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ARTERIAL BLOOD GASES

Dr. Aidah Abu Elsoud AlkaissiAn-Najah National UniversityNursing College

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Arterial Blood Gases

In an ABG test, a sample of arterial blood is drawn and analyzed to help determine the quality and extent of pulmonary gas exchange and acid–base status.

The ABG test measures PaO2, SaO2, PaCO2, pH, and the bicarbonate (HCO3) level.

The procedure involves obtaining arterial blood from a direct arterial puncture or from an arterial line often placed in the radial artery.

More recent technology allows the continuous monitoring of ABGs using a fiberoptic sensor placed in the artery.

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Normal ABG values

Normal Values for an Arterial Blood Gas

PaO2: 80–100 mm Hg SaO2: 93%–99% pH: 7.35–7.45 PaCO2: 35–45 mm Hg HCO3: 22–26 mEq/L

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MEASURING OXYGEN IN THE BLOOD

Oxygenation may be measured using an ABG by evaluating the PaO2 and the SaO2.

As mentioned previously, only 3% of oxygen is dissolved in the arterial blood, and the remaining 97% is attached to hemoglobin in the red blood cells.

The normal PaO2 is 80 to 100 mm Hg at sea level (barometric pressure of 760 mm Hg).

For people living at higher altitudes, the normal PaO2 is lower because of the lower barometric pressure. PaO2 tends to decrease with age.

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MEASURING OXYGEN IN THE BLOOD

Patients who are 60 to 80 years of age, a PaO2 of 60 to 80 mm Hg is normal.

An abnormally low PaO2 is referred to as hypoxemia.

Hypoxemia may result from many conditions, which are most commonly grouped according to their origin: intrapulmonary (disturbances in the lung), intracardiac (disturbance of flow to or from the heart, which impedes pulmonary flow or function), or perfusion deficits (inadequate perfusion of the lung tissues, which causes decreased oxygen uptake from the alveoli).

The normal SaO2 ranges between 93% and 97%.

SaO2 is an important oxygenation value to assess because most oxygen supplied to tissues is carried by hemoglobin.

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MEASURING pH IN THE BLOOD

The pH is a measure of the hydrogen ion concentration in the blood and provides information about the acidity or alkalinity of the blood.

A normal pH is 7.35 to 7.45. As hydrogen ions accumulate, the pH drops, resulting in acidemia.

Acidemia refers to a condition in which the blood is too acidic.

Acidosis refers to the process that caused the acidemia.

A decrease in hydrogen ions results in an elevation of the pH and alkalemia.

Alkalemia refers to a condition in which the blood is too alkaline.

Alkalosis refers to the process that causes the alkalemia.

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Clinical Terminology: Acid–Base

Acid: A substance that can donate hydrogen ions (H+). Example: H2CO3 (an acid) → H+ + HCO3

Base: A substance that can accept hydrogen ions, H+; All bases are alkaline substances. Example: HCO3 (base) + H+ → H2CO3

Acidemia: Acid condition of the blood in which the pH is <7.35

Alkalemia: Alkaline condition of the blood in which the pH is >7.45

Acidosis: The process causing acidemia

Alkalosis: The process causing alkalemia

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Acids

An acid is a substance that can donate a hydrogen ion (H+) to a solution.

There are two different types of acids, volatile acids and nonvolatile acids.

Volatile acids are those that can move between the liquid and gaseous states.

Once in the gaseous state, these acids can be removed by the lungs.

The major acid in the blood serum is carbonic acid (H2CO3).

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Acids

This acid is broken down into carbon dioxide and water by an enzyme produced in the kidneys.

Nonvolatile acids are those that cannot change into a gaseous form and therefore cannot be excreted by thel ungs.

They can only be excreted by the kidneys (a metabolic process).

Examples of nonvolatile acids are lactic acid and ketoacids.

An acid–base disorder may be either respiratory or metabolic in origin.

