Stewart's Quantitative approach

Question 1

Quantitative approach

Answer 1

Mathematical explantion of the relevant variable that control H+ in body fluids.

Treats body fluids as a system containing multiple interacting constituents.

Question 2

Principles

Answer 2

1. Manitainence of electircal neutralitiy
2. Satisfaction of dissociation equilibria for weak elecrolytes
3. Conservation of mass

Question 3

How is different to Henderson Hasselbach approach?

Answer 3

HH considers only few variables in system. pH, pCO2 & bicarbonate.

Stewarts approach takes many variables into account.

Question 4

Basic concept of Stewarts physiochemical method

Answer 4

Dependent variables:

H+, OH-, HCO3-, CO3^2-, HA,A^-. ( There values depend on independent variables)

Independent variables: ( can be altered from outside the system without affecting each other).

1. Strong ion difference
2. Total weak nonvolatile acid concentration (ATOT)
3. PCO².

Question 5

Strong ion difference

SID = (Na+ + K+ + Ca2+ + Mg2+) – (Cl- – other strong anions)

Answer 5

Strong ions: ions that dissociate totally at the pH of interest in a particular solution.

strong cations: Na+, K+, Ca2+, Mg2+

strong anions : Cl- and SO42-

SID = (Na+ + K+ + Ca2+ + Mg2+) – (Cl- – other strong anions)

Question 6

SID

‘Two methods”

Answer 6

(1) Concentration change:

Dehydration: concentrates the alkalinity and increases SID

Overhydration: dilutes the alkaline state (dilutional acidosis) and decreases SID.

(2) Strong Ion changes:

Decreased Na+: decreased SID and acidosis

Increased Na: increased SID and alkalosis

Increased Cl-: decreased SID and acidosis (NAGMA)

increased in organic acids (lactate, formate, ketoacids): decreased SID and acidosis (HAGMA))

Question 7

Total weak acid concentration (ATOT)

it is A_tot not ATOT

Answer 7

Total plasma concentration of inorganic phosphate, serum proteins and albumin (weak non-volatile acids)

ATOT = [PiTOT] + PrTOT] + albumin

Question 8

PCO2

Answer 8

Partial pressure of CO2

Question 9

PROS OF THE STEWART METHOD

Answer 9

Acknowledgement of the importance of other factors controlling pH

Diminishes the importance of the HCO3- ion which is just a dependent variable

Question 10

CRITICISMS OF THE STEWART METHOD

Answer 10

1. complex
2. calculation of small differences between large numbers of variables -> decreases accuracy
3. SID only reflect plasma (where as SBE reflects the whole body and Hb’s influence)
4. lack of clinical correlation to validate benefit
5. standard base excess accuracy has been well validated and accepted in clinical correlation

Question 11

Classification of Primary A-B disturbance

Answer 11

Respiratory : ↑ PCO2 ↓ PCO2

Non-Respiratory
a. Abnormal SID
i .Water excess/deficit ↓ SID, ↓ [Na+] ↑ SID, ↑ [Na+]
ii. Imbalance of strong anions

• Chloride excess/ deficit ↓ SID, ↑ [Cl-] ↑ SID, ↓ [Cl-]
• Unidentified anion excess ↓ SID, ↑ [XA-]
• *b. Non-volatile weak acids**
  i. Serum albumin ↑ [Alb] ↓ [Alb]
  ii. Inorganic phosphate ↑ [Pi] ↓ [Pi]

Question 12

Base excess

Answer 12

The base excess is defined as the amount of H+ ions that would be required to return the pH of the blood to 7.35 if the pCO2 were adjusted to normal.

Question 13

ATOT

Answer 13