Acid-Base Balance
Janis Rusin APN, MSN, CPNP-AC
Pediatric Nurse Practitioner
Lurie Children’s Transport Team
Objectives
• Discuss the mechanisms for maintaining normal acid-base
balance
• Define respiratory and metabolic acidosis and alkalosis
• Identify the common causes of acid base imbalance
• Define and differentiate between respiratory distress and
failure
• Discuss interventions on transport for a patient with acid-base
imbalance
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Acid-Base Balance
• The human body must be maintained in a very narrow range
of acid-base balance
• We use pH as our measure of acidity or alkalinity
• pH stands for “power” of hydrogen
• Normal pH is 7.35-7.45-Not a whole lot of wiggle room!
• Normal cellular metabolism occurs within this range
• The 2 major organs responsible for maintaining acid base
balance are:
– The lungs-Respiratory balance
– The kidneys-Metabolic balance
3
Chemistry Flashback!
• An acid is a substance that
releases hydrogen ions (when it
dissociates)
• A base is a substance that
accepts the hydrogen ions
• A buffer is a substance that
protects the pH from
derangements by binding with
hydrogen ions
HA  H+ + A-
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The Bicarbonate Buffer System
• The bicarbonate buffer system is what we monitor clinically to
assess acid base balance
• This system works in the plasma
• Relationship of carbon dioxide (CO2) to bicarbonate (HCO3-)
• CO2 is the acid and HCO3- is the base
5
Balancing Act
• Lungs
– CO2 is an end product of normal
cellular metabolism
– The lungs regulate the CO2 level
through respiration
– Rapid response-quick fix!
– The lungs cannot regulate
bicarbonate levels
• Kidneys
– The renal tubules reabsorb
bicarbonate
– Excess hydrogen ions are excreted in
the urine
– Slower process
– The kidneys cannot regulate CO2
levels
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Clinical Applications
• Acidosis (blood pH < 7.35)
– A pathologic condition that causes an increase in the hydrogen ion
concentration
• Alkalosis (blood pH > 7.45)
– A pathologic condition that causes a decrease in the hydrogen ion
concentration
• A simple acid base disorder has just one disturbance
• The respiratory and metabolic systems compensate for each
others deficiencies
• If there is more than one disturbance, the patient is said to
have a mixed acid base disorder
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Types of Acid Base Disorders
•
•
•
•
Metabolic Alkalosis
Metabolic Acidosis
Respiratory Alkalosis
Respiratory Acidosis
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Metabolic Alkalosis
• An elevation in the serum pH associated with a decrease in
hydrogen ion concentration and increase in bicarbonate ion
concentration
• Chloride plays a big role
• 2 main categories
– Chloride Responsive
• Chloride levels are < 10 mEq/L
– Chloride Resistant
• Chloride levels are > 20 mEq/L
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Metabolic Alkalosis
• Chloride Responsive
– Hydrogen ions are lost
– Vomiting
• Loss of HCL from stomach contents, as well as Na and K
• Excessive NG suctioning
• Loss of both Hydrogen and Chloride ions
• The kidneys retain Na and K instead of H in order to maintain the Na-K pump
function
– Diuretics
• Pull H2O from the extracellular space which is low in bicarb
• Results in an increased concentration of bicarb
• More bicarb available to bind with Hydrogen
– Post hypercapnia
• Compensation by kidneys to retain bicarb in presence of hypercapnia
• Metabolic alkalosis occurs transiently once PaCO2 levels corrected
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Metabolic Alkalosis
• Chloride Resistant
– Bicarbonate is retained
• Hypokalemia
– Low serum K causes K to shift out of the cells and H to shift into the cells
• Excessive base intake
– Antacids
• Hypertension
– Aldosterone levels are elevated
– Results in Na and H2O retention
– Hydrogen and excess K are dumped by kidney
– K shifts into cells
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Metabolic Acidosis
• A decrease in pH associated with a low serum bicarbonate
concentration
• Three primary mechanisms:
– Bicarbonate is lost form the body
– Kidney function is impaired and acid cannot be excreted properly
– Endogenous or exogenous addition of acid to the body
• Common Diagnoses leading to MA
– Diarrhea
– Insulin Dependent Diabetes Mellitus (IDDM)
– Lactic Acidosis
• Poor perfusion and shock
– Renal Failure
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Metabolic Acidosis
• Diarrhea
– Most common cause of MA
– Bicarbonate is lost in excessive stool
– The kidneys are unable to keep up
with the losses
– Potassium is also lost in the stool
– Volume depletion results in
aldosterone release
– Sodium is retained leading to further
loss of K
– Hypokalemia results
