Acid-Base Imbalance

NRS 440

2010

What is pH?

• pH is the concentration of hydrogen (H+) ions

• The pH of blood indicates the net result of normal acid-base regulation, any acid-base imbalance, and the body’s compensatory mechanisms

• The human body must maintain a very narrow pH range– 7.35-7.45

What is pH?

• In terms of the human body:

• acidosis<------7.4------>alkalosis– Carbon dioxide is the “acid” (CO2)

• Normal: 35-45 mmHg

– Bicarbonate is the “base” (HCO3)• Normal: 22-26 mEq/L

How does the body maintain pH?

• Buffer systems– Prevent major changes in pH by removing or

releasing a hydrogen (H+) ion– Act chemically to change strong acids into

weaker acids or to bind acids to neutralize their effects

1. Carbonic acid (H2C03) buffer system neutralized hydrochloric acid

2. Phosphate buffer system neutralizes strong acids

How does the body maintain pH?

• Buffer systems3. Intracellular and extracellular proteins act as

buffer systems

4. The cell can act as a buffer by shifting hydrogen in and out of the cell

How does the body maintain pH?

• Kidneys– Regulate bicarbonate in the ECF– The kidneys will retain or excrete H+ ions or HCO3

ions as needed– Normally acidic urine

• Lungs– Control CO2– Adjust rate and depth of ventilation in response to

amount of CO2 in the blood• A rise in arterial blood CO2 stimulates respiration• Oxygen content of arterial blood will also stimulate

respiration

Acidosis and Alkalosis

• Metabolic acidosis – Decreased HCO3 or increase in other acids

• Metabolic alkalosis– Increased HCO3 and excess loss of acids

• Respiratory acidosis– Increased PaCO2 due to hypoventilation

• Respiratory alkalosis– Decreased PaC02 due to hyperventilation

Imbalances

• Imbalances in PaCO2 are influenced by respiratory causes

• Imbalances in HCO3 are influenced by metabolic processes

Metabolic Acidosis• Low pH (<7.35)

• Low HCO3 (<22 mEq/L)

• Body may attempt to compensate by increasing respirations to decrease CO2

High anion gap acidosis– Results from excessive accumulation of fixed

acid

Normal anion gap acidosis– Results from direct loss of bicarbonate

Metabolic acidosis• Primary feature is decrease in serum HCO3• Hyperkalemia may also occur due to shift of

potassium out of the cells– Hypokalemia may occur once the acidosis is

corrected

• Treatment is aimed at correcting the metabolic defect– IV bicarbonate– Potassium management– Dialysis

Metabolic Alkalosis• High pH (>7.45)• High serum HCO3 (>26)• Body may attempt to compensate by decreasing

respirations to increase CO2• Treatment is aimed at treating the underlying

disorder– Chloride supplementation– Restore normal fluid volume– Maintain potassium– Carbonic anhydrase inhibitor if unable to tolerate

volume resuscitation

Respiratory Acidosis• Low pH (<7.35)• High serum CO2 (>42)• Body may attempt to compensate through

renal retention of HCO3 (does not happen quickly - hours to days)– Chronic respiratory acidosis occurs with chronic

pulmonary disease (eg, emphysema, OSA)• Pt. will often be asymptomatic, as the body has time to

compensate

– Acute respiratory acidosis may be severe and will produce symptoms

Respiratory Acidosis

• Treatment is directed at improving ventilation --> treat the underlying cause– Pulmonary hygiene to clear respiratory

tract– Adequate hydration to help clear

secretions– Supplemental oxygen– Adjustment of mechanical ventilation as

appropriate

Respiratory Alkalosis• High pH (>7.45)• Low PaCO2 (<35)• Always due to hyperventilation• Body may compensate through increased

kidney excretion of bicarbonate (does not happen quickly - hours to days)

• Treatment is aimed at correcting the cause of hyperventilation– If anxiety-related, may breathe into a closed

system (rebreathe CO2)

Interpreting Arterial Blood Gases

• pH (7.35-7.45)

• PaO2 (80-100 mmHg on room air)

• O2 saturation (95-100%)

• PaCO2 (35-45 mmHg)

• HCO3 (22-26 mEq/L)

• Base excess (or deficit) (+2 to -2 mEq/L)– Sum of bases (alkalis)

Interpreting Arterial Blood Gases

• 1. Determine if acidosis or alkalosis– *use 7.40 as normal in this step

• 2. Determine the component that caused the abnormality in step 1

• 3. Determine if the gas is compensated– If the pH is 7.35-7.45, it is compensated– If the pH is <7.35 or >7.45, it is

uncompensated

Case Study• Alan

– 17 years old– History of:

• Feeling “bad”• Fatigue• Constant thirst• Frequent urination

Case Study

• Alan– Blood glucose is 484 mg/dL– Respirations are 28, lungs are clear to

auscultation– Breath has a fruity odor

Case Study

• Alan– What acid-base disorder would you

expect?– What is the treatment for the disorder?

Practice

Low <---- Neutral ----> High

pH 7.46

CO2 30

HCO3 22

Practice

Low <---- Neutral ----> High

pH 7.38

CO2 51

HCO3 29

Practice

Low <---- Neutral ----> High

pH 7.28

CO2 35

HCO3 18

Case Study

• Susan’s ABG results are: – pH 7.20– PaCO2 58 mm Hg– PaO2 59 mm Hg – HCO3

24 mEq/L

1.Describe a patient who would have these ABGs, including history and assessment.

2.What is the treatment?