Acid-Base Physiology
2012
Objectives
Understand normal mechanisms and regulation
of acid-base balance
 Interpret blood gases
 Understand the effects of acidosis and alkalosis
 Evaluate and manage acidosis and alkalosis

Normal Physiology


Acid-base balance maintained by normal
pulmonary excretion of CO2 and renal excretion
of acid
Organic buffers: HCO3-, HPO4, protein anions,
carbonate



90% of bicarb is reabsorbed by kidney
Renal excretion – H+ combines with urinary
titratable acids (phosphates) or ammonia to form
ammonium
Henderson-Hasselbach equation:
pH = 6.1 + log (HCO3 ÷ (0.03 x PCO2))
Compensatory Mechanisms
 pH
is determined by ratio of HCO3 and
PCO2
 Body responds to changes in pH by
attempting to normalize the pH
Buffering
 Respiratory – alterations in paCO2
 Renal – alterations in bicarbonate excretion

Compensatory Mechanisms

Compensated metabolic acidosis:


Compensated metabolic alkalosis:


0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3
Compensated respiratory acidosis:



1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3
Acute- 1 meq/L  for every 10 mmHg  in pCO2
Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2
Compensated respiratory alkalosis:


Acute- 2 meq/L  for every 10 mmHg  in pCO2
Chronic- 4 meq/L  for every 10 mmHg  in pCO2
Blood Gas Interpretation

General guidelines:



1) Is it acidosis or alkalosis?
 Acidosis – pH < 7.38
 Alkalosis – pH > 7.42
2) Is it primary respiratory or metabolic?
 Evaluate paCO2 and bicarbonate
3) Is there compensation?
 Calculations from previous slides
Blood Gas Interpretation

4) If respiratory disturbance, is it acute or chronic?



Respiratory acidosis:
 Acute decrease in pH = 0.08 x (paCO2-40)/10
 Chronic decrease in pH = 0.03 x (paCO2-40)/10
Respiratory alkalosis
 Acute increase in pH = 0.08 x (40-paCO2)/10
 Chronic increase in pH = 0.03 x (40-paCO2)/10
5) If metabolic disturbance, is there an anion gap?
 Check
serum Na, Cl, HCO3
Case #1
pH 7.16, pCO2 70, HCO3 24
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical Scenario?

Case #1
pH 7.16, pCO2 70, HCO3 24
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical Scenario?

Case #1
pH 7.16, pCO2 70, HCO3 24
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical Scenario?

Case #1
pH 7.16, pCO2 70, HCO3 24
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



No … likely acute
Clinical Scenario?
Case #1
pH 7.16, pCO2 70, HCO3 24
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical Scenario?


2 yo receiving deep sedation by the adult ED
attending who gives him 4 mg morphine,
respiratory rate is 6
Acute Respiratory Acidosis

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Respiratory pathophysiology – airway obstruction,
severe pneumonia, chest trauma, pneumothorax
Acute drug intoxication (narcotics, sedatives)
Residual neuromuscular blockade
CNS disease (head trauma, decreased consciousness)
Bicarbonate is often normal, kidneys have not had
time to compensate
Case #2
pH 7.6, pCO2 23, HCO3 22
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #2
pH 7.6, pCO2 23, HCO3 22
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #2
pH 7.6, pCO2 23, HCO3 22
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #2
pH 7.6, pCO2 23, HCO3 22
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



No … likely acute
Clinical scenario?
Case #2
pH 7.6, pCO2 23, HCO3 22
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


4 mo mechanically ventilated pt who was bagged
during transport to CT scan by an overeager intern
Respiratory Alkalosis





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Pain, anxiety
Hypoxemia
Interstitial lung disease
Severe congestive heart
failure (pulmonary
edema)
Pulmonary emboli
Drugs – salicylates,
methylxanthines,
nicotine





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Sepsis, fever
Hepatic failure –
encephalopathy
Pregnancy
Thyrotoxicosis
CNS hemorrhage
Overagressive
mechanical
ventilation
Case #3
pH 7.29, pCO2 26, HCO3 12
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #3
pH 7.29, pCO2 26, HCO3 12
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #3
pH 7.29, pCO2 26, HCO3 12
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Compensatory Mechanisms