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Metabolic Acidosis
• Diabetic Ketoacidosis
– Insulin deficiency occurs stimulating the release of excess glucagon
– Glucagon stimulates the release of fatty acids from triglycerides
– Fatty acids are oxidized in the liver to ketone bodies, betahydroxybutrate and aceto-acetic acid
– These acids result in MA
– In addition, the DKA patient become volume depleted due to excessive
urination
– Shock develops and further exacerbates the acidosis
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Metabolic Acidosis
• Lactic acidosis
– Hypoxia or poor tissue perfusion
– Cells are forced into anaerobic
metabolism producing lactic acid
• Shock
• Excessive exercise
• Ethanol toxicity
– Ethanol interferes with
gluconeogenesis
– Anaerobic metabolism
• Renal Failure
– Distal RTA
• Failure of the distal tubule to
properly excrete hydrogen ions
– Fanconi syndrome
• Failure of the proximal renal tubule
to reabsorb bicarbonate,
phosphate and glucose
• Causes include:
– Genetics
– Medications such as tetracycline
and antiretrovirals
– Lead poisoning
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Anion Gap
• Calculation that determines the gap between concentrations
of positive (cations) and negative (anions) ions
• Useful in determining the cause of metabolic acidosis
• Calculated by:
– (Na+ + K+) – (HCO3- + Cl-) = 10-12mEq/L
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Anion Gap
• Normal Anion Gap
• The loss of bicarbonate is
compensated for by the
retention of chloride
• Also known as Hyperchloremic
Metabolic Acidosis
– Diarrhea
– Renal Failure, Proximal RTA
• Elevated Anion Gap
• MA due to increased H+ load
• MUDPILES
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–
–
–
–
–
–
–
Methanol
Uremia
DKA
Propylene Glycol
Isoniazid
Lactic Acid
Ethylene Glycol (antifreeze)
Salicylates
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Respiratory Alkalosis
• A condition in which the carbon dioxide content is significantly
reduced (hypocapnia)
• Caused by:
–
–
–
–
–
Hyperventilation
Occurs within minutes of onset of hyperventilation
Pulmonary disease
CHF
Hypermetabolic states
• Fever
• Anema
• Hyperthyroid
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Respiratory Acidosis
• Occurs when ventilation of CO2
is inadequate and CO2 is
retained (hypercapnia)
• Causes include airway
obstruction, respiratory
depression, pneumonia,
asthma, pulmonary edema,
chest trauma
• The renal buffer system is not
effective for acute RA
• Chronic respiratory acidosis can
be well compensated for by the
kidneys
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So, how do we make the
diagnosis?
• Arterial Blood Gas-Normal
Values
•
•
•
•
•
pH (7.35-7.45)
PCO2 (35-45)
PO2 (80-100)
HCO3 (22-26)
Base Excess/Deficit (-2 to +2)
• Venous Blood Gas-Normal
Values
•
•
•
•
•
pH (7.31-7.41)
PCO2 (40-50)
PO2 (35-40)
HCO3 (22-26)
Base Excess/Deficit (-2 to +2)
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Blood Gas Analysis
• Step 1: Look at the pH
– < 7.35 is acidic
– > 7.45 is alkalotic
• Step 2: Look at the PCO2
– <35 is alkalotic
– > 45 is acidic
• Step 3: Look at the HCO3
– < 22 is acidic
– > 26 is alkalotic
• Step 4:Match the pH to either
the PCO2 or HCO3
– Whichever one goes in the same
direction as pH determines the
primary disorder
– Respiratory = CO2
– Metabolic = HCO3
• Step 5:Which one goes in the
opposite direction of the pH?
– This is the compensatory system
• Step 6: Look at the PO2
– Determines presence of hypoxia
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Blood Gas Analysis
Blood Gas Interpretation
Respiratory
Acidosis
Metabolic
Alkalosis
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HCO3
45
Normal Values
22
PaCO2
35
Metabolic
Acidosis
Respiratory
Alkalosis
pH 7.35-7.45
Acidemia
Alkalemia
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Mixed Acid Base Disorders
• When to suspect a mixed acid base disorder:
– The expected compensatory response does not occur
– Compensatory response occurs, but level of compensation is inadequate
or too extreme
– Whenever the PCO2 and HCO3 become abnormal in the opposite
direction.
– In simple acid base disorders, the direction of the compensatory
response will always be in the same as the direction of the initial
abnormal change.
– pH is normal but PCO2 or HCO3- is abnormal
• General rule:
– If the pCO2 is elevated and HCO3 is reduced, then both respiratory and
metabolic acidosis are present
– If the pCO2 is reduced and the HCO3 is elevated, then both respiratory
and metabolic alkalosis are present
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Respiratory Distress
• A compensated state in
which oxygenation and
ventilation are maintained
– Define oxygenation and
ventilation
– How will the blood gas look?
• Characterized by any
increased work of breathing
– Flaring, retractions, grunting
– What is grunting?
