Chapter 14
Assessment and Care of
Patients with Acid-Base
Imbalances
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Normal Blood pH
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Keeping pH within the normal range
involves balancing acids and bases in
body fluids.
Normal pH for arterial blood—7.35 to 7.45.
Normal pH for venous blood—7.31 to 7.41.
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Normal Blood pH (Cont’d)
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Changes from normal blood pH interfere
with many normal functions by:
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Changing the shape of hormones and
enzymes.
Changing the distribution of other electrolytes,
causing fluid and electrolyte imbalances.
Changing of excitable membranes.
Decreasing the effectiveness of many
hormones and drugs.
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Introduction to Acid-Base
Chemistry
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Acids
Bases
Buffers
Body fluid chemistry:
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Bicarbonate ions
Relationship between carbon dioxide and
hydrogen ions
Calculation of free hydrogen ion level
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Sources of Acids
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Incomplete breakdown of glucose
Destruction of cells
Bicarbonate
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Respiratory Acid-Base Control
Mechanisms
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When chemical buffers alone cannot
prevent changes in blood pH, the
respiratory system is the second line of
defense against changes:
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Hyperventilation
Hypoventilation
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Respiratory Acid-Base Control
Mechanisms (Cont’d)
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Renal Acid-Base Control
Mechanisms
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The kidneys are the third line of defense
against wide changes in body fluid pH.
Stronger for regulating acid-base balance
but take longer than chemical and
respiratory mechanisms to completely
respond.
Kidney movement of bicarbonate.
Formation of acids.
Formation of ammonium.
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Compensation
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The body attempts to correct changes in blood
pH.
pH below 6.9 or higher than 7.8 is usually fatal.
Respiratory system is more sensitive to acid-base
changes; can begin compensation efforts within
seconds to minutes.
Renal compensatory mechanisms are much more
powerful and result in rapid changes in ECF
composition not fully triggered unless imbalance
continues for several hours to days.
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Respiratory Compensation
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Lungs compensate for acid-base
imbalances of a metabolic origin.
Example: Prolonged running causes
buildup of lactic acid, hydrogen ion levels
in the ECF increase, pH drops; breathing
is triggered in response to the increased
carbon dioxide levels to bring the pH level
back to normal.
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Renal Compensation
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A healthy kidney can correct or
compensate for changes in blood pH
when the respiratory system either is
overwhelmed or is not healthy.
Example: Person has chronic obstructive
pulmonary disease, retains carbon dioxide
in the blood, blood pH level falls (becomes
more acidic); kidney excretes more
hydrogen ions and increases the
reabsorption of bicarbonate back into the
blood.
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Acid-Base Imbalances
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Metabolic acidosis
Respiratory acidosis
Combined metabolic and respiratory
acidosis
Metabolic alkalosis
Respiratory alkalosis
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Metabolic Acidosis
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Overproduction of hydrogen ions
Under-elimination of hydrogen ions
Underproduction of bicarbonate ions
Over-elimination of bicarbonate ions
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Respiratory Acidosis
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Retention of CO:
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Respiratory depression
Inadequate chest expansion
Airway obstruction
Reduced alveolar-capillary diffusion
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Combined Metabolic and
Respiratory Acidosis
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Uncorrected respiratory acidosis always
leads to poor oxygenation and lactic
acidosis.
Combined acidosis is more severe than
metabolic or respiratory acidosis alone.
Cardiac arrest is an example of a problem
leading to combined metabolic and
respiratory acidosis.
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Collaborative Care
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History
CNS changes
Neuromuscular changes
Cardiovascular changes
Respiratory changes:
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Kussmaul respiration
Skin changes
Psychosocial assessment
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Laboratory Assessment
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Metabolic acidosis:
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pH <7.35
Bicarbonate <21 mEq/L
PaO2 normal
PaCO2 normal or slightly decreased
Serum potassium high
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Laboratory Assessment
(Cont’d)
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Respiratory acidosis:
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pH <7.35
PaO2 low
PaCO2 high
Serum bicarbonate variable
Serum potassium levels elevated if acidosis is
acute
Serum potassium levels normal or low if renal
compensation is present
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Interventions—Metabolic
Acidosis
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Hydration
Drugs:
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Insulin to treat DKA
Antidiarrheal drugs
Bicarbonate only if serum bicarbonate levels
are low
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Interventions—Respiratory
Acidosis
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Maintain a patent airway, and enhance
gas exchange
Drug therapy
Oxygen therapy
Pulmonary hygiene
Ventilation support
Prevent complications
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Alkalosis
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Metabolic alkalosis:
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Base excesses—excessive intake
bicarbonates, carbonates, acetates, and
citrates
Acid deficit—prolonged vomiting, excess
cortisol, hyperaldosteronism, thiazide diuretics,
prolonged NG suction
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Alkalosis (Cont’d)
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Respiratory alkalosis:
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Hyperventilation—anxiety, fear, improper vent
settings, stimulation of central respiratory
center due to fever, DNS lesion, and
salicylates
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Collaborative Care
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Assessment
CNS changes—positive Chvostek’s and
Trousseau’s signs
Neuromuscular changes—tetany
Cardiovascular changes
Respiratory changes
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Interventions
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Prevent further losses of hydrogen,
potassium, calcium, and chloride ions.
Restore fluid balance.
Monitor changes.
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