Compensated metabolic acidosis:
 1.2 mmHg  in pCO2 for every 1 meq/L  in
HCO3

Compensated metabolic alkalosis:
 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3
Compensated respiratory acidosis:
 Acute- 1 meq/L  for every 10 mmHg  in pCO2
 Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2
Compensated respiratory alkalosis:
 Acute- 2 meq/L  for every 10 mmHg  in pCO2
 Chronic- 4 meq/L  for every 10 mmHg  in pCO2


Case #3
pH 7.29, pCO2 26, HCO3 12
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



Yes … 1.2 mmHg decrease in pCO2 for every 1
meq/L decrease in HCO3
Clinical scenario?
Case #3
pH 7.29, pCO2 26, HCO3 12
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


10 yo dev delayed pt admitted with diarrhea, med
list reveals mom has been giving Miralax every 4
hours
Metabolic Acidosis

Anion gap
Metabolic Acidosis

Anion gap




Lactic acidosis
DKA
Toxic ingestions (salicylates, ethylene glycol, ethanol,
isopropyl alcohol, paraldehyde, methanol)
Renal failure – uremia
Metabolic Acidosis

Nonanion gap
Metabolic Acidosis

Nonanion gap

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RTA
Diarrhea
Hypoaldosteronism
Potassium-sparing diuretics
Pancreatic loss of bicarbonate
Ureteral diversion
Carbonic anhydrase inhibitors
Acid administration (ArgCl, NaCl)
Case #4
pH 7.47, pCO2 46, HCO3 32
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #4
pH 7.47, pCO2 46, HCO3 32
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #4
pH 7.47, pCO2 46, HCO3 32
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Compensatory Mechanisms

Compensated metabolic acidosis:
 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3

Compensated metabolic alkalosis:
 0.7 mmHg  in pCO2 for every 1 meq/L  in
HCO3

Compensated respiratory acidosis:
 Acute- 1 meq/L  for every 10 mmHg  in pCO2
 Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2
Compensated respiratory alkalosis:
 Acute- 2 meq/L  for every 10 mmHg  in pCO2
 Chronic- 4 meq/L  for every 10 mmHg  in pCO2

Case #4
pH 7.47, pCO2 46, HCO3 32
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



Yes … 0.7 mmHg increase in pCO2 for every 1
meq/L increase in HCO3
Clinical scenario?
Case #4
pH 7.47, pCO2 46, HCO3 32
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


5 yo s/p appendectomy with NG tube left to
suction on 7CH for 5 days
Metabolic Alkalosis

Chloride-responsive (urine Cl < 10 meq/L)



Gastric acid loss (vomiting, NG suction)
Contraction alkalosis (often due to loop or thiazide
diuretics)
Posthypercapnia syndrome
Metabolic Alkalosis

Chloride-resistant



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Mineralocorticoid excess
Renal chloride wasting (Bartter syndrome)
Exogenous alkali (milk-alkali syndrome, massive
blood transfusion)
Hypokalemia
Case #5
pH 7.30, pCO2 89, HCO3 42
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clincial scenario?

Case #5
pH 7.30, pCO2 89, HCO3 42
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clincial scenario?

Case #5
pH 7.30, pCO2 89, HCO3 42
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clincial scenario?

Compensatory Mechanisms


Compensated metabolic acidosis:
 1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3
Compensated metabolic alkalosis:
 0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3

Compensated respiratory acidosis:
 Acute- 1 meq/L  for every 10 mmHg  in pCO2
 Chronic- 3.5 meq/L  for every 10 mmHg  in
pCO2

Compensated respiratory alkalosis:
 Acute- 2 meq/L  for every 10 mmHg  in pCO2
 Chronic- 4 meq/L  for every 10 mmHg  in pCO2
Case #5
pH 7.30, pCO2 89, HCO3 42
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



Yes … 3.5 meq/L increase in HCO3 for every 10
mmHg increase in CO2
Clincial scenario?
Case #5
pH 7.30, pCO2 89, HCO3 42
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clincial scenario?


35 yo CF patient on the Peds floor with end-stage
lung disease
Chronic Respiratory Acidosis
Chronic lung diseases (BPD, CF)
 Neuromuscular disorders
 Severe restrictive lung disease
 Severe obesity

Case #6
pH 6.84, pCO2 82, HCO3 14
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #6
pH 6.84, pCO2 82, HCO3 14
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Blood Gas Interpretation

4) If respiratory acidosis or alkalosis, is it acute or
chronic?


