Acid-Base Balance and Arterial Blood Gases

ABG Introduction

• Arterial Blood Gases (ABGs) provide crucial information about acid-base balance and oxygenation.

• Essential in assessing respiratory and metabolic function in critically ill patients.

Acids and Bases

• Acids and bases are electrolytes.

  • An electrolyte is any molecule or chemical compound that can release ions when dissolved into a liquid/solution such as water.

  • These ions have either a positive or a negative electrical charge.

• Acid:

  • Can release a proton or a hydrogen ion.

  • Can also accept electrons.

  • Most acids contain a hydrogen ion that can be released into a liquid/solution.

  • The release of the hydrogen ion by the acid is called dissociation.

  • The higher the concentration (percentage) of hydrogen ions released by an acid into a liquid/solution, the higher the acidity and the lower the pH of the solution.

  • If an acid in a solution does not release its hydrogen ions, the pH of the solution remains unchanged.

  • Acids are classified as either strong acids or weak acids.

Acids and Bases (Continued)

• Bases are the chemical opposite of acids.

  • They are molecules or chemical compounds that can accept protons or yield hydroxide ions (OH^-) in solution.

  • Bases can counteract acids by accepting the H^+ ions released by the acid. This process is called hydrolysis.

  • Bases can also be classified as either weak or strong.

Chemical Buffering Systems

• The body has three chemical buffering systems that respond immediately to pH changes:

  • The carbonic acid buffer system,

  • The phosphate buffer system,

  • The protein buffer system.

• The carbonic acid buffer system regulates pH in the intracellular fluid (ICF).

• The phosphate buffer system regulates pH in the extracellular fluid (ECF).

• The protein buffer system regulates pH in both the ICF and ECF.

Carbonic Acid Buffer System

• The carbonic acid buffer system maintains the balance in the blood between bicarbonate (HCO3^-), which is a weak base, and carbonic acid (H2CO_3^-), which is a weak acid.

• Bicarbonate levels in the blood are regulated by chemical reactions involving sodium.

• Carbonic acid levels in the blood are regulated by the expiration of carbon dioxide from the lungs.

• The balancing act between bicarbonate and carbonic acid that occurs in the blood to maintain pH is directly related to several interactions related to the ratio between bicarbonate and carbonic acid.

Role of the Respiratory System in Acid-Base Balance

• Volatile acids are acids that can be converted into a gaseous form and then eliminated by the lungs.

• The body produces between 13,000 and 20,000 mmol per day of volatile acid in the form of carbonic acid (H2CO3), which is excreted through respiration.

• Central and peripheral chemoreceptors in the body monitor changes in arterial PCO_2 levels and report changes to the respiratory center in the medulla.

• This portion of the brain then directs the appropriate response to the respiratory musculature.

Physiologic Interactions of Acids and Bases in the Body

• The body maintains pH balance through buffer systems, respiration, and renal function.

• Key buffers: bicarbonate (HCO_3^-), proteins (hemoglobin), and phosphates.

• Normal pH range: 7.35 – 7.45

Base Excess/Deficit

• Reflects metabolic component of acid-base balance.

  • Base Excess (BE) > +2 indicates metabolic alkalosis.

  • Base Deficit (BE) < -2 indicates metabolic acidosis.

Role of the Renal System in Acid-Base Balance

• The renal system is made up of the kidneys, bladder, ureters, urethra, and urinary meatus.

• The kidneys are bean-shaped organs located on either side of the abdominal cavity just above the waistline in the lumbar area.

• Highly vascular, the kidneys are approximately 11.5 cm (4.5 inches) long and 6.5 cm (2.5 inches) wide.

• The primary function of the kidneys is the filtration and excretion of metabolic waste products; regulation of electrolytes, fluids, and acid-base balance; and stimulation of red blood cell production.

• The kidneys also play a role in the regulation of blood pressure via the renin–angiotensin–aldosterone system by controlling reabsorption of water and maintaining intravascular volume.

Role of the Renal System in Acid-Base Balance (Continued)

• Approximately 20% of the cardiac output passes through the kidneys each minute.

• Adults have 0.8 to 1.5 million nephrons in each kidney.

• The nephrons are the structural centers of the kidney where filtration occurs.

• Each nephron is composed of a renal corpuscle and a renal tubule.

• Filtration occurs in the renal corpuscle, which is made up of a knot of capillaries called a glomerulus and a surrounding Bowman’s capsule.

• Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerulus.

Role of the Renal System in Acid-Base Homeostasis

• Metabolic Acidosis: Excess acid or loss of HCO_3^-.

• Metabolic Alkalosis: Excess HCO_3^- or loss of acid.

• Kidneys regulate pH by excreting H^+ and reabsorbing HCO_3^-.

Identifying Acid-Base Status

• Step 1: Assess pH (Acidosis <7.35 or Alkalosis >7.45).

• Step 2: Evaluate PCO_2 (Respiratory cause if abnormal).

• Step 3: Evaluate HCO_3^- (Metabolic cause if abnormal).

• Step 4: Determine Compensation (Full, Partial, or Uncompensated).

Role of the Respiratory System in Acid-Base Balance

• Lungs regulate pH by controlling CO_2 elimination.

• Hypoventilation increases CO_2 → Respiratory Acidosis.

• Hyperventilation decreases CO_2 → Respiratory Alkalosis.

Conditions Causing Acid-Base Disturbances

• Respiratory Acidosis: COPD, drug overdose, neuromuscular disorders.

• Respiratory Alkalosis: Anxiety, pain, high altitude, fever.

• Metabolic Acidosis: DKA, renal failure, lactic acidosis.

• Metabolic Alkalosis: Vomiting, diuretics, excessive bicarbonate intake.

Differences Between Arterial and Venous Blood Gases

• Arterial Blood Gases (ABG): Measures oxygenation and acid-base status.

• Venous Blood Gases (VBG):

  • Less accurate for oxygenation but useful for pH and CO_2 trends.

  • ABGs are preferred in critically ill patients.

Case Study Example

• Case: A patient presents with pH 7.30, PaCO2 50 mmHg, HCO3^- 24 mEq/L.

• Interpretation: Respiratory Acidosis (low pH, high CO2, normal HCO3^-).

• Likely Cause: COPD exacerbation.

Summary

• ABGs are vital for assessing respiratory and metabolic function.

• The respiratory and renal systems work together to maintain pH balance.

• Recognizing acid-base disorders is crucial for proper treatment.