

calibration.

The measurement at the current developed at the oxygen electrode due to the partial pressure of

oxygen presents special problems. The difficulty arises because of the extremely small size of the

electrical signal. The sensitivity is typically of the order of 20pA per Torr for most instruments.

Measurement of oxygen electrode current is made by using high input impedance, low noise, and,

low current amplifiers. Field effect transistors usually form the input stage of the pre-amplifiers.

Elevated pO2 levels are associated with:

Increased oxygen levels in the inhaled air

Polycythemia

Decreased pO2 levels are associated with:

Decreased oxygen levels in the inhaled air

Anemia

Heart decompensation

Chronic obstructive pulmonary disease

Restrictive pulmonary disease

Hypoventilation

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3.1 COMPLETE BLOOD GAS ANALYZER:

Blood gas analyzers quantify and analyze the amount of various gases within blood. They

operate in a similar way to blood glucose monitors. A chemical reagent is mixed with a sample of

blood, which is examined using either photo-optical sensors or electrochemical sensors. The

readings of the blood sample are compared against a calibration reagent to determine the result.

Block Diagram of Blood Gas Analyzer:

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3.1.1 Signal Path:

Blood gas analyzers have multiple sensors that are driven through an amplifier and a

multiplexer to an analog-to-digital converter (ADC). The data is processed in the microcontroller,

which is connected to a PC or other instruments through RS-232, USB, or Ethernet. A digital-to-

analog converter (DAC) is often used to calibrate the sensor amplifiers to maximize the sensitivity

of the electrodes.

3.1.2 Applications:

Blood gas analyzers are often used for simple blood tests, as well as for sophisticated suites

of tests that allow physicians to monitor patient health in various settings. In addition to clinical

diagnostics, blood gas analyzers are finding use in respiratory therapy and point-of-care

diagnostics. These markets require device miniaturization and sophistication. Small, sometimes

handheld, form factors are needed that integrate multiple testing capabilities, such as blood glucose

and electrolyte analysis. This testing versatility increases the cost effectiveness of the device.

Blood gas analyzers are used in most pathology and biochemistry laboratories in hospitals to

analyze blood samples for CO2 and Oxygen levels in order to detect respiratory and metabolic

issues. The routine calibration of the Analyzer using a calibration gas mixture is essential to ensure

its continued accuracy.

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3.1.3 Display:

Modern blood gas analyzers increasingly employ a touch screen in combination with a

graphical user interface (GUI) to make the programming process more intuitive. Visible, audible,

and haptic responses to user inputs help designers improve the user experience. Advanced touch-

screen controllers from Maxim offer haptic feedback, touch processing to reduce bus traffic, and

autonomous modes for precision gesture recognition.

3.1.4 Precautions:

The syringe used to collect the sample for a blood gas analysis must contain a small

amount of heparin to prevent clotting of the blood. It is very important that air be excluded from

the syringe both before and after the sample is collected. The syringe must be filled completely and

never exposed to air. For transportation, the syringe should be capped with a blind hub, placed on

ice, and immediately sent to the laboratory for analysis to guarantee the accuracy of the results.

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A blood gas analysis requires a sample of arterial blood in order to evaluate gas exchange by the

lungs. Arterial puncture is associated with a greater risk of bleeding than vein puncture. The test

may be contraindicated in persons with a bleeding disorder such as hemophilia or low platelet

count. During the arterial puncture, the patient may feel a brief throbbing or cramping at the

puncture site. In cases where the primary concern is ascertaining that the blood is adequately

oxygenated, a pulse oximeter may be used in lieu of arterial blood gas analysis. Medical personnel

must follow standard precautions for prevention of exposure to blood borne pathogens when

performing arterial blood collection.

3.2 PROCESS OF TESTING THE SAMPLE:

The sample of choice for blood gas analysis is arterial blood. This is usually collected fromthe radial artery in the wrist, but in cases where no radial pulse is obtained, the femoral or brachial

artery may be used. The sample may also be collected from an arterial line after flushing the line to

remove excess anti-coagulant and fluid. In neonates and in adults when arterial puncture is

contraindicated or unsuccessful, a capillary blood sample may be used.

The sample is inserted into an analytical instrument that uses electrodes to measure the

concentration of hydrogen ions (H+), which is reported as pH, and the partial pressures of oxygen

[PO2] and carbon dioxide PO2gases. The pH-measuring electrode consists of a special glassmembrane that is selectively permeable to hydrogen ions. An electical potential develops across

the inner and outer surfaces of this membrane that is related to the log of hydrogen ion activity in

the sample. A Severinghaus electrode is used to measure PCO2 . The measuring principle is the

same as for hydrogen ions, except that the electrode tip is covered with a gas permeable

membrane, so that the pH change is proportional to carbon dioxide diffusing from the sample to

the electrode surface. The PO2 is measured using a polarographic (Clark) electrode. Oxygen

diffuses from the sample to the cathode, where it is reduced to peroxide ions. The electrons come

from a silver anode that is oxidized, generating current in proportion to oxygen concentration at the

cathode. Electrode signals are dependent upon temperature as well as concentration, and all

measurements are performed at 37C. Since the in-vivo pH and levels of oxygen and carbon

dioxide are temperature dependent, results may need to be adjusted for the patient's actual

temperature. Portable blood gas analyzers are available that can be used at the bedside.

