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Respiratory regulation of Acid-Base Balance
DR. SUMREENA MANSOOR
ASSISTANT PROF OF BIOCHEMISTRY
DEPT OF BIOCHEMISTRY & MOLECULAR BIOLOGY
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Arterial Blood Gas (ABG)
pH, pCO2, pO2 – Measured directly
HCO3-, O2 saturation (usually) – Calculated from pH,
pCO2, and pO2
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Normal Arterial Blood Gas Values
pH 7.35 - 7.45
PaCO2 35-45 mmHg(4.7-6.0kpa)
PaO2 70-100mmHg(11.3-
12.6kpa)
SaO2 93 - 98%
HCO3¯ 22 - 26 mmol/L
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Arterial Blood Gases
Condition State of
PaCO2 in blood alveolar ventilation
> 45 mm Hg Hypercapnia Hypoventilation
35 - 45 mm Hg Eucapnia Normal ventilation
< 35 mm Hg Hypocapnia Hyperventilation
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Practical Approach
1. Check the pH
If the pH < 7.35 acidemia is present.
If the pH > 7.45 alkalemia is present.
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Practical Approach
2. Check the pCO2
pH < 7.35 and pCO2 > 40 respiratory acidosis
pH > 7.45 and pCO2 < 40 respiratory alkalosis
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17 year-old female patient while hyperventilating
(frequent, deep, sighing respiration to sustained, obvious,
rapid, deep breathing) fell unconscious to the floor. An
attending Physician diagnosed her condition as
"hyperventilation syndrome" caused by her anxiety and
mental stress. He successfully treated her by rebreathing
expired CO2 from a paper bag and she completely
recovered.
What happened to the acid-base balance in her body?
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Respiratory alkalosis
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54 year-old man becomes critically unwell with
confusion and hypotension
Test Results Normal Range
PaO2 7.9 kPa 11.3-12.6
PaCO2 12.8 kPa 4.7-6.0
pH 7.16 7.36-7.44
HCO-3 42 mmol/l 20-28
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Respiratory acidosis with renal
compensation
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A 25 year-old patient has acute
exacerbation of bronchial asthma.
Which abnormality on the arterial
blood gas evaluation does this
patient most likely have?
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High pH suggesting low
Hydrogen [H+] ions*
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17 year old w/severe kyphoscoliosis, admitted for
pneumonia
pH: 7.37
pCO2: 25 mmHg
pO2: 60 mmHg
HCO3: 14 mmol/L
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Compensated respiratory alkalosis
due to chronic hyperventilation
secondary to hypoxia
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9 year old with H/O asthma, audibly wheezing x 1
week, has not slept in 2 nights; presents sitting up
and using accessory muscles to breath with audible
wheezes
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pH: 7.51
pCO2: 25 mmHg
pO2 35 mmHg
HCO3: 22 mEq/L
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Uncompensated respiratory
alkalosis with severe hypoxia due to
asthma exacerbation
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57-year-old male with COPD, home oxygen
dependence, daily chronic sputum production, is
treated with several types of metered dose inhalers.
No other medications. He is in his usual state.
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Chronic respiratory acidosis
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The patient described in above problem develops increasing
shortness of
breath, purulent sputum production, and increased weakness
requiring
emergency room visit. He appears to be in moderate respiratory
distress
peripheral cyanosis.
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Acute respiratory acidosis and chronic respiratory acidosis
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MALE, AGED 70 YEARS, WITH RESPIRATORYAND METABOLIC FAILURE DUE TO COPD. HE WAS
ADMITTED TO ICU AND MECHANICALLY VENTILATED
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ON ADMISSION THIS PATIENT HAD A PARTIALLY
COMPENSATED RESPIRATORY ACIDOSIS, i. e PCO2 AND
[HCO-3]. VENTILATION WAS OVER-ENTHUSIASTIC AND
PRODUCED A RESPIRATORY ALKALOSIS (P CO2 <35).
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