Acid-base Flashcards
What is an acid?
A H+ donor
What is a strong acid?
A H+ donor that is completely dissociated at physiological pH
What is a weak acid?
A H+ donor that is incompletely dissociated at physiological pH
What is pH?
The reverse logarithmic representation of H+ activity
What is a base?
A H+ recipient
What does base excess represent?
A reflection of the metabolic component of acid-base status.
What is HCO3-?
A non-linear representation of the metabolic component. It is not measured in gas samples but is derived from pH and PaCO2
What is standard base excess?
The acid required when Hb= 50g/L
The reduced buffering capacity of this Hb depleted blood is more representative of extracellular acid-base status
What is actual base excess?
Actual base excess is the amount of acid that must be added to return the pH in vitro to 7.40 under the standard conditions of T=37 and PaCO2=5.33Kpa
What are the two approaches to acid-base balance
Siggard-Anderson
Stewart approach - this offers more understanding of the underlying mechanism
What is the Siggaard-Anderson approach?
It’s based upon the Henderson-Hasselbach equation, which arises from the dissociation equation for carbonic acid: CO2+H20H+ +HCO3-
It’s based on the premise that HCO3 and PaCO2 are the main determinants of pH
A negative BE suggests a metabolic acidosis is contributing to the acid-base state
Identification of a metabolic acidosis should prompt identification of the precipitating biochemical abnormality - calculate anion gap.
What is the Henderson-Hasselbach equation?
pH= 6.1 +log HCO3/ (0.03x PaCO2)
How do you calculate anion gap?
(Na+K)-(Cl+HCO3)
What are the mechanisms behind normal anion gap metabolic acidosis?
Bicarbonate loss
Decreased renal excretion of H
Ingestion/infusion of acid
What is the mechanism behind raised anion gap acidosis?
Strong acid accumulation
What are the causes of raised anion gap acidosis?
Lactic acidosis Ketoacidosis AKI/CKD Toxins - methanol, ethylene glycol Chronic paracetamol use Salicylate poisoning (MUDPILES)
What are the causes of normal anion gas acidosis?
Diarrhoea Ileostomy Renal tubular acidosis TPN Dilutional Colonic ureteric implant/diversion
What are the concepts that the Stewart approach to acid-base balance are based on?
- Acids increase the [H+] and bases decrease the [H+] of a solution
- HCO3- is not independent of pH/H+, rather it is dependent on it
- Acids are said to be stong if the fully dissociate in plasma and weak if they are only partially dissociated
- It takes into account the influence of albumin on acid-base disturbances
According to the Stewart theory, what are the 3 major contributors to acid-base balance?
- Strong ion difference
- Concentration of non-volatile weak acids in the plasma
- PaCO2
What is the strong ion difference?
Reflects the difference between the number of strong cations and the number of strong anions
Strong cations: Na, K, Ca, Mg
Strong anions: Cl, lactate
How do you calculate the ‘apparent’ SID?
([H+] + [K+] + [Ca2+] + [Mg2+]) - ([Cl-] + [lactate-])
What does an increase in SID mean?
An increase in strong cations relative to anions leads to an alkalosis
What does a decrease in SID mean?
Acidosis
What is the ‘apparent’ SID?
SID is referred to as apparent if it only include the strong ions which are routinely measured
What is the ‘effective’ strong ion difference?
Can be calculated using a different equation that takes into account PaCo2 and albumin
What is the strong ion gap?
The difference between the SIDeff and the SIDapp
What is the significance of the SIG?
If elevated (>2) it signifies the presence of unmeasured strong cations contributing to the acidosis e.g. ketones, salicylates, paraldehyde
According to the ‘Emcrit’ method how do you interpret an ABG?
- Look at pH - acidosis or alkalosis
- Look at PaCO2 - respiratory or metabolic
- Calculate SID (Na-Cl)
- > if < 38 this is a metabolic acidosis
- > if > 38 this is a metabolic alkalosis
- Look at the lactate if > 2 then think sepsis, or ischaemia, or drug induced
- Calculate the strong ion gap
- > if > 2 this is a SIG acidosis (MUDPILES)
- > if negative causes include high ca, mg or K, immunoglobulins, nitrates, lithium OD
- Think about compensations
How do you calculate the SIG?
(the emcrit method)
[base deficit] + [SID - 38] + 2.5(42-albumin) - lactate
Compensation:
If the primary defect is respiratory and you think it’s chronic how do you calculate the expected metabolic compensation?
Expected change in BE = 0.4 x chronic change in CO2
If the primary problem is a metabolic acidosis how do you calculate the expected decrease in CO2?
= base deficit
If the primary problem is a metabolic alkalosis how do you estimate the expected increase in CO2?
0.6 x base excess
