Lecture 6: Osmo Blood Gases

Osmotic Pressure

Force that pulls water across membranes, primarily influenced by dissolved ions like sodium and small proteins to maintain fluid balance.

Four Main Colligative Properties

Osmotic pressure, lowered vapor pressure, raised boiling point, and lowered freezing point.

Osmolality vs. Osmolarity

Osmolality measures osmoles per kilogram of solvent (unaffected by temperature), whereas osmolarity measures osmoles per liter of solution.

Primary Lab Colligative Property

Freezing point depression is the primary colligative property used in clinical laboratory osmometry.

Osmolality Reference Ranges

Serum/plasma reference range is 275–300 mOsm/kg H2O; urine reference range is 300–900 mOsm/kg H2O.

Electrolytes vs. Nonelectrolytes Dissociation

Non-electrolytes (e.g., glucose, urea) do not dissociate (1 mmol = 1 mOsm), whereas electrolytes dissociate into multiple ions (1 mmol NaCl = 2 mOsm; 1 mmol CaCl2 = 3 mOsm).

Nonideal Osmolality Equation

Osmolality = φnC, where φ is the osmotic activity coefficient, n is the number of particles per molecule, and C is concentration/molality.

Calculated Serum Osmolality Formula

Calculated mOsm/kg = (2 × Na) + (Glucose / 18) + (Urea / 2.8).

Osmolal Gap Definition & Normal Value

The difference between directly measured and calculated osmolality; normal reference interval is < 10 mOsm/kg.

Elevated Osmolal Gap Causes

Caused by exogenous osmotic substances like ethanol or pseudohyponatremia due to the electrolyte exclusion effect.

Freezing Point Depression Constant

Added solute yielding 1 Osm/kg H2O depresses the freezing point of pure water by -1.86°C.

Osmometer Calculation Formula

Osmolality (mOsm/kg) = (Observed Freezing Point / -1.86) × 1000.

Freezing Point Osmometer Key Parts

Cooling chamber (supercools sample to ~-7°C), agitation/stirring mechanism (initiates ice crystallization), thermistor (detects temperature changes), and digital display.

Heat of Fusion / Equilibrium Plateau

Agitation triggers ice crystal formation in supercooled sample, releasing heat of fusion so temperature rises to a plateau where freezing and thawing balance.

BTPS Conditions

Body Temperature (37°C), Ambient barometric Pressure, and Saturated water vapor pressure (47 mmHg).

Humidified Inspired pO2 Formula

pO2 = (pAmb - pH2O) × fO2; at sea level, pO2 = (760 mmHg - 47 mmHg) × 0.21 = 149.7 mmHg.

Room Air Exposure Effect on ABG

Air exposure causes sample pO2 to rise toward room air (~150 mmHg), pCO2 to decrease toward room air (~0.3 mmHg), and pH to increase.

Dalton's Law

Total pressure of a gas mixture equals the sum of the partial pressures of all individual gases in the mixture.

Henry's Law

Concentration of a gas dissolved in liquid is directly proportional to its partial pressure above the liquid (c = α × P).

Dissolved Gas Values at Standard Pressures

At pO2 of 100 mmHg, dissolved O2 = 0.140 mmol/L; at pCO2 of 40 mmHg, dissolved CO2 = 1.224 mmol/L.

Henderson-Hasselbalch Equation for Blood pH

pH = 6.1 + log([HCO3-] / (0.03 × pCO2)), maintaining a normal 20:1 ratio of bicarbonate to dissolved CO2.

Calculated HCO3- vs. Measured Total CO2 Discrepancy

Discrepancies > 7.4 mmol/L prompt investigation for uncapped tubes (CO2 loss), air contamination, liquid heparin dilution, or sample collection timing errors.

Hemoglobin Oxygen Binding

Oxygen reversibly binds to ferrous iron (Fe2+) in hemoglobin, but does not bind to ferric iron (Fe3+).

Dyshemoglobins Examples

Methemoglobin (MetHb), Carboxyhemoglobin (COHb), and Sulfhemoglobin (SulfHb), which cannot transport oxygen normally.

Pulse Oximetry Wavelengths & Limitations

Measures light absorption at 660 nm (deoxyhemoglobin) and 940 nm (oxyhemoglobin); cannot distinguish dyshemoglobins like COHb, leading to falsely normal SpO2 during carbon monoxide poisoning.

Co-Oximetry Advantage

Uses spectrophotometry across multiple wavelengths to directly measure all individual hemoglobin species (O2Hb, HHb, COHb, MetHb).

Functional SO2 vs. Fractional FO2Hb

Functional SO2 = [O2Hb / (O2Hb + HHb)] × 100 (ref: 94–98%); Fractional FO2Hb = [O2Hb / (O2Hb + HHb + COHb + MetHb + SulfHb)] × 100 (ref: 90–95%).

P50 Definition & Significance

The pO2 at which hemoglobin is 50% saturated; an increased P50 indicates decreased O2 affinity and easier oxygen release to peripheral tissues.

Right Shift Causes (Decreased O2 Affinity)

Increased temperature, decreased pH (acidosis), increased pCO2, and increased 2,3-BPG.

Left Shift Causes (Increased O2 Affinity)

Decreased temperature, increased pH (alkalosis), decreased pCO2, decreased 2,3-BPG, fetal hemoglobin, and carbon monoxide binding.