Pathophysiology Resources

Pulmonary and Critical Care Pathophysiology Resources

A. Acid-base disorders

Resources for Medical Student pathophysiology small group sessions.

DISCLAIMER: while the Henderson-Hasselbach approach provides a sufficiently broad understanding of many disorders,

it is overly reductivist and dependent on analysis of derived variables. WHen you have mastered this, take an opportunity to review the linked full text reviews for a more modern and scientifically robust approach based on the work of Stewart and Fencl - termed the physico - chemical approach.

• • Disorders of Acid Base Balance - Physicochemical approach;

  • Acid Base Review I. Epidemiology

  • Acid Base Review II. Clinical

  • Acid Base Review III. Fluid

  • Review of Stepwise approach to Acid-base disturbances

  • Online Acid-Base Physiology tutorial

• Clinical Pearls in Acid-base Disorders

Stepwise approach to Acid-base Analysis (Detailed on the ATS website by David Kauffman, MD: Acid-Base)

• 1. Careful and detailed history & examination

  2. Obtain ABG and check for internal consistency

  3. Determine the primary disturbance (If MA - Evaluate the gaps - AG, delta - delta)

  4. Establish the presence and effectiveness of any compensatory met/resp state. (See table)

  5. What does not fit? Superimposed or concomitant secondary disorder.

1. Evaluate gas -exchange efficiency - calculate Aa gradient

Expected changes in Acid-base disorders

Metabolic Acidosis ΔpCO2= 1.2 ΔHCO3

Metabolic Alkalosis Δ pCO2 = 0.9 Δ HCO3

Acute Respiratory Acidosis Δ HCO3 = 0.1 Δ pCO2

Chronic Respiratory Acidosis Δ HCO3 = 0.35 Δ pCO2

Acute Respiratory Alkalosis Δ HCO3 = 0.2 Δ pCO2

Chronic Respiratory Alkalosis Δ HCO3 = 0.5 Δ pCO2

For an alternative and physicochemically rational approach to acid-base balance:

• • Quantitative acid–base physiology using the Stewart model

  • Disorders of acid-base balance - Physciochemical analysis

  • Stewart-Fencl approach tutorial with java applet for calcuations

B. Laminar and turbulent flow through rigid tubes

Liquid flow through a pipe is dependent on viscosity of the fluid and can be modeled using the

Hagen-Poiseuille equation. (Link to Wikipedia description)

In standard fluid dynamics notation:

ΔP = (8 μ L Q) / (π r4)

Where:

ΔP is the pressure drop

L is the length of pipe

μ is the dynamic viscosity

Q is the volumetric flow rate

r is the radius

d is the diameter

π is the mathematical constant (approximately 3.1416).

The key point is that for small reductions in diameter of the conducting tube, there is a large

(to the 4th power) increase in the resistive pressure drop for flow. For this reason small amounts of narrowing of central airways (or large vessels) cause big reductions in air (or blood) flow.

The critical Reynolds number

The transition between laminar and turbulent flow is often indicated by a critical Reynolds number which depends on the exact flow configuration and must be determined experimentally. Within circular pipes the critical Reynolds number is generally accepted to be ~2400, where the Reynolds number is based on the pipe diameter and the mean velocity vs within the pipe.

This provides the basis for the use of Helium:Oxygen 80:20 or 70:30 mixtures in the management of acute large airway obstruction.

(reviewed by Dean Hess: History and Physics of Heliox)

The density of heliox is 1/3 that of Nitrogen containing room air and results in a transition of the driving gas Reynolds number to laminar flow.

Coming soon:

Pulmonary Gas Exchange