Acid-Base Balance

By: Hannah Coakley2/27/2014

Quick Review: AcidsAcids are compounds which function as

hydrogen (H+) donors in biochemical equations/solutes

The more free H+ ions available for donation, the more acidic the compound

Many foods that are “acidic” in taste are actually metabolized into basic compounds in the body

Quick Review: Bases

Bases are compounds which can accept H+ ions.

This is accomplished by having an excess of OH- (hydroxide) ions

The terms “basic” and “alkaline” are used interchangeably

Quick Review: pH

pH is the –log of H+ concentration in any given solute

Its range spans from 0 – 14

A lower pH implies a high H+ concentration (acidic). A higher pH implies a low H+ concentration (basic)

The pH Scale

Role of pH in the BodyIntra and extracellular pH levels are tightly

regulated:

Acids & Bases in the Body

The body naturally produces more H+ than OH- ions

This occurs in several ways:-- The metabolism of fats (fatty acids) and proteins (amino acids)

-- The byproduct of cellular respiration: where carbonic acid breaks down into CO2 and H20 to be breathed out by the lungs

The Chemical Buffer System

Works to regulate pH by taking up or releasing H+ ions accordingly

Protects neutrality, usually by pairing a weak acid with a base

Also functions by substituting a strong acid or base for a weak one

Methods of Excretion

The other primary way to maintain pH homeostasis is through the excretion of excess acids or bases.

Respiratory excretion of CO2 using rate and depth of breath

Renal excretion, which eliminates acids and can also regulate the amount of circulating bicarbonate (HCO3

-)

pH Balance Visualized

Defining Acidosis and Alkalosis

Acidosis:• pH< 7.35 • Primary effect is in suppression of the CNS decreased Ca

binding to protein, high I-Cal• Weakness, coma, death

Alkalosis:• pH > 7.45• Primary effect is in overstimulation of CNS & PNS

increased Ca binding to protein, low I-Cal• Lightheadedness, spasms/tetany, death

Metabolic vs Respiratory

Metabolic Acidosis loss of relative concentration of bicarbonate ion (< 22 mEq/L)

(Symptoms: Headache, lethargy, N/V/D, coma)

Metabolic Alkalosis excess of relative concentration of bicarbonate ion (> 26 mEq/L)

(Symptoms: electrolyte depletion, tetany, slow and/or shallow breathing, tachycardia)

Metabolic vs Respiratory

Respiratory Acidosis carbonic acid excess leading to hypercapnia (pCO2 > 45 mm Hg)

(Symptoms: warm, flushed skin vasodilation, breathlessness, hypoventilation, disorientation, tremors)

Respiratory Alkalosis carbonic acid deficit leading to hypocapnia (pCO2 < 35 mm Hg)

(Symptoms: dizziness, lightheaded, numbness of extremities)

Compensatory Mechanisms

Depending upon the primary acid-base

imbalance, the body will compensate using a

secondary mechanism in order to return pH

homeostasis to the body

Compensation: Metabolic

Metabolic Acidosis Increased Ventilation to eliminate excess CO2

(Hyperventilation)

K+ & PO4 shifting from ICF to ECF to function as a buffer (H+ shifts into the cells)

Metabolic Alkalosis Decreased ventilation (Hypoventilation)

limited by constraints of hypoxia

Compensation: Respiratory

Respiratory Acidosis Kidneys eliminate excess H+ ions, retain

Bicarbonate ions

Respiratory Alkalosis Kidneys conserve H+ ions and excrete excess

Bicarb ions K+ shift from ECF to ICF to increase circulating

H+ (sudden low serum K+)

Calculating the Anion Gap

Interpreting the Anion GapIf the Anion gap is > than 26 mEq/L this is

considered normochloremic acidosis

If the anion gap is WNL (6 – 12 mEq/L), this is considered hyperchloremic acidosis

Understanding Base Excess

Mixed Acid-Base Disorders

More than one acid/base disturbance can occur concomitantly in the body

If unexpected lab values are noted, there is good reason to suspect a mixed acid-base

disorder

Treatment Strategies: Metabolic

Acidosis If Hyperchloremic: IV-Lactate solution is given, this is converted

to bicarb in the liver, thus raising the relative concentration of bicarbonate in the blood (shift of K+ back into ICF and may cause a need for it to be repleted)

If Normochloremic:Identify and correct sources of excess acids

Treatment Strategies: Metabolic

AlkalosisSaline Responsive (urine Cl- < 10 mEq/L)IV-NaCl solution is given to physiologically replace the excess bicarbonate ions in the blood with Cl

** Administration of KCl is also essential, as adequate K+ buffer in the ECF is essential to fully correcting the alkalosis

Treatment Strategies: Respiratory

Acidosis-- Treat the underlying dysfunction or disease

-- Restore appropriate ventilation

-- Add IV-lactate to aid in compensatory bicarb production

-- Ensure that the patient is not being overfed, as this will prolong acidosis (via excess CO2 production)

Treatment Strategies: Respiratory

Alkalosis-- Treat underlying dysfunction or disease

-- Attempt to slow respiration

-- Add IV-Cl to aid in compensatory replacement of excess bicarb

-- Replete K+ as needed, since K+ shifts intracellularly in exchange for H+ in the ECF

Correcting Acid/Base Imbalance:Step By Step

1) Analyze the pH

2) Analyze the pCO2

3) Analyze the HCO3

4) Match the pCO2 or the HCO3 with the pH

5) Assess AG and BE

6) Assess directionality/compensation

7) Analyze the pCO2 and O2 saturation

Check Your Knowledge

Scenario #1a:

pH: acidic // CO2: high // HCO3: high

What is the primary imbalance? Is there evidence of compensation?

#1b: What if HCO3 was normal? Low?

Take Home:What is the RD’s Role?

Monitoring

Treatment (Repletion of electrolytes and fluids)

Maintenance (appropriate TF or TPN)

Thank You!

ReferencesBrantley, Susan. The ABCs of ABGs. Support Line. UT Medical Center, Knoxville, TN.

Langley, Ginger. Fluid, Electrolytes, and Acid-Case Disorders. A.S.P.E.N Nutrition Support Core Curriculum, 2007.

Gilmore, Diane M. Acid Base Balance and Imbalance. Arkansas State University, Dept of Pathophysiology, 2012.

Ebihara, L. & West, John. Acid-Base Balance, A Respiratory Approach. Repiratory Physiology, The Essentials. 2011

Jaber, Bertrand. Metabolic Acidosis. Tufts University Open Courseware, Renal Pathophysiology. 2007.

Kibble, Jonathan D.& Colby R. Halsey, Medical Physiology: The Big Picture. 2009.

Skujor, Mario & Mira Milas. Endocrinology. Cleveland Clinic: Center for Continuing Education. 2013