Clinical Approach to Acid-base Disorders - University at Buffalo

Clinical Approach to Acid-base
Disorders
Nasir M. Khan, MD
Department of Medicine
University at Buffalo

Basic Concepts
• Henderson-Hasselbach equation:
[H + ]=24x(P CO / [HCO 3 - a 2 3] ) expressed in
nEq/L
• Normal pH=7.36-7.44
•P a CO 2 = 36-44 mm Hg
• [HCO 3 ] - =22-26 26 mEq/L

Acid-Base Disorders
• Metabolic Disorders are caused by a disturbance
in [HCO 3 ] - . Metabolic Acidosis is present when
[HCO 3 ] - is low and metabolic alkalosis is present
when [HCO 3 ] - is high
• Respiratory disorders d are caused dby changes in the
P a CO 2. When the P a CO 2 is high, respiratory
acidosis is said to be present, and when P a CO 2 is
low, respiratory alkalosis is present.

Compensatory Changes
• These changes tend to correct the pH but do not
completely normalize it.
• An alteration in one component of the P a CO 2 /
[HCO 3 ] - ) ratio changes the other component in the
same direction i to limit i changes in [H + ]
concentration (and thus the pH).
• Ventilatory t response of the lungs to alter P a CO 2 is
prompt but the kidney may take many hours to
compensate.

Maintenance of Acid-Base
Balance
• Kidneys alter the rate of absorption of [HCO 3 ] - by
increasing its absorption when P a CO 2 increases
(respiratory acidosis) and decreasing the rate of
absorption of [HCO 3 ] - when P a CO 2 decreases
(respiratory alkalosis)
• Lungs compensate for a change in [HCO 3 ] - by
increasing the ventilation to reduce P a CO 2 in
metabolic acidosis and decreasing the ventilation
to reduce P a aCO 2 in metabolic alkalosis.

Rules of Acid-Base Problem
Interpretation
1. Check for internal consistency
2. Examine the pH
3. Determine if the primary problem is
metabolic or respiratory
4. Calculate the anion gap
5. Is it a simple or a mixed disturbance?
6. Determine the underlying clinical problem

Rule #1: Check for Internal
Consistency
• The pH and P a CO 2 is measured from an
arterial blood gas (ABG) sample and the
[HCO 3 ] - is then calculated using the
Henderson-Hasselbach equation.
• Serum measured [HCO 3 ] - should be within
2 mmol/L of the calculated [HCO -.
3 ]

Internal Consistency (contd)
• pH can be calculated as follows:
– At pH 7.0, [H + ]=100 nmol/L
– At each increment of 0.1 pH unit above 7.0,
multiply pythe previous value by 0.8
– For pH less than 7.0, multiply each 0.1 change
in pH by 1.25

Rule #2: Examine the pH
• A reduction below the normal range
suggests acidosis, while an elevation is
indicative of alkalosis

Rule #3: Determine if the
primary problem is metabolic or
respiratory
• A primary metabolic disorder is present if
the [HCO 3 ] - is abnormal. The pH and P a CO 2
change in the same direction.
• A primary respiratory disorder is present if
the P a CO 2 is abnormal. The pH and P a CO 2
change in the opposite direction.

Rule #4:Calculate the anion gap
• The anion gap (AG) is an estimate of unmeasured
anions and is calculated as follows:
AG = Na - (Cl + HCO 3 ).
Normal range is 6-12 mEq/L.
Reduction in plasma proteins, particularly albumin may
decrease the AG by about 1/3.
• AG helps to determine if the metabolic acidosis is
due to accumulation of [H + ] e.g. DKA or a loss of
[HCO 3 ] - as in diarrhea.
• Calculate ΔAG/ Δ [HCO - 3 ] ratio or add ΔAG to
[HCO 3 ] - to obtain ‘corrected’ [HCO 3 ] -

Rule #5: Is it a simple or a mixed
disturbance?
• A simple acid-base disorder is present if an
expected compensatory response is seen.
• A mixed d( (acidosis i and alkalosis) l )disorder d is
present if the P a CO 2 or AG is abnormal and the pH
is normal or unchanged, or if the pH is abnormal
and the P a CO 2 is normal or unchanged. These can
be.
– Mixed metabolic and respiratory disorders.
– Mixed metabolic disorders.
– Complex mixed disorders.

