Normal Values

pH = 7.38 - 7.42

[H+]  = 40 nM/L for a pH of 7.4

PaCO₂  = 40 mm Hg

[HCO₃] = 24 meq/L

Acid base definitions

Acid base disorder is considered present when there is abnormality in HCO₃ or PaCO₂ or pH.

Acidosis and alkalosis refer to in-vivo derangement's and not to any change in pH.

Acidemia (pH < 7.38) and Alkalemia  (pH >7.42) refer to derangement's of blood pH.

Kidney and Respiratory system play a key roles in maintaining the acid base status.

Primary Acid base disorders

Recquired lab values/information 

• Arterial blood gases: pH, PaCO₂,PaO₂,Sat,CO
• Electrolytes: Na, K, Cl, HCO₃
• BUN, Glucose, Creatinine
• FIO2 and Clinical history

Anion and Cations

Electrochemical balance means that the total anions are the same as total Cations. For practical purposes anion gap is calculated using only Sodium, Chlorides and Total CO₂.((140-(104+24)) = 12.

Compensatory measures

• Buffering---occurs immediately

• Respiratory regulation of pCO₂ is intermediate (12-24 hours)

• Renal regulation of [H] and [HCO₃] occurs more slowly (several days)

Extracellular

• almost entirely through bicarbonate whose concentration highest of all buffers
• small contribution from phosphate

Intracellular

• Hemoglobin can directly buffer protons
  • H+ entry into RBC matched by exit of Na and K+
• Hemoglobin can directly buffer dissolved
• intracellular conversion of

Buffer systems

• Hemoglobin can directly buffer protons
  • H+ entry into RBC matched by exit of Na and K+
• Hemoglobin can directly buffer dissolved
• intracellular conversion of

Bicarbonate buffer system

Calculated bicarbonate:

• calculated using the H-H equation

• From arterial blood

Total CO₂:

• measured from venous blood

• includes bicarbonate and dissolved carbon dioxide

• runs slightly higher than calculated value from nomogram

Primary changes in bicarbonate leads to metabolic acid base disorder.

Metabolic compensation 

• Respiratory acidosis
  • Acute: Expected decrease in pH = 0.08 x (measured PaCO₂ - 40)
  • Chronic: Expected drop in pH = 0.03 x (measured PaCO₂ - 40)
• Respiratory alkalosis
  • Acute
  • Chronic

Compensation is almost complete

Anion gap (AG)

• Sum of Cations minus anions
• (Na+K)-(CL+HCO₃)
• Use the measured total CO2 from venous blood as HCO₃
• anion gap is an artifact because some anions are not measured
• gap is mainly due to unmeasured proteins, phosphates and sulfates
• Normal anion gap is 8-12 meq/L (Varies from Lab to Lab)

The bicarbonate gap

• useful in identifying mixed acidbase disorders
• in single acidbase disorder the difference between anion gap and the change in total CO₂ should  be negligible
• in other words change in total CO₂  (Normal total CO₂-observed total CO₂) should be equal to anion gap.
• Excess bicarbonate gap suggests metabolic alkalosis
• Decrease in the gap suggests metabolic acidosis

Renal regulation of  [H+] and [HCO₃]

Two major functions 

1. Reclamation of filtered bicarbonate
2. Excretion of Acid
   • titratable acidity
   • ammonium formation
   • free H+ excretion

Bicarbonate change for PaCO₂ changes

Hypoventilation:

• Expected bicarbonate elevation for chronic CO₂ retention  (Renal compensation) is
  • 0.35 x (current PaCO2-normal PaCO₂)  
  • 0.35 x( 60-40)=0.35x20=7 meq/l
• Expected bicarbonate decrease for acute CO₂ retention  (Buffering) is

Hyperventilation:

• The expected fall in bicarb for  acute decrease in PaCO₂   (Buffering) is 0.2 x (normal PaCO2-observed PaCO2)
• The expected fall in bicarb for  chronic decrease in PaCO2 (Renal compensation) is : 0.5 x (normal PaCO2-observed PaCO2)

 Ventilation and pCO₂ relationship 

• PaCO₂ is inversely proportional to ventilation
• Ventilation increases in response to a drop in pH
  • respiratory center in medulla
  • responds to pH "intermediate" between that of CSF and plasma
  • response is rapid
  • response is more predictable for falls in pH
  • for increase in pH ventilation decreases only when it is severe and protracted

Interrelationship between pH and pCO2

This formula is usable because in the range of pH values we usually deal with, there is nearly linear relationship between pH and pCO2.

