NRAN 80323 Diffusion and Dilution.pptx

Diffusion and Dilution NRAN 80323 Casey Crow DNP, CRNA Objectives • Diffusion • Graham’s Law • Osmosis • Osmotic Pressure • Osmolarity/osmolality • Fick’s Law Diffusion • The net movement of one type of molecule through space as a result of random motion to minimize a concentration gradient •...

Diffusion and Dilution NRAN 80323 Casey Crow DNP, CRNA Objectives • Diffusion • Graham’s Law • Osmosis • Osmotic Pressure • Osmolarity/osmolality • Fick’s Law Diffusion • The net movement of one type of molecule through space as a result of random motion to minimize a concentration gradient • Space- liquid or gas • Brownian Motion: Random, uncontrolled movement of particles in a fluid as they constantly collide with other molecules • Random walk • Driven by the inherent kinetic energy of the molecules Diffusion • Diffusion constant (D) decreases with • Increasing molecular mass • Heavy molecular mass diffuses a shorter distance over time xrms= distance • Increasing density of solvent (root meter square) D = diffusion constant assigned per particular molecule • Diffusion constant (D) increases with • Increasing temperature • Molecules moving quicker, then diffusion distance increases over time Diffusion • Graham’s Law • Rate of diffusion of a gas is inversely proportional to the square root of the molecular weight • Think: molecular weight and rate of diffusion • Important factor: molecular weight r = rate of diffusion mw = molecular weight Diffusion • Diffusion of solute through a permeable membrane is dependent on 5 factors • How solutes can pass through a membrane Directly proportional Inversely proportional Concentration Gradient Membrane Thickness Tissue Area Molecular Weight Fluid:Tissue Solubility Diffusion • Fick’s Law: explains the directly proportional and indirectly proportional factors of diffusion DIFFUSION = DP x (area /thickness ) x (solubility gas gas molecular weightgas) mem mem gas / 1. Delta P- concentration gradient, directly proportional, example: overpressurizing VA 2. Tissue area/membrane area, directly proportional, example: lungs, COPD, atelectasis, surface area of alveoli 3. Fluid/tissue solubility, directly proportional, example: lipophilicity of drugs 4. Membrane thickness, indirectly proportional, example: pulmonary edema, ARDs, pulmonary fibrosis 5. Molecular weight, indirectly proportional, example: heavy Diffusion • Fick’s Law: cellular respiration Diffusion • Fick’s Law: movement of O2 and CO2 at alveolar capillary membrane Diffusion • Nitrous Oxide (N2O) • Highly diffusible: across the membrane • Low molecular weight • Inversely proportional, increased rate of diffusion • Low blood:gas partition coefficient Diffusion into closed air spaces Diffusion Hypoxia • Almost* insoluble in blood Diffusion • Diffusion into closed spaces • While N2O has low blood solubility, it is still 34 times more soluble than N • B:G partition coefficient is 0.46 • N2O will rapidly diffuse into closed spaces containing “air” Middle Ear Bowel Pneumothorax Air Embolism Bowel Obstruction ETT Cuff PA Catheter Cuff • N2O inspired at 70% will double the size of a pneumothorax in 10 min • Compliant spaces will continue to expand • Non-compliant spaces will increase in pressure Diffusion • Diffusion Hypoxia • Discontinuation of N2O on emergence results in rapid diffusion of N2O from bloodstream into alveolus • Results in significant, but transient, dilution of O2 and CO2 in alveolus • Attenuated by administration of 100% O2 We need CO2 to diffuse out from capillary to alveolus We need O2 to diffuse in from the alveolus to capillary to be delivered to the tissue LIMITED AMOUNT of molecules of gas that we can put into the alveolus, set partial pressure SO, this leads to increased concentration of N20 and decreased concentration of O2 leading Diffusion • Second-Gas Effect • Applies to coadministration of volatile anesthetic (more soluble, less diffusible) with N2O (less soluble, more diffusible) • Rapid