Mixed Disturbance
• Acidosis
• Alkalosis
• Normal pH

Mixed Disturbance
• Acidosis.
– If the pH is low or AG is high , and the P a CO 2 is low or
normal, a primary metabolic acidosis is present. The
difference between the measured and the expected
P a CO 2 is then used to identify a superimposed
respiratory disorder.
d
– If the pH is low and the P a CO 2 is high, a primary
respiratory acidosis is present. The difference between
the measured and the expected [HCO 3 ] - is then used to
identify a superimposed metabolic disorder. A low
[HCO 3 ] - may also indicate a metabolic disorder with
superimposed respiratory acidosis.

Mixed Disturbance (contd)
• Alkalosis.
– If the pH is high, a normal or high P a CO 2 indicates a
primary metabolic alkalosis. The difference between
the measured and the expected P a CO 2 is then used to
identify a superimposed respiratory disorder.
– If the pH is high, a low P a CO 2 indicates a primary
respiratory alkalosis. The difference between the
measured and the expected d[HCO 3 ] - is then used to
identify a superimposed metabolic disorder.

Mixed Disturbance (contd)
• Normal pH.
– Low P a CO 2 indicates mixed respiratory
alkalosis-metabolic acidosis.
– High P aCO 2 indicates mixed respiratory
acidosis-metabolic alkalosis.
– Normal P aCO 2 indicates metabolic acidosis co-
existing with metabolic alkalosis. Anion gap
will be elevated.

Expected Compensatory
Responses
• Metabolic Acidosis: P a CO 2 =1.5 [HCO 3 ] - +8±2
• Metabolic alkalosis:ΔP aCO 2 =0.7× Δ[HCO[ 3 3]
-
• Respiratory Acidosis:
– Acute: ↑[HCO - 3 ] = 1mEq/L×Δ Δ P a CO 2 /10
– Chronic: ↑[HCO 3 ] - = 3.5 mEq/L ×Δ P a CO 2 /10
• Respiratory alkalosis:
– Acute: ↓[HCO 3 ] - = 2 mEq/L ×Δ P a CO 2 /10
– Chronic: c:↓[ ↓[HCO - 3 ] = 4 mEq/L ×Δ P a aCO 2 /10

Metabolic Acidosis
1. Hyperchloremic or normal AG acidosis
• Renal causes e.g. RTA, renal insufficiency
• Urine AG is positive
• Urine AG= Na+K-Cl (spot urine electrolytes)
• Non-renal causes e.g. diarrhea
• Urine AG is negative

Metabolic Acidosis
2. High AG acidosis:
1. Lactic acidosis,
2. Ketoacidosis,
3. Ingested toxins,
• Aspirin
• Ethanol, methanol, ethylene glycol (osmolal
gap)
4. Acute or chronic renal failure

“MUD PILES”
• M-Methanol
• U-Uremia
• D-Diabetic Ketoacidosis
• P-Paraldehyde
• I-Infection, Iron, Isoniazid
• L-Lactic Lactic acidosis
• E-Ethylene Glycol, Ethanol
• S-Salicylates

Metabolic Alkalosis
1. Chloride sensitive: urine Cl < 15 mEq/L.
Causes include volume depletion, gastric
drainage, diuretics, villous adenoma etc.
2. Chloride resistant: urine Cl > 25 mEq/L.
Causes include mineralocorticoid excess,
Bartter’s syndrome, Cushing’s s syndrome etc.

Respiratory Acidosis
Primary problem is inadequate ventilation.
1. Acute: acute airway obstruction, drugs,
cardiopulmonary arrest, acute severe lung
disease.
2. Chronic: COPD, neuromuscular diseases,
restrictive lung diseases.

Respiratory Alkalosis
1.Problem caused by voluntary or mechanical
hyperventilation or increased neurochemical
stimulation of ventilation by central or
peripheral p causes e.g. anxiety, head trauma,
CNS tumors/infections, drugs, pain, fever,
pulmonary embolism, pneumonia, CHF,
asthma etc.