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ConditionRespiratory AcidosisPaCO2 >45 mm HgpH <7.35

Possible causesCentral nervous system depressionHead traumaOversedationAnesthesiaHigh cord injuryPneumothoraxHypoventilationBronchial obstruction and atelectasisSevere pulmonary infectionsHeart failure and pulmonary edemaMassive pulmonary embolusMyasthenia gravisMultiple sclerosis

Signs and symptoms DyspneaRestlessnessHeadacheTachycardiaConfusionLethargyDysrhythmiasRespiratory distressDrowsinessDecreased responsiveness

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Respiratory AlkalosisPaCO2 <35 mm HgpH >7.45

Possible causes Anxiety and nervousness Fear Pain Hyperventilation Fever Thyrotoxicosis Central nervous system

lesions Salicylates Gram-negative septicemia Pregnancy

Signs and symptoms Light-headednessConfusionDecreased concentrationParesthesiasTetanic spasms in the arms and legsCardiac dysrhythmiasPalpitationsSweatingDry mouthBlurred vision

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Metabolic AcidosisHCO3 <22 mEq/LpH <7.35

Possible causes Increased acids Renal failure Ketoacidosis Anaerobic metabolism Starvation Salicylate intoxication Loss of base Diarrhea Intestinal fistulas

Signs and symptomsHeadacheConfusionRestlessnessLethargyWeaknessStupor/comaKussmaul respirationNausea and vomitingDysrhythmiasWarm, flushed skin

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Metabolic AlkalosisHCO3 >26 mEq/LpH >7.45

Possible causes Gain of base Excess use of bicarbonate Lactate administration in dialysis Excess ingestion of antacids Loss of acids Vomiting Nasogastric suctioning Hypokalemia Hypochloremia Administration of diuretics Increased levels of aldosterone

Signs and symptomsMuscle twitching and crampsTetanyDizzinessLethargyWeaknessDisorientationConvulsionsComaNausea and vomitingDepressed respiration

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Acidosis & Alkalosis

An excess of either kind of acid results in acidemia.

If carbon dioxide accumulates, then respiratory acidosis exists.

If nonvolatile acids accumulate, then metabolic acidosis exists.

Alkalemia may be the result of losing too many acids from the serum.

If too much carbon dioxide is lost, the result is respiratory alkalosis.

If there are less than normal amounts of nonvolatile acids, the result is metabolic alkalosis

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Bases

A base is a substance that can accept a hydrogen ion (H+), thereby removing it from the circulating serum.

The main base found in the serum is bicarbonate (HCO3).

The amount of bicarbonate that is available in the serum is regulated by the kidney (a metabolic process).

If there is too little bicarbonate in the serum, the result is

metabolic acidosis.

If there is too much bicarbonate in the serum, the result is metabolic alkalosis

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Conditions leading to acidemia or alkalemia Conditions leading to acidemia or alkalemia

are influenced by a multitude of physiological Some of these processes include respiratory

and renal function or dysfunction, tissue oxygenation, circulation, lactic acid production, substance ingestion, and electrolyte loss from the gastrointestinal tract.

The identification of a pH abnormality should lead to the investigation of possible contributing factors.

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MEASURING CARBON DIOXIDE IN THE BLOOD

The PaCO2 refers to the pressure or tension exerted by dissolved carbon dioxide gas in arterial blood.

Carbon dioxide is the natural byproduct of cellular metabolism.

Carbon dioxide levels are regulated primarily by the ventilatory function of the lung.

The normal PaCO2 is 35 to 45 mm Hg.

In interpretation of ABGs, PaCO2 is thought of as an “acid.” Elimination of carbon dioxide from the body is one of the main functions of the lungs, and an important relationship exists between the amount of ventilation and the amount of carbon dioxide in blood.

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Respiratory acidosis and alkalosis If a patient hypoventilates, carbon dioxide accumulates, and

the PaCO2 value increases above the upper limit of 45 mm Hg.

The retention of carbon dioxide results in respiratory acidosis. Respiratory acidosis may occur even with normal lungs if the

respiratory center is depressed and the respiratory rate or quality is insufficient to maintain normal carbon dioxide concentrations.

If a patient hyperventilates, carbon dioxide is eliminated from the body, and the PaCO2 value decreases below the lower limit of 35 mm Hg.

The loss of carbon dioxide results in respiratory alkalosis.

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MEASURING BICARBONATE IN THE BLOOD

Bicarbonate (HCO3), the main base found in the serum, helps the body regulate pH because of its ability to accept a hydrogen ion (H+).

The concentration of bicarbonate is regulated by the kidneys and is referred to as a metabolic process of regulation.

The normal bicarbonate level is 22 to 26 mEq/L.

Bicarbonate may be thought of as a “base” (alkaline).

When the bicarbonate level increases above 26 mEq/L, a metabolic alkalosis exists.