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Blood gas analyzers calculate blood bicarbonate concentration using the formula: pH = 6.1 + Log

bicarbonate/.0306 x PCO2 . They also calculate oxygen content, total carbon dioxide, base excess,

and percent oxygen saturation of hemoglobin. These values are used by physicians to assess the

extent of hypoxia and acid-base imbalance.

3.2.1 Aftercare:

After the blood sample has been taken, the health care practitioner or patient applies

pressure to the puncture site for about 10 minutes or until bleeding has stopped, after which a

dressing is applied. The patient should rest quietly while applying pressure to the puncture site and

be observed for signs of bleeding or impaired circulation at the puncture site.

3.2.2 Complications:

Complications posed by the arterial puncture are minimal when the procedure is performed

correctly, but may include bleeding or delayed bleeding or bruising at the puncture site, or, rarely,

impaired circulation around the puncture site.

3.3 TEST RESULTS:

3.3.1 Normal Results:

The following normal results are for arterial blood at sea level (at altitudes of 3,000 feet and

above, the values for oxygen are lower) for an assumed fluid sample of a patient;

Partial pressure of oxygen (pO2 7500 mm Hg). Note that PO2values normally decline

with age.

Partial pressure of carbon dioxide pCO2 355 mm Hg.

pH: 7.35 - 7.45

Oxygen content (O2CT): 153 volume%.

Oxygen saturation (SaO2): 94%00%.

Concentration of bicarbonate (HCO3/sup>): 226 millimols per liter (mEq/liter).

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Total CO2 is often reported with blood gas analysis results and is defined as the sum of carbonic

acid and bicarbonate concentrations. Normally, the ratio of bicarbonate to carbonic acid at

physiological pH is about 20:1, thus, the total CO2 is normally about 5% higher than the

bicarbonate value.

3.3.2 Abnormal results:

Values that differ from the normal values may indicate the presence of respiratory,

metabolic, or renal diseases.

For most clinical decisions, the bicarbonate value, PCO2 , and pH are used to evaluate acid-base

status. The pH value defines the magnitude of the disturbance and the bicarbonate and PCO2

determine the cause. The bicarbonate level is under the control of the kidneys, which may increaseor decrease bicarbonate blood levels in response to pH changes. Bicarbonate is also the principal

blood buffer anion, and it functions as the conjugate base to increase pH. PCO2 is the respiratory

component because it is regulated by the lungs. It is determined by the concentration of dissolved

carbon dioxide (anhydrous carbonic acid) and is the principal acid component of the blood.

Abnormal results are classified on the basis of pH and whether the abnormal pH is caused by the

metabolic or respiratory component. pH 7.45 indicates alkalosis.

Metabolic or non-respiratory acidosis is characterized by pH


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severe pneumonia and pulmonary fibrosis; thoracic conditions such as multiple broken ribs.

Respiratory acidosis is also caused by neuromuscular disease, and by depression of the respiratory

center in the brain due to drugs, head trauma, or cranial tumor. The blood gas analysis results may

deviate only slightly from normal values, and pH may even fall within the normal range

(compensated respiratory acidosis) in cases of chronic compared to acute acidosis.

Metabolic alkalosis is caused by excess blood bicarbonate and usually involves a renal factor.

Metabolic alkalosis is characterized by pH >7.45 and elevated [HCO3-]. The PCO2 is usually

elevated due to respiratory compensation. Metabolic alkalosis can be caused by mineralcorticoid

excess (e.g. Cushing's or Conn's syndromes), which promotes increased acid excretion and

bicarbonate retention by the kidney. Other causes are diuretic therapy, vomiting,

severe dehydration, hypokalemia (low blood potassium), and hypo parathyroidism.

Respiratory alkalosis is caused by hyperventilation. The pH is >7.45 and the PCO2 is low. If the

kidneys are functioning normally and given sufficient time, the HCO3- will be decreased in

compensation. Respiratory alkalosis may be caused by hyperventilation psychologically induced

(anxiety), by drugs that stimulate the respiratory center, excessive ventilation therapy, and mild

hypoxia.

A decrease in PO2 is a sensitive measure of respiratory function and hypoxia. In addition to

ventilation defects that also result in increased PCO2, PO2 will be low in persons with poor ratios

of ventilation to perfusion; mild emphysema and other gas diffusion defects; pulmonary arterial-

venous shunts; and those breathing air with a low oxygen content. Elevated PO2 is caused by

excessive administration of oxygen which can lead to optic nerve damage and acidosis by

displacing hydrogen ions from hemoglobin.

It is important to note that in cases of carbon monoxide poisoning the PO2: will be normal, but

life-threatening hypoxia may be present. Blood gas analyzers calculate the oxygen saturation of

hemoglobin from PO2, temperature, and pH. In cases of CO poisoning, the calculation will be

falsely elevated. Accurate assessment of hypoxia in CO poisoning requires direct measurements of

carboxy hemoglobin and oxygen saturation of hemoglobin by oximetry or colorimetry methods.

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