uptake (immediate diffusion) of N2O from alveolus into bloodstream concentrates the amount of volatile anesthetic in alveolus • Greater concentration of alveolar volatile anesthetic speeds onset- overpressurizing, increase FA increase Fa • Also applies on emergence when rapid diffusion of N2O from bloodstream into alveolus concentrates amount of volatile anesthetic in bloodstream (“pulling” from tissue compartments) • Emptying the physiologic compartments more quickly Diffusion Diffusion • Factors affecting diffusion for NON-GASES across membranes • Concentration gradient from NON-IONIZED molecules • Electrochemical gradient for IONIZED molecules • Lipid Solubility • Size of Molecule Diffusion • Semipermeable Membrane • Process in which molecules of a solvent (what something is dissolved in) pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentration on each side of the membrane • NOT permeable to the solute • Solvent is freely permeable and moves to dilute the solute and establish equilibrium • When WATER is solvent, process is known as OSMOSIS Osmosis • Osmotic Pressure • Pressure, exerted by the solutes, required to stop osmosis • Related to the number of non-permeable molecules (solute) • Physiologic osmotic pressure in capillaries results from #1 factor for osmotic pressure in plasma proteins CAPILLARIES: PLASMA PROTEINS • ONCOTIC Pressure • Normal: 24-27 mmHg • Osmole Plasma proteins hold fluid in even though we have a very semipermeable membrane • One-gram molecular weight of undissociated solute • Can be measured as osmoles per liter of solvent or osmoles per kilogram of solvent Osmosis Osmolarity: Concentration of solute in terms of osmoles per liter VOLUME Osmolality: Concentration of solute in terms of osmoles per •kilogram ClinicalWEIGHT Application • Lab typically reports serum osmolality • Normal: 285-295 mOsm/kg • Osmolarity can be calculated at bedside: 2(Na+ + K+) + glucose + BUN • Difference is Osmotic Gap • Normally <10-15 mmOsm/kg • Value >15 is indicative of large presence of foreign SOLUTE Osmosis • Osmolarity • Concentration of solute expressed in terms of osmoles per liter VOLUME • Avogadro’s Hypothesis: One mole of solute in molar volume (22.4L) will exert 1 atmP at 0 degrees C • In mixed solution, osmotic pressure is the SUM of individual molarities: the added osmotic pressures of Na, K, Glucose and BUN • Over 99% of plasma osmolarity due to serum electrolytes, remainder is plasma protein, exception: CAPILLARIES • Plasma protein play role in capillaries • Oncotic pressure 24-27 mmHg or 285-2295 mOsm/kg • In dilute solutions such as plasma, differences between osmolality and osmolarity are generally <1% • RBC’s will lyse at osmolarities below 200 mOsm/L Osmosis • Physiologic Membranes • For osmosis to occur • Across cell membranes • Membranes must be impermeable to one or more solutes • Must have difference in concentration of solutes • Across capillary walls • Most capillary walls are permeable to small solutes (Na+, Cl-, etc) • Not permeable to albumin/plasma proteins, which is why they contribute to oncotic pressure Summary • Define diffusion and Brownian Motion • Describe diffusion in terms of molecular mass, density of solvent and temperature • Graham’s Law considers which particular characteristic of diffusion? • Describe diffusion across a permeable membrane • Define Fick’s Law in terms of the 5 factors of diffusion across a permeable membrane • How does Fick’s Law apply to physiologic gas exchange in lungs? • Apply diffusion into closed spaces, diffusion hypoxia, and second-gas effect to the application of N2O • What are the factors affecting diffusion of non-gas particles across physiologic membranes? • What is osmosis? • Define and describe osmotic pressure, osmolarity, and osmolality • What are the physiologic implications of serum osmolarity? • Describe osmosis across human cell membranes and capillary walls References • Shubert / Chapter 6 / 152-153 • Nagelhout / Chapter 15