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Metobolic acidosis and alkalosis Metabolic alkalosis results from a gain of

base (alkaline) substances or a loss of metabolic acids.

When the bicarbonate level decreases below 22 mEq/L, a metabolic acidosis exits.

Metabolic acidosis results from a loss of

base (alkaline) substances or a gain of metabolic acids.

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ALTERATIONS IN ACID–BASE BALANCE

Disturbances in acid–base balance result from an abnormality of the metabolic or respiratory system.

If the respiratory system is responsible, it is detected by the carbon dioxide in the serum. If the metabolic system is responsible, it is detected by the bicarbonate in the serum.

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Respiratory Acidosis Respiratory acidosis is defined as a PaCO2 greater

than 45 mm Hg and a pH of less than 7.35. Respiratory acidosis is characterized by inadequate

elimination of carbon dioxide by the lungs and may be the result of inefficient pulmonary function or excessive production of carbon dioxide.

Respiratory Alkalosis Respiratory alkalosis is defined as a PaCO2 less

than 35 mm Hg and a pH of greater than 7.45. Respiratory alkalosis is characterized by excessive elimination of carbon dioxide from the serum.

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Metabolic Acidosis Metabolic acidosis is defined as a bicarbonate level of

less than 22 mEq/L and a pH of less than 7.35. Metabolic acidosis is characterized by an excessive

production of nonvolatile acids or an inadequate concentration of bicarbonate for the concentration of acid within the serum.

Metabolic Alkalosis Metabolic alkalosis is defined as a bicarbonate level of

greater than 26 mEq/L and a pH of greater than 7.45. Metabolic alkalosis is characterized by excessive loss

of nonvolatile acids or excessive production of bicarbonate.

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INTERPRETING ARTERIAL BLOOD GAS RESULTS

When interpreting ABG results, three factors must be considered:

(1) oxygenation status, (2) acid–base status, (3) degree of compensation. Evaluating Oxygenation It is necessary to examine the patient’s oxygenation

status by evaluating the PaO2 and the SaO2. If the PaO2 value is less than the patient’s norm,

hypoxemia exists. If the SaO2 is less than 93%, inadequate amounts of oxygen are bound to hemoglobin.

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Evaluating Acid–Base Status The first step in evaluating acid–base status is the

examination of the arterial pH. If the pH is less than 7.35, acidemia exists. If the pH is greater than 7.45, alkalemia exists.

The second step in evaluating acid–base status is examination of the PaCO2. A PaCO2 of less than 35 mm Hg indicates a respiratory alkalosis, whereas a PaCO2 of greater than 45 mm Hg signifies a respiratory acidosis.

The third step in evaluating acid–base status is examination of the bicarbonate level. If the bicarbonate value is less than 22 mEq/L, metabolic acidosis is present. If the bicarbonate value is greater than 26 mEq/L, metabolic alkalosis exists.

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Interpretation of Arterial Blood Gas (ABG) Results Approach 1. Evaluate oxygenation by examining the

PaO2 and the SaO2. 2. Evaluate the pH. Is it acidotic, alkalotic,

or normal? 3. Evaluate the PaCO2. Is it high, low, or

normal? 4. Evaluate the HCO3. Is it high, low, or

normal? 5. Determine if compensation is occurring.

Is it complete, partial, or uncompensated?

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Sample blood gas: Case 2

PaO2 80 mm Hg Normal SaO2 95% Normal pH 7.30 Acidemia PaCO2 55 mm Hg Increased

(respiratory cause) HCO3 25 mEq/L Normal Conclusion: Respiratory acidosis

(uncompensated)

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Sample blood gas: Case 2

PaO2 85 mm Hg Normal SaO2 90% Low saturation pH 7.49 Alkalemia PaCO2 40 Normal HCO3 29 mEq/L Increased (metabolic

cause) Conclusion: Metabolic alkalosis with a

low saturation (uncompensated)

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Occasionally, patients present with both respiratory and metabolic disorders that together cause an acidemia or alkalemia.

For example, alkalosis could result from an Increase in bicarbonate and a decrease in carbon dioxide, or an acidosis could result from a decrease in bicarbonate and an increase in carbon dioxide.

A patient with metabolic acidosis from acute renal failure could also have a very slow respiratory rate that causes the patient to retain carbon dioxide, creating a respiratory acidosis.

