ACID-BASE BALANCE
AND IMBALANCE
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pH
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Types of Acids: Volatile Acid
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Volatile acids: can be converted into gas & eliminated by lungs
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Primary volatile acid produced in body = CO2
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@ tissue
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•
•
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Metabolically generated
Diffuses into blood
Largely travels as HCO3- in blood
@ lungs
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HCO3- releases CO2
Blow off CO2
Types of Acids: Fixed Acids
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Fixed acids: generated by physiological & pathophysiological mechanisms,
eliminated from the body by urinary via kidneys
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Catabolism of proteins & phospholipids generates sulfuric acid & phosphoric acid,
respectively
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Vigorous exercise generates lactic acid
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Path ex: diabetic ketoacidosis generates beta-hydroxybutyric acid & acetoacetic
acid
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Overproduction of fixed acids causes metabolic acids
Control of Acids
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Body produces more acids than bases
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Acid-base homeostasis works to maintain balance between acids & bases
Described as body pH
• Outside acceptable pH range, proteins are denatured, enzymes lose
functionality, & may result in death
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The Body and pH
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pH range compatible w/ life = 6.8 to 8.0
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Intracellular pH (7.2) is lower than extracellular (7.4) due to transporters in
cell membranes
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Na-H pumps move H out of cells to alkalinize intracellular fluids
Cl-HCO3- exchange moves HCO3- out of cells to acidify intracellular fluids
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Changes in pH
• Normal arterial pH range = 7.35 to 7.45
• Acidemia = pH < 7.35
• Alkalemia = pH > 7.45
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Acid-Base Basics
• Acids dissociate into H+
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Results in lower pH values
• Bases dissociate into OH•
Results in higher pH values
• Buffers absorb excess ions to maintain pH
Arterial Blood Gases: Normal Values
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pH: 7.35-7.45
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Partial pressure of oxygen (PaO2): 75 to 100 mmHg
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Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
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Bicarbonate (HCO3): 22-26 mEq/L
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Oxygen saturation (O2 Sat): 94-100%
Regulatory Systems
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Chemical buffers
Bicarbonate
• Phosphate
• Protein
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Respiratory system (Lungs)
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Lungs regulate CO2 in blood
Renal system
Produces bicarbonate
• Absorbs acids & bases as
needed
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Rates of correction
• Fastest to slowest
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Chemical buffers
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Respiratory
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Responds in sec to min
Respond in mins to hours
Renal
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Responds in hours to days
Do have strongest response
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Chemical Buffers
• Three chemical buffering systems in the body
Bicarbonate buffering system in extracellular fluid*
• Phosphate intracellular & urinary buffer
• Protein intracellular buffer
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Bicarbonate buffer
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When acidic substance enters blood, HCO3- neutralizes by forming
carbonic acid & water
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Prevents blood from becoming too acidic
When basic substances enters blood, carbonic acid reacts w/ H+ by forming
HCO3- & water
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Prevents blood from becoming too basic
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Protein Buffers
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Positively charged amino groups & negatively charges carboxyl groups of
amino acids bind H & hydroxyl ions to function as buffers
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Nearly all proteins can act as buffers
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Accounts for most buffering w/in cells & a middling amount of buffering power
in blood
Hemoglobin as a buffer
Main protein in RBCs
• During conversion of CO2 to HCO3-, H+ are freed
• H+ accepted by hemoglobin, by dissociation of Oxygen
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Process revered in lungs to blow off CO2
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Acidosis
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Process that causes increased acidity in blood & tissues
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pH <7.35
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Metabolic acidosis results from increased production of metabolic acids or
failure to excrete acid in renal system
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Compensated for in lungs
Respiratory acidosis results from build up of CO2 in blood due to
hypoventilation
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A results of pulmonary dysfunction, head trauma, drugs, etc.
Can occur as a compensatory response to chronic metabolic alkalosis
Alkalosis
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Process that reduces acid (H+) in arterial blood
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pH > 7.45
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Respiratory alkalosis results in loss of CO2
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Hyperventilation
Compensation is releasing H from tissue buffers & excreting HCO3- (renal)
Metabolic alkalosis results in loss of H+ or a gain in HCO3•
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Vomiting, diarrhea, diuretics
Compensation is hypoventilation, increase CO2
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COMPENSATORY
MECHANISMS
Respiratory and Renal
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Compensatory Mechanisms
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Compensation is the body’s way of restoring a normal blood pH
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Compensation DOES NOT treat the root of the problem – the reason for the acidbase imbalance is STILL THERE!!!
