Dr aidah abu elsoud alkaissi an najah national university nursing college
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Dr . Aidah Abu Elsoud Alkaissi An- Najah National University Nursing College. Arterial Blood Gases. 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.

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Dr aidah abu elsoud alkaissi an najah national university nursing college

Dr. AidahAbu Elsoud Alkaissi

An-Najah National University

Nursing College

ArterialBloodGases


Arterial blood gases

ArterialBloodGases

  • 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.


Normal abg values

Normal ABG values

Normal Values for an ArterialBlood Gas

  • PaO2: 80–100 mm Hg

  • SaO2: 93%–99%

  • pH: 7.35–7.45

  • PaCO2: 35–45 mm Hg

  • HCO3: 22–26 mEq/L


Measuring oxygen in the blood

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.


Measuring oxygen in the blood1

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.


Measuring ph in the blood

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.

  • Acidemiarefers 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.

  • Alkalemiarefers to a condition in which the blood is too alkaline.

  • Alkalosis refers to the processthat causes the alkalemia.


Arterial blood gases

  • 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+;

  • Allbases 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


Acids

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 nonvolatileacids.

  • 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).


Acids1

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 thelungs.

  • 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.


Condition respiratory acidosis paco2 45 mm hg ph 7 35

ConditionRespiratoryAcidosisPaCO2 >45 mm HgpH <7.35

Signs and symptoms Dyspnea

Restlessness

Headache

Tachycardia

Confusion

Lethargy

Dysrhythmias

Respiratorydistress

Drowsiness

Decreasedresponsiveness

Possiblecauses

Central nervous system depression

Head trauma

Oversedation

Anesthesia

High cord injury

Pneumothorax

Hypoventilation

Bronchialobstruction and atelectasis

Severepulmonaryinfections

Heart failure and pulmonary edema

Massive pulmonaryembolus

Myasthenia gravis

Multiple sclerosis


Respiratory alkalosis paco2 35 mm hg ph 7 45

RespiratoryAlkalosisPaCO2 <35 mm HgpH >7.45

  • Possiblecauses

  • Anxietyand nervousness

  • Fear

  • Pain

  • Hyperventilation

  • Fever

  • Thyrotoxicosis

  • Central nervoussystem lesions

  • Salicylates

  • Gram-negativesepticemia

  • Pregnancy

Signs and symptoms

Light-headedness

Confusion

Decreasedconcentration

Paresthesias

Tetanic spasms in the arms and legs

Cardiacdysrhythmias

Palpitations

Sweating

Drymouth

Blurred vision


Metabolic acidosis hco3 22 meq l ph 7 35

MetabolicAcidosisHCO3 <22 mEq/LpH <7.35

  • Possiblecauses

  • Increasedacids

  • Renalfailure

  • Ketoacidosis

  • Anaerobic metabolism

  • Starvation

  • Salicylateintoxication

  • Loss of base

  • Diarrhea

  • Intestinalfistulas

Signs and symptoms

Headache

Confusion

Restlessness

Lethargy

Weakness

Stupor/coma

Kussmaul respiration

Nausea and vomiting

Dysrhythmias

Warm, flushed skin


Metabolic alkalosis hco3 26 meq l ph 7 45

MetabolicAlkalosisHCO3 >26 mEq/LpH >7.45

  • Possiblecauses

  • Gainof base

  • Excess use of bicarbonate

  • Lactate administration in dialysis

  • Excess ingestion of antacids

  • Loss of acids

  • Vomiting

  • Nasogastricsuctioning

  • Hypokalemia

  • Hypochloremia

  • Administration of diuretics

  • Increasedlevels of aldosterone

Signs and symptoms

Muscletwitching and cramps

Tetany

Dizziness

Lethargy

Weakness

Disorientation

Convulsions

Coma

Nausea and vomiting

Depressed respiration


Acidosis alkalosis

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


Bases

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 resultis metabolicalkalosis


Conditions leading to acidemia or alkalemia

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 possiblecontributingfactors.


Measuring carbon dioxide in the blood

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.


Respiratory acidosis and alkalosis

Respiratory acidosis and alkalosis

  • If a patient hypoventilates, carbondioxideaccumulates, 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 resultsin respiratoryalkalosis.


Measuring bicarbonate in the blood

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.


Metobolic acidosis and alkalosis

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.


Alterations in acid base balance

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.


Arterial blood gases

  • RespiratoryAcidosis

  • 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.

  • RespiratoryAlkalosis

  • 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 dioxidefrom the serum.


Arterial blood gases

  • MetabolicAcidosis

  • 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.

  • MetabolicAlkalosis

  • 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.


Interpreting arterial blood gas results

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.

  • EvaluatingOxygenation

  • 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 boundto hemoglobin.


Evaluating acid base status

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, alkalemiaexists.

  • 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 alkalosisexists.


Interpretation of arterial blood gas abg results approach

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?


Sample blood gas case 2

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)


Sample blood gas case 21

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)


Arterial blood gases

  • 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.


Arterial blood gases

Arterial Blood Gases in Mixed Respiratory and Metabolic Disorders

MIXED ACIDOSIS MIXED ALKALOSIS

pH: 7.25 pH: 7.55

PaCO2: 56 mm Hg PaCO2: 26 mm Hg

PaO2: 80 mm Hg PaO2: 80 mm Hg

HCO3: 15 mEq/LHCO3: 28 mEq/L


Determining compensation

DeterminingCompensation

  • 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.


Arterial blood gases

  • It maytake 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 respiratoryinduced problem.

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


Arterial blood gases

  • 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 bicarbonatereabsorption

  • ■ Respiratory alkalosis: decrease in hydrogen secretion and bicarbonatereabsorption


Arterial blood gases

  • 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.


Arterial blood gases

  • Next, an evaluationis 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.


Compensatory status of arterial blood gases

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


Compensatory status of arterial blood gases1

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 beencompletelysuccessful.

  • 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 withinthe normal range.

  • PaO2: 94 mm Hg Normal

  • pH: 7.48 Alkalotic

  • PaCO2: 25 mm Hg Decreased

  • HCO3: 20 mEq/LDecreased


Compensatory status of arterial blood gases2

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, tendingtowardalkalosis

  • PaCO2: 25 mm Hg Decreased, primary problem

  • HCO3: 18 mEq/L Decreased, compensatory response


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