Therefore, the ABG reflects a mixed respiratory and metabolic acidosis.

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Arterial Blood Gases in Mixed Respiratory and Metabolic DisordersMIXED ACIDOSIS MIXED ALKALOSISpH: 7.25 pH: 7.55PaCO2: 56 mm Hg PaCO2: 26 mm HgPaO2: 80 mm Hg PaO2: 80 mm HgHCO3: 15 mEq/L HCO3: 28 mEq/L

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Determining Compensation

If the patient presents with an alkalemia or acidemia, it is important to determine if the body has tried to compensate for the abnormality.

If the buffer systems in the body are

unable to maintain normal pH, then the renal or respiratory systems attempt to compensate.

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It may take as little as 5 to 15 minutes for the lungs to recognize a metabolic presentation and start to correct it.

It may take up to 1 day for the kidneys to correct the respiratoryi nduced problem.

One system will not overcompensate; that is, a compensatory mechanism will never make an acidotic patient alkalotic or an alkalotic patient acidotic.

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The respiratory system responds to metabolic-based pH imbalances in the following manner:

■ Metabolic acidosis: increase in respiratory rate and depth ■ Metabolic alkalosis: decrease in respiratory rate and depth

The renal system responds to respiratory-based pH imbalances in the following manner:

■ Respiratory acidosis: increase in hydrogen secretion and bicarbonate reabsorption

■ Respiratory alkalosis: decrease in hydrogen secretion and bicarbonate reabsorption

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ABGs are defined by their degree of compensation:

uncompensated, partially compensated, or completely compensated.

To determine the level of compensation, the pH, carbon dioxide, and bicarbonate are examined.

First, it is determined whether the pH is acidotic or alkalotic.

In some cases, the pH is not within the normal range, indicating an acidosis or alkalosis.

If it is within the normal range, it is important to determine on which side of 7.40 (midpoint of the normal pH range) the pH lies.

For example, a pH of 7.38 is tending toward acidosis, whereas a pH of 7.41 is tending toward alkalosis.

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Next, an evaluation is made to see whether carbon dioxide or bicarbonate has changed to account for the acidosis or alkalosis.

Finally, it is determined whether the opposite system (metabolic or respiratory) has worked to try to shift back toward a normal pH.

The primary abnormality (metabolic or respiratory) is correlated with the abnormal pH (acidotic or alkalotic).

The secondary abnormality is an attempt to correct the primary disorder.

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Compensatory Status of Arterial Blood Gases Uncompensated: pH is abnormal, and either the CO2

or HCO3 is also abnormal. There is no indication that the opposite system has tried to

correct for the other. In the example below, the patient’s pH is alkalotic as a result

of the low (below the normal range of 35–45 mm Hg) CO2 concentration.

The renal system value (HCO3) has not moved out its normal range (22–26 mEq/L) to compensate for the primary respiratory disorder.

PaO2: 94 mm Hg Normal pH: 7.52 Alkalotic PaCO2: 25 mm Hg Decreased HCO3: 24 mEq/L Normal

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Compensatory Status of Arterial Blood Gases Partially compensated: pH is abnormal, and both the CO2 and HCO3 are also abnormal; this indicates that one

system has attempted to correct for the other but has not been completely successful.

In the example below, the patient’s pH remains alkalotic as a result of the low CO2 concentration.

The renal system value (HCO3) has moved out its normal range (22–26 mEq/L) to compensate for the primary respiratory disorder but has not been able to bring the pH back within the normal range.

PaO2: 94 mm Hg Normal pH: 7.48 Alkalotic PaCO2: 25 mm Hg Decreased HCO3: 20 mEq/L Decreased

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Compensatory Status of Arterial Blood Gases Completely compensated: pH is normal and both the CO2

and HCO3 are abnormal; the normal pH indicates that one system has been able to compensate for the other.

In the example below, the patient’s pH is normal but is tending toward alkalosis (>7.40).

The primary abnormality is respiratory because the PaCO2 is low (decreased acid concentration).

The bicarbonate value of 18 mEq/L reflects decreased concentration of base and is associated with acidosis, not alkalosis.

In this case, the decreased bicarbonate has completely compensated for the respiratory alkalosis.

PaO2: 94 mm Hg Normal pH: 7.44 Normal, tending toward alkalosis PaCO2: 25 mm Hg Decreased, primary problem HCO3: 18 mEq/L Decreased, compensatory response