Respiratory Compensation
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Used to compensate for metabolic imbalances only
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Chemoreceptors respond to changes in [H+]  alters respiratory rate and depth
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Exhalation of carbon dioxide
Body pH can be adjusted by changing rate and depth of breathing
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Slower breathing (hypoventilation)  hold on to CO2 (↑ PaCO2) and lower pH
Faster breathing (hyperventilation)  blow off CO2 (↓ PaCO2) and raise pH
Respiratory Compensation
• Respiratory Rate will…
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Increase when blood [H+] is increased
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acidic pH/lower pH
CO2 is “blown off”
Amount of acid in blood is decreased
Decrease when [H+] is decreased
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alkaline pH/higher pH
CO2 is retained
Amount of acid in blood is increased
Respiratory Compensation
• This means
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Metabolic acidosis causes an increase in rate and depth of ventilation as
the body attempts to get rid of acid (CO2)
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Metabolic alkalosis causes a decrease in rate and depth of ventilation as
the body attempts to retain acid (CO2)
Renal Role in (normal) Acid-Base Balance
• Reabsorption of filtered HCO3•
Proximal tubule
• Excretion of excess H+
Buffered by phosphate
• As NH4+
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Renal Compensation
• Used to compensate for respiratory imbalances
• Remember:
HCO3- is a base
• Kidneys respond to changes in blood pH
Excrete H+ and retain HCO3- when acidemia is present
• Retain H+ and excrete HCO3- when alkalemia is present
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Renal Compensation
• This means
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A respiratory acidosis will make the kidneys excrete acid (H+) and retain
base (HCO3-)
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A respiratory alkalosis will make the kidneys excrete base (HCO3-) and
retain acid (H+)
Renal Compensation
• This is the slowest, but most effective regulator of pH
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If kidneys fail, pH balance fails
• May take hours to days
• Ineffective in patients with renal failure
Compensated Imbalance
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The body is correcting the imbalance
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Blood pH is normal
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Other blood gas values remain abnormal until the root cause is treated and
corrected
Uncompensated Imbalance
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pH abnormal
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Either PaCO2 OR HCO3- abnormal
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All other values normal
ACID-BASE
IMBALANCES
And Their Compensations
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Acid-Base Imbalances
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pH < 7.35 acidosis
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pH > 7.45 alkalosis
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Metabolic or Respiratory
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The body’s response to acid-base imbalance is called compensation
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Respiratory Imbalances
• Primary disorders of CO2 (disorders of respiration)
Respiratory acidosis: increase in CO2  decrease in pH
• Respiratory alkalosis: decrease in CO2  increase in pH
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Metabolic Imbalances
• Primary disorders involving HCO3-
Acidosis: decrease in [HCO3-]  decrease in pH
• Alkalosis: increase in [HCO3-]  increase in pH
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• Metabolic mechanisms involve
Renal function alteration
• Production of acidic metabolic products
• Loss of acid from the body
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RESPIRATORY
IMBALANCES
Respiratory Acidosis, Respiratory Alkalosis and their Compensations
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Uncompensated Respiratory Acidosis
• pH < 7.35
• PaCO2 >45 (due to CO2 retention)
• HCO3-: WNL
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Respiratory Acidosis
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Any compromise in breathing can result in respiratory acidosis
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Hypoventilation  CO2 retention  decrease in pH
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Can result from neuromuscular dysfunction, depression of the brain’s
respiratory center, lung disease or airway obstruction
Respiratory Acidosis
• Acute conditions:
Adult Respiratory Distress Syndrome
• Pulmonary edema
• Pneumothorax
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• Chronic conditions:
Depression of respiratory center in brain that controls breathing rate –
drugs or head trauma
• Paralysis of respiratory or chest muscles
• Emphysema
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Compensation
• Kidneys eliminate H+ and retain HCO3-
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ABGs
• Uncompensated
pH < 7.35
• PaCO2 >45
• HCO3 Normal
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• Compensated
pH Normal
• PaCO2 >45
• HCO3 > 26 (increased; retaining
the base to reestablish the acid
base balance)
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Uncompensated Respiratory Alkalosis
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pH > 7.45
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PaCO2 < 35 (loss of CO2)
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HCO3: Normal
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Respiratory Alkalosis
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Most common acid-base imbalance
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Most commonly results from hyperventilation caused by overstimulation of
the respiratory center: pain, hypermetabolic states or acute hypoxia
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Other conditions that stimulate respiratory center:
Oxygen deficiency at high altitudes
• Pulmonary disease
• Congestive heart failure
• Acute anxiety
• Fever, anemia
• Cirrhosis
• Sepsis
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Compensation
• Kidneys conserve H+ and excrete HCO3-
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ABG Results
• Uncompensated
pH > 7.45
• PaCO2 < 35
• HCO3 Normal
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• Compensated
pH Normal
• PaCO2 < 35
• HCO3 < 22 (decreased; excreting
the base to reestablish the
balance)
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Summary: UNCOMP. Resp. Imbalances
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Uncompensated respiratory
alkalosis
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Uncompensated respiratory
acidosis
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pH > 7.45
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pH < 7.35
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PaCO2 < 35
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PaCO2 > 45
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HCO3- WNL
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HCO3- WNL