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Respiratory Failure
• Compensatory mechanisms
are no longer effective
• Inadequate oxygenation
and/or ventilation resulting in
acidosis
– Abnormal blood gas with
hypercapnia and/or hypoxia
– Will begin to see decreasing LOC
due to hypercapnia
• Medical emergency! Must
protect airway!
• Strongly consider intubation
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Respiratory Failure-Causes
• Pulmonary Causes
–
–
–
–
–
–
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Diffusion impairment
Atelectasis
Pneumonia
Bronchiolitis
Acute lung injury
Pulmonary edema
Shunting and V/Q mismatch
• Non-Pulmonary Causes
– Respiratory muscle compromise or
fatigue
– Impairment of the nervous systems
control of breathing
• Guillain-Barre
• Muscular Dystrophy
• Central hypoventilation syndrome
– Sedatives
– Head injury
– Upper airway obstructions
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Indications for intubation
• Inability to protect airway
– No cough or gag
•
•
•
•
•
Decreasing LOC
GCS < 8
Cardiac or respiratory arrest
Acute respiratory acidosis
Refractory hypoxemia despite
100% FiO2
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Goals of ventilation
• Correct acidosis
• Rest the respiratory muscles
• Correct hypoxemia
– Allows for delivery of high FiO2
– PEEP
• Improves cardiac function
– Decreases preload
– Decreases metabolic demand
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Initial Ventilator settings
Volume Control
Pressure Control
Rate
Normal for age
Normal for age
Tidal Volume
8-10 cc/kg
PEEP
Start at 5cm H2O and
increase as clinically
indicated
i-Time
Pressure Control
Start at 5cm H2O and
increase as clinically
indicated
1:2
(Must increase E-time in
obstructive processes to
avoid air trapping)
Set pressure to produce
adequate chest rise and
TV’s (8-10/kg)
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Correction of hypoxia and
hypercarbia
To increase PaO2
To decrease PaCO2
Increase FiO2
Increase Rate
Increase PEEP
Increase Tidal Volume or
Pressure control
Increase I-Time
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Match the Gas
• Which patient does this gas belong to?
• pH 7.09 PCO2 98 PO2 218 HCO3 30
– A) 22 y/o with Muscular Dystrophy. Severe and worsening muscle
weakness
– B) 9 y/o with new onset Diabetic Ketoacidosis
– C) A 30 y/o patient presenting with a panic attack
– D) A 25y/o in a skiing accident presenting in respiratory distress
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Match the Gas
• pH 7.09 PCO2 98 Po2 218 HCO3 30
– A) 22 y/o with Muscular Dystrophy. Severe and worsening muscle
weakness
– Chronic Respiratory Failure
– Uncompensated Respiratory Acidosis
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Match the Gas
• Which patient does this gas belong to?
• pH 7.55 PCO2 28 PO2 63 HCO3- 23
– A) 22 y/o with Muscular Dystrophy. Severe and worsening muscle
weakness
– B) 9 y/o with new onset Diabetic Ketoacidosis
– C) A 30 y/o patient presenting with a panic attack
– D) A 25y/o in a skiing accident presenting in respiratory distress
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Match the Gas
• Which patient does this gas belong to?
• pH 7.55 PCO2 28 PO2 63 HCO3- 23
– C) A 30 y/o patient presenting with a panic attack
– Hyperventilation
– Uncompensated Respiratory alkalosis
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Match the Gas
• Which patient does this gas belong to?
• pH 6.94 PCO2 26.6 PO2 55.7 HCO3 5.7 BD -27
– A) 22 y/o with Muscular Dystrophy. Severe and worsening muscle
weakness
– B) 9 y/o with new onset Diabetic Ketoacidosis
– C) A 30 y/o patient presenting with a panic attack
– D) A 25y/o in a skiing accident presenting in respiratory distress
35
Match the Gas
• pH 6.94 PCO2 26.6 PO2 55.7 HCO3 5.7 BD -27
– B) 9 y/o with new onset Diabetic Ketoacidosis
– DKA
– Uncompensated Metabolic Acidosis
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Match the Gas
• Which patient does this gas belong to?
• pH 7.27 PCO2 54.8 PO2 70 HCO3 26 BD -1
– A) 22 y/o with Muscular Dystrophy. Severe and worsening muscle
weakness
– B) 9 y/o with new onset Diabetic Ketoacidosis
– C) A 30 y/o patient presenting with a panic attack
– D) A 25y/o in a skiing accident presenting in respiratory distress
37
Match the Gas
• pH 7.27 PCO2 54.8 PO2 70 HCO3 26 BD -1
– D) A 25y/o in a skiing accident presenting in respiratory distress
– Acute Respiratory Distress
– Uncompensated Respiratory Acidosis
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Questions?
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