Respiratory acidosis:
 Acute decrease in pH = 0.08 x (paCO2-40)/10
 Chronic decrease in pH = 0.03 x (paCO2-40)/10
Respiratory alkalosis
 Acute increase in pH = 0.08 x (40-paCO2)/10
 Chronic increase in pH = 0.03 x (40-paCO2)/10
Case #6
pH 6.84, pCO2 82, HCO3 14
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #6
pH 6.84, pCO2 82, HCO3 14
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?



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No …
Combined acidosis
Clinical scenario?
Case #6
pH 6.84, pCO2 82, HCO3 14
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


2 mo found down at home, hypoperfusion leading
to lactic acidosis, hypoventilation leading to
respiratory acidosis
Case # 7
pH 7.46, pCO2 24, HCO3 16
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case # 7
pH 7.46, pCO2 24, HCO3 16
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case # 7
pH 7.46, pCO2 24, HCO3 16
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case # 7
pH 7.46, pCO2 24, HCO3 16
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?

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Well … 2 meq/L decrease in HCO3 for every 10
mmHg decrease in CO2
Combined respiratory alkalosis & metabolic
acidosis
Clinical scenario?
Case # 7
pH 7.46, pCO2 24, HCO3 16
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


Anxious 5 yo who is hyperventilating and has a
history of RTA
Case #8
pH 7.45, pCO2 54, HCO3 36
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #8
pH 7.45, pCO2 54, HCO3 36
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #8
pH 7.45, pCO2 54, HCO3 36
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?

Case #8
pH 7.45, pCO2 54, HCO3 36
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?




No … 0.7 mmHg increase for every 1 meq/L
predicts pCO2 of 48
Combined metabolic alkalosis and respiratory
acidosis
Clinical scenario?
Case #8
pH 7.45, pCO2 54, HCO3 36
 Acidosis or alkalosis?
 Respiratory or metabolic?
 Compensated?
 Clinical scenario?


1 yo with vomiting for 3 days who presents to the
ED with lethargy and decreased arousal,
hypoventilating
Physiologic Effects of Acidosis

Shifts the oxygen-hemoglobin dissociation
curve to the right

Decreased affinity for O2
Physiologic Effects of Acidosis
Pulmonary effects – vasoconstriction
decreases pulmonary blood flow
 Cardiac effects – depressed contractility
 Neurologic effects – increased cerebral blood
flow, increased ICP
 Extracellular shift of K+  hyperkalemia
 Sympathetic overactivity, resistance to
catecholamines

Physiologic Effects of Alkalosis

Shifts the oxygen-hemoglobin dissociation
curve to the left


Stronger bond between Hb and O2
Decreased O2 delivery to tissues
Physiologic Effects of Alkalosis
Cardiac arrhythmias
 Lungs – vasodilation increases pulmonary
blood flow
 Neurologic effects – headache, seizures, altered
mental status


Decreased cerebral blood flow from vasoconstriction
Decreased levels of ionized Ca++
 Intracellular shift of potassium  severe
hypokalemia

Management - Respiratory Acidosis


Treat the underlying disorder
Assist or increase ventilation





Secure airway if necessary
Increase tidal volume or respiratory rate if mechanically
ventilated
Noninvasive ventilation
Bronchodilators
Reverse sedative medications
Management – Respiratory Alkalosis


Treat the underlying disorder
Decrease ventilation




Decrease respiratory rate
Decrease tidal volume
Sedation and pain control
Reassurance for anxious patients
Management - Metabolic Disorders

Acidosis




Treat the underlying disorder
Consider bicarb administration depending on etiology
Dialysis in the setting of renal failure
Alkalosis



Treat the underlying disorder
Chloride-responsive: replete chloride (NaCl, KCl, ArgCl)
Carbonic-anydrase inhibitors if diuresis needed
Conclusion





The body has compensatory mechanisms to maintain
acid-base balance.
Blood gases should be interpreted in a systematic way to
determine the etiology of the acid-base disturbance.
Acidosis causes pulmonary vasoconstriction, cardiac
depression, arterial vasodilation, & decreased O2 affinity.
Alkalosis causes pulmonary vasodilation, arterial
vasoconstriction, & increased O2 affinity.
Management of acid-base disorders primarily involves
treatment of the underlying disorder.
Everyone always has slides of their kids …
QUESTIONS?
References
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www.uptodate.com
www.emedicine.com
Morganroth ML. Six steps to acid-base analysis: clinical
applications. The Journal of Critical Illness. 1990;5:460-69.
Morganroth ML. An analytic approach to diagnosing acidbase disorders. The Journal of Critical Illness. 1990;5:138-50.