Summary: COMPENSATED Resp. Imbalances
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Compensated Respiratory Alkalosis
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Compensated Respiratory Acidosis
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pH = WNL but closer to 7.45
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pH = WNL but closer to 7.35
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PaCO2 < 35
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PaCO2 > 45
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HCO3- < 22
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HCO3- > 26
METABOLIC
IMBALANCES
Metabolic Acidosis, Metabolic Alkalosis and their Compensations
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Uncompensated Metabolic Acidosis
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pH < 7.35
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PaCO2: Normal
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HCO3 < 22 (direct or relative; loss of HCO3- or gain of H+)
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Metabolic Acidosis
• Characterized by gain of acid or loss of bicarb
Loss of bicarbonate through diarrhea or renal dysfunction
• Gain of acid
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Production of ketone bodies as with diabetes mellitus, alcoholism, starvation, &
hyperthyroidism
Failure of kidneys to excrete H+
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Compensation
• Increased ventilation
• Renal excretion of hydrogen
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ABG Results
• Uncompensated
pH < 7.35
• PaCO2 Normal
• HCO3 < 22
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• Compensated
pH Normal
• PaCO2 < 35 (decreased: loss of
acid/CO2 to reestablish the
balance)
• HCO3 < 22
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Uncompensated Metabolic Alkalosis
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pH > 7.45
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PaCO2: Normal
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HCO3 >26 (direct or relative: increase in HCO3- or loss of H+)
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Metabolic Alkalosis
• Causes:
Excess vomiting = loss of stomach acid
• Excessive use of alkaline drugs
• Certain diuretics
• Endocrine disorders
• Heavy ingestion of antacids
• Severe dehydration
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Compensation
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Respiratory compensation difficult – hypoventilation limited by hypoxia
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ABG Results
• Uncompensated
pH > 7.45
• PaCO2 Normal
• HCO3 >26
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• Compensated
pH Normal
• PaCO2 > 45 (increased; retaining
CO2/acid to reestablish the
balance)
• HCO3 > 26
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Summary: UNCOMP. Metabolic Imbalances.
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Uncompensated metabolic alkalosis
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Uncompensated metabolic acidosis
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pH > 7.45
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pH < 7.35
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PaCO2 = WNL
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PaCO2 = WNL
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HCO3- > 26
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HCO3- < 22
Summary: COMP. Metabolic Imbalances
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Compensated Metabolic Alkalosis
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Compensated Metabolic Acidosis
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pH = WNL but closer to 7.45
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pH = WNL but closer to 7.35
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PaCO2 > 45
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PaCO2 < 35
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HCO3- > 26
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HCO3- < 22
INTERPRETING ABGS
Normal ABG
• pH and PaCO2 move in opposite directions
As PaCO2 increases, pH decreases
• As PaCO2 decreases, pH increases
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• HCO3- and PaCO2 move in the same direction
H2O + CO2  H2CO3  H+ + HCO3• As PaCO2 increases, HCO3- increases
• As PaCO2 decreases, HCO3- decreases
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If pH is ABNORMAL…
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If pH and PaCO2 move in the SAME direction, the primary problem is METABOLIC
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Increased pH and increased PaCO2 = Metabolic alkalosis
Decreased pH and decreased PaCO2 = Metabolic acidosis
If pH and PaCO2 move in the OPPOSITE direction, the problem is RESPIRATORY
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Increased pH and decreased PaCO2 = Respiratory alkalosis
Decreased pH and increased PaCO2 = Respiratory acidosis
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Mixed Disorders ARE Possible
• If HCO3- and PaCO2 change in opposite directions (which they
normally should not) then it’s a mixed disorder
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We won’t get into this scenario
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Identify the Primary Disorder: Look at pH
• < 7.35 is acidosis
• > 7.45 is alkalosis
http://www.rnceus.com/abgs/abgmethod.html
Look at the PaCO2
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If it changes in the SAME direction as the pH, the primary disorder is
METABOLIC
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If it is high (>45) it is METABOLIC alkalosis
If it is low (< 35) it is METABOLIC acidosis
If it changes in the OPPOSITE direction as the pH, the primary disorder is
RESPIRATORY
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http://www.thoracic.org/clinical/critical-care/clinical-education/abgs.php
Look for Compensation
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http://www.thoracic.org/clinical/critical-care/clinical-education/abgs.php
Example
• A patient is in intensive care because he suffered a severe
myocardial infarction 3 days ago. The lab reports the following
values from an arterial blood sample:
pH 7.3
• HCO3- = 20 mEq / L ( 22 - 26)
• PaCO2 = 32 mm Hg (35 - 45)
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Check the pH
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pH = 7.3
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acidosis
What is the PaCO2?
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PaCO2 = 32 mm Hg (35 - 45)
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Low PaCO2
Move is in the same direction as pH
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Low pH, low PaCO2
Metabolic
Look at the HCO3•
HCO3- = 20 mEq / L ( 22 - 26)
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Low
Means there must be a compensation going on
Look for Compensation
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If a change is seen in BOTH PaCO2 and HCO3-, the body is trying to compensate
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HCO3- = 20 mEq / L ( 22 - 26)
PaCO2 = 32 mm Hg (35 - 45)
Diagnosis
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Metabolic acidosis
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With compensation
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