BLOOD GASES AND
ACID–BASE DISORDERS
Professor Magdy Amin RIAD
Professor of Otolaryngology.
Ain shames University
Senior Lecturer in Otolaryngology
University of Dundee
CAPILLARY BLOOD GASES
• pH: Same as arterial or slightly lower
(Normal = 7.35-7.40)
• pCO2: Same as arterial or slightly higher
(Normal = 40-45)
• pO2: Lower than arterial (Normal = 45-60)
• O2 Saturation: >70% is acceptable.
CAPILLARY BLOOD GASES
• Saturation is probably more useful than the
pO2 itself when interpreting a CBG.
• The heel is the most commonly used site
• The CBG is often used for pediatric patients
because it is easier to obtain than the ABG
• less traumatic (no risk of arterial
thrombosis, hemorrhage).
blood gas machines
• The blood gas machines in most labs
actually measure the pH ,the pCO2 and the
pO2.
• The [HCO3-] and the base difference are
calculated values using the HendersonHasselbalch equation
For a rough estimate of [H+]
•
•
•
•
[H+] = (7.80 -pH) x100.
This is accurate from a pH 7.25 to 7.48;
40 mEq/L = [H+] at the normal pH of 7.40.
pH is a log scale, and for every change of 0.3 in
pH from 7.40 the [H+] doubles or halves.
• For pH 7.10 the [H+] = 2 x 40, or 80 nmol/L, and
for pH 7.70 the [H+] = 1/2 x40, or 20 nmol/L.
pCO2 and pH
• A change in pCO2 up or down 10 mm Hg is
associated with an increase or decrease in
pH of 0.08 units.
• As the pCO2 decreases, the pH increases;
as the pCO2 increases, the pH decreases.
base deficit and base excess
• A pH change of 0.15 is equivalent to a base
change of 10 mEq/L.
• A decrease in base (i.e, [HCO3-]) is termed
a base deficit, and an increase in base is
termed a base excess.
Acidosis and alkalosis
• Acid-base disorders are very common clinical
problems.
• Acidemia is a pH <7.37, and
• alkalemia is a pH >7.44.
• Acidosis and alkalosis are used to describe how
the pH changes.
• The primary causes of acid-base disturbances are
abnormalities in the respiratory system and in the
metabolic or renal system.
normal compensatory response
• Any primary disturbance in acid-base homeostasis
invokes a normal compensatory response.
• A primary metabolic disorder leads to respiratory
compensation, and a primary respiratory disorder
leads to an acute metabolic response due to the
buffering capacity of body fluids.
• A more chronic compensation (1-2 days) due to
alterations in renal function.
Mixed acid-base disorder
• Most acid-base disorders result from a single primary disturbance
with the normal physiologic compensatory response and are called
simple acid-base disorders.
•
In certain cases, however, particularly in seriously ill patients, two
or more different primary disorders may occur simultaneously,
resulting in a mixed acid-base disorder.
•
The net effect of mixed disorders may be additive (eg, metabolic
acidosis and respiratory acidosis) and result in extreme alteration
of pH;
•
or they may be opposite (eg, metabolic acidosis and respiratory
alkalosis) and nullify each other’s effects on the pH.
INTERPRETATION OF BLOOD GASES
Step 1:
• Determine if the numbers fit.
• The right side of the equation should be within about
10% of the left side.
• If the numbers do not fit, you need to obtain another
ABG
INTERPRETATION OF BLOOD GASES
Step 2:
determine if an acidemia (pH <7.37)
or an alkalemia (pH >7.44) is present.
Step 3: Identify the primary disturbance as
metabolic or respiratory.
•
For example, if acidemia is present, is the pCO2 >44 mm Hg (respiratory
acidosis), or is the [HCO3 -] <22 mmol/L (metabolic acidosis).
•
In other words, identify which component, respiratory or metabolic, is
altered in the same direction as the pH abnormality.
• If both components act in the same direction (eg, both respiratory [pCO2 >
44 mm Hg] and metabolic [HCO3 - <22 mmol/L] acidosis are present),
then this is a mixed acid-base problem.
• The primary disturbance will be the one that varies from normal the
greatest, that is, with a [HCO3 -] = 6 mmol/L and pCO2 = 50 mm Hg, the
primary disturbance would be a metabolic acidosis, the [HCO3 -] is about
one-quarter normal, whereas the increase in pCO2 is only 25%.
Step 4:
• Calculate the anion gap.
• Anion gap = Na+ - (Cl- + HCO3 -).
• Normal anion gap is 8-12 mmol.
Step 5:
If the anion gap is elevated
• Then compare the changes from normal between the
anion gap and [HCO3 -].
• If the change in the anion gap is greater than the
change in the [HCO3 -] from normal, then a metabolic
alkalosis is present in addition to a gap metabolic
acidosis.
• If the change in the anion gap is less than the change in
the [HCO3 -] from normal, then a non gap metabolic
acidosis is present in addition to a gap metabolic
acidosis.
METABOLIC ACIDOSIS: DIAGNOSIS AND
TREATMENT
• Metabolic acidosis represents an increase in
acid in body fluids .
• Reflected by a decrease in [HCO3 -] and a
compensatory decrease in pCO2.