Relationship between pH and acute CO₂ changes

We can expect an almost leniar relationship between pH and acute CO₂ changes

Respiratory compensation  for Metabolic acid base disturbance

• Metabolic acidosis
  • PaCO2 decrease by 1.2 x the drop in [H+]
  • You can use the following crude formula  0.1 change in pH 10 mm change of PaCO₂.
  • If all else is well the PaCO₂ should be the same as decimal values of pH i.e. for a pH of 7.28, the CO₂ levels would be 28 mm Hg.   

  Acidosis increases respiratory drive, alveolar ventilation and gets rid of Carbonic acid.

  Respiratory system can never completely compensate for a metabolic defect.

  Respiratory compensation attempts to maintain pH in a reasonable range.

• Metabolic alkalosis
  • Only when it is severe and protracted
  • It is unusual to see CO₂ retention (I don't agree with Books and others)
  • Less predictable

Compensation is never complete. If the pH is normal there is probably a superimposed second acid base disturbance. Estimation of expected PaCO₂ for a given acidic pH also enables us to determine whether respiratory compensation is appropriate.

High anion gap metabolic Acidosis
Usually from addition of acid

• Ketoacidosis
  • DKA
  • Alcoholic
  • Starvation
• Lactic acidosis
  • Hypoperfusion
• Ingestion's
  • ASA
  • Ethylene glycol
  • Methanol
• Renal insufficiency
  • inability to excrete acid

Normal anion gap metabolic acidosis

• Renal disease

  Proximal or distal RTA
  Renal insufficiency (HCO3 loss)
  Hypoaldosteronism / K+ sparing diuretics

• Loss of alkali

  Diarrhea
  Ureterosigmoidostomy

• Ingestions

  Carbonic anhydrase inhibitors

Metabolic alkalosis

• Vomiting/gastric suction
• Contraction alkalosis
• Posthyercapnic alkalosis
• Hyperreninism
• Hypokalemia
• Cirrhosis with ascites
• Corticoid excess
• Massive blood transfusion

Respiratory Acidosis

Obstructive pulmonary disease
Respiratory center depression (drugs, anesthesia)
Pickwickian/sleep apnea syndrome
Kyphoscoliosis
End stage restrictive pulmonary disease

Respiratory Alkalosis

Anxiety/Pain
Aspirin
Fever
Sepsis
Hypoxemia
Pregnancy
Hepatic insufficiency
Ventilator induced
Diffuse interstitial fibrosis

Renal regulation of  [H+] and [HCO₃]  

• a dumb kidney is usually better than a smart Doctor
• Reclamation of filtered bicarbonate
  • 4000 meq / day in normal persons
  • by far the greatest use of secreted acids

Renal regulation of  [H+] and [HCO₃]

Excretion of Acid 

• Titratable acidity
  • mainly with phosphate
  • 10-30 meq / day

• Ammonia formation
  • H+ combines with  N3HO from NH4+ which is trapped in tubule
  • baseline 30-60 meq / day, but may rise to as much as 250-400 meq / day

Renal regulation of  [H+] and [HCO₃]

Excretion of  free [H] 

• maximally acidified urine -- pH = 4.5  -- will account for only 0.04 meq +H / L

• urinary acidification is important in order to titrate protons into phosphate and ammonium

Renal regulation of  [H+] and [HCO₃]

Excretion of  acid stimulated by 

• Acidemia
• Hypercapnea
• Volume depletion
• Chloride depletion
• ? Hypokalemia
• Aldosterone

Renal regulation of  [H+] and [HCO₃]

Excretion of  acid inhibited by 

• Alkalemia
• Elevated [HCO3]
• Hypocapnea
• ? Hyperkalemia

Renal regulation of  [H+] and [HCO₃]

Two major functions 

1. Reclamation of filtered bicarbonate
2. Excretion of Acid
   • Titratable acidity
   • ammonium formation
   • free H+ excretion

Approach to interpreting Acid-base disturbance 

• Is the patient Acidemic or Alkalemic
• Identify the primary Acidbase disorder by evaluating HCO₃ and PaCO₂

  What is the [HCO₃]

  • Elevated------Metabolic alkalosis if alkalotic
  • Decreased ---- Metabolic acidosis if acidotic

  What is the PaCO₂

  • Elevated ---Respiratory acidosis if acidotic
  • Decreased --- Respiratory alkalosis if alkalotic
• What is the anion gap (to determine etiology of Metabolic acidosis)
• What is the bicarbonate gap (to evaluate mixed disorders)
• Is the degree of compensation what you expect (appropriate)?