Anion Gap Acidosis:
• Anion gap >12 mmol/L; caused by a
decrease in [HCO3 -]
• balanced by an increase in an unmeasured
acid ion from either endogenous production
or exogenous ingestion (normochloremic
acidosis).
Non anion Gap Acidosis:
• Anion gap = 8-12 mmol/L; caused by a decrease in [HCO3 -] balanced
by an increase in chloride (hyperchloremic acidosis). Renal tubular
acidosis is a type of non gap acidosis
• The anion gap is helpful in identifying metabolic gap acidosis, non gap
acidosis, mixed metabolic gap and non gap acidosis. If an elevated
anion gap is present, a closer look at the anion gap and the bicarbonate
helps differentiate among
(a) a pure metabolic gap acidosis
(b) a metabolic non gap acidosis
(c) mixed metabolic gap and non gap acidosis, and
(d) a metabolic gap acidosis and metabolic alkalosis.
Treatment of Metabolic Acidosis
1. Correct any underlying disorder (control diarrhea, etc).
2. Treatment with bicarbonate should be reserved for severe
metabolic gap acidosis.
If the pH <7.20, correct with sodium bicarbonate.
The total replacement dose of [HCO3 -] can be calculated as
follows:
3. Replace with one-half the total amount of bicarbonate over 812 h and reevaluate.
• Be aware of sodium and volume overload during replacement.
• Normal or isotonic bicarbonate drip is made with 3 ampules
NaHCO3 (50 mmol NaHCO3/ampule) in 1 L D5W.
METABOLIC ALKALOSIS:
• Metabolic alkalosis represents an increase
in [HCO3 -] with a compensatory rise in
pCO2.
Differential Diagnosis
• In two basic categories of diseases the
kidneys retain [HCO3 -]
• They can be differentiated in terms of
response to treatment with sodium chloride
• and also by the level of urinary [Cl-] as
determined by ordering a Spot,or random
urinalysis for chloride (UCl).
Chloride-Sensitive (Responsive)
Metabolic Alkalosis:
• The initial problem is a sustained loss of chloride out of
proportion to the loss of sodium (either by renal or GI )
• This chloride depletion results in renal sodium
conservation leading to a corresponding reabsorption of
[HCO3 -] by the kidney.
• In this category of metabolic alkalosis, the urinary [Cl-]
is <10 mEq/L,
• and the disorders respond to treatment with intravenous
NaCl.
Chloride-Insensitive (Resistant)
Metabolic Alkalosis:
• The pathogenesis in this category is direct
stimulation of the kidneys to retain
bicarbonate irrespective of electrolyte
intake and losses.
• The urinary [Cl-] >10 mEq/L, and these
disorders do not respond to NaCl
administration.
Treatment of Metabolic Alkalosis
• Correct the underlying disorder.
1. Chloride-responsive
a. Replace volume with NaCl if depleted.
b. Correct hypokalemia if present.
c. NH4Cl and HCl should be reserved for extreme
cases.
2. Chloride-resistant
a. Treat underlying problem, such as stopping
exogenous steroids.
RESPIRATORY ACIDOSIS:
DIAGNOSIS AND TREATMENT
• Respiratory acidosis is a primary rise in pCO2
with a compensatory rise in plasma [HCO3 -].
• Increased pCO2 occurs in clinical situations in
which decreased alveolar ventilation occurs.
Differential Diagnosis
1. Neuromuscular Abnormalities with Ventilatory Failure
2. Central Nervous System Drugs, Sedative,,Central sleep apnea
3. Airway Obstruction
a. Chronic (COPD)
b. Acute (asthma)
c. Upper airway obstruction
d. Obstructive sleep apnea
4. Thoracic/Pulmonary Disorders
a. Bony thoracic cage: Flail chest, kyphoscoliosis
b. Parenchymal lesions: Pneumothorax, pulmonary edema,
c. Large pleural effusions
d. Scleroderma
e. Marked obesity (Pickwickian syndrome)
Treatment of Respiratory Acidosis
• Improve Ventilation:
Intubate patient and place on ventilator,
increase ventilator rate, reverse narcotic
sedation with naloxone (Narcan), etc
RESPIRATORY ALKALOSIS:
• Respiratory alkalosis is a primary fall in
pCO2 with a compensatory decrease in
plasma [HCO3 -].
• Respiratory alkalosis occurs with increased
alveolar ventilation.
Differential Diagnosis
1. Central stimulation
a. Anxiety, hyperventilation syndrome, pain
b. Head trauma or CVA with central neurogenic hyperventilation
c. Tumors
d. Salicylate overdose
e. Fever, early sepsis
2. Peripheral stimulation
a. PE
b. CHF (mild)
c. Interstitial lung disease
d. Pneumonia
e. Altitude
f. Hypoxemia:
3. Miscellaneous
a. Hepatic insufficiency
b. Pregnancy
c. Progesterone
d. Hyperthyroidism
e. Iatrogenic mechanical overventilation
Treatment of Respiratory
Alkalosis
• Correct the underlying disorder.
• Hyperventilation Syndrome: Best treated
by having the patient rebreathe into a paper
bag to increase pCO2, decrease ventilator
rate