acid base 2013

Introduction to acid-base

Joel Topf, M.D. Assistant Clinical Professor of Medicine

Wayne State University School of Medicine

http://www.pbfluids.com

Getting acid-base

•  Acid base physiology is the regulation of hydrogen ion concentration

•  A normal hydrogen concentration is 40 nmol/L

•  This is .00004 mmol/L

So •  It is measured on a negative log

scale called pH, normal is 7.4

40 nanomol/L = 0.00004 milimol/L Every change of 0.3 pH units represents a change in H+ by a factor of 2

Grand mal seizure

Is this patient sick?

pH = 6.8

Methanol toxicity

It’s the disease, stupid.

•  Hydrogen ion concentration can dramatically impact protein structure and enzyme function.

•  The absolute pH is less important than the etiology of the acid-base disturbance.

Since the disease is important

•  It is imperative to rapidly assess the cause of an acid-base disturbance.

•  Using an arterial blood gas and an electrolyte panel, one can quickly classify a patient’s primary and compensatory acid-base physiology.

•  Patients may have multiple, simultaneous acid-base disorders. This should be determined.

Determine the primary Acid-Base disorder

Metabolic acidosis

Metabolic alkalosis

Respiratory acidosis

Respiratory alkalosis

Determine the anion gap

Non-Anion gap Anion gap

Determine the osmolar gap

Determine the bicarbonate before

Osmolar gap Non-osmolar gap

Pre-existing met. alkalosis Pre-existing NAGMA No pre-existing acid-base disorders

Determine the urinary anion gap Positive gap

(RTA) Negative gap

(GI, IVF)

Winter’s formula

⅓ the Δ HCO3 1:10 acute 3:10 chronic

2:10 acute 4:10 chronic

Determine if the compensation is appropriate

Step 1: determine the primary disorder

The Henderson-Hasselbalch formula is the mantra of acid-base physiology

There are 4 primary ways that pH can change

Increase in HCO3, increases pH. Metabolic alkalosis



Decrease in HCO3, decreases pH. Metabolic acidosis




Increase in pCO2, decreases pH. Respiratory acidosis




Increase in pCO2, decreases pH. Respiratory acidosis

Decrease in pCO2, increases pH. Respiratory alkalosis

•  In respiratory disorders, the kidney modifies the serum bicarbonate to return pH toward normal.

Patients with primary acid-base disorders compensate to restore normal pH.

•  In metabolic disorders, breathing is altered to change the pCO2 in order to return pH toward normal.

Compensation minimizes changes in pH

Increased HCO3, increases pH.

Increased CO2 compensates to reduce the change in pH.


Decreased HCO3, decreases pH.

Decreased CO2 compensates to reduce the change in pH.


Increased CO2, decreases pH.

Increased HCO3 compensates to reduce the change in pH.

Compensation is always in the same direction as the primary disorder.

pCO2 HCO3 Metabolic acidosis

pCO2 HCO3 Metabolic alkalosis

HCO3 pCO2 Respiratory acidosis

HCO3 pCO2 Respiratory alkalosis

Primary Compensation

If all three variables move in the same direction the disorder is metabolic; �if they move in discordant directions it is respiratory

Primary Compensation

pCO2 HCO3 Metabolic acidosis

pCO2 HCO3 Metabolic alkalosis

HCO3 pCO2 Respiratory acidosis

HCO3 pCO2 Respiratory alkalosis

pH

Determine the primary disorder

1.  Acidosis or alkalosis –  If the pH is less than 7.4 it is acidosis –  If the pH is greater than 7.4 it is alkalosis

2.  Determine if it is respiratory or metabolic –  If the pH, bicarbonate and pCO2 all move in the same direction (up or

down) it is metabolic

–  If the pH, bicarbonate and pCO2 move in discordant directions (up and down) it is respiratory

pH / pO2 / pCO2 / HCO3

Determine the primary disorder


2.  Determine if it is respiratory or metabolic –  If the pH, bicarbonate and pCO2 all move in the same direction (up or

down) it is metabolic


7.2 / 78 / 25 / 16 pH / pO2 / pCO2 / HCO3


7.2 / 78 / 25 / 16 pH / pO2 / pCO2 / HCO3

2.  Determine if it is respiratory or metabolic –  If the pH, bicarbonate and pCO2 all move in the same

direction (up or down) it is metabolic


Metabolic Acidosis

1.  Respiratory acidosis 2.  Metabolic acidosis

3.  Respiratory alkalosis

4.  Respiratory alkalosis

7.5 / 55 / 24 / 36 pH / pO2 / pCO2 / HCO3

1.  Respiratory acidosis 2.  Metabolic acidosis

3.  Respiratory alkalosis 4.  Metabolic alkalosis

1.   Respiratory acidosis 2.  Metabolic acidosis

3.   Respiratory alkalosis 4.  Metabolic alkalosis

Respiratory alkalosis Determine the primary disorder

Now let’s do some questions


Metabolic acidosis

Metabolic alkalosis



Winter’s formula




Step 2: is there the correct degree of compensation?

•  The direction of the compensation is always in the same direction as the primary disorder.

•  The magnitude of the compensation is determined solely by the magnitude of the primary disorder.

–  If, in a case of metabolic acidosis, the bicarbonate falls to 10 then the pCO2 should fall to 23±2 to compensate.

–  If the pCO2 is not in that range a second primary disorder is present

•  If the pCO2 is less than 21, then the patient also has a respiratory alkalosis

•  If the pCO2 is over 25, the patient has an additional respiratory acidosis

•  Each primary acid base disorder has its own formula for prediction:

–  Metabolic acidosis: Winter’s Formula

•  1.5 × HCO3 + 8 ± 2

–  Metabolic alkalosis:

•  pCO2 rises 0.7 per mmol rise in HCO3

–  Respiratory acidosis:

•  1 or 3 mmol rise in HCO3 for 10 rise in pCO2

–  Respiratory alkalosis:

•  2 or 4 mmol fall in HCO3 for 10 fall in pCO2

Predicting pCO2 in metabolic acidosis

•  In metabolic acidosis the expected pCO2 can be estimated from the HCO3

Expected pCO2 = (1.5 x HCO3) + 8 ± 2

•  If the pCO2 is higher than predicted then there is an addition respiratory acidosis

•  If the pCO2 is lower than predicted there is an additional respiratory alkalosis

•  Example:

–  Expected pCO2 = (1.5 x HCO3) + 8 ±2 –  Expected pCO2 = 18-22 –  Actual pCO2 is 19, which is within the predicted range,

indicating a simple metabolic acidosis


7.23 / 78 / 19 / 8 pH / pO2 / pCO2 / HCO3

•  Example:

–  Expected pCO2 = (1.5 x HCO3) + 8 ±2 –  Expected pCO2 = 16-20 –  Actual pCO2 is 34, which is above the predicted range,

indicating an additional respiratory acidosis


7.15 / 112 / 34 / 12 pH / pO2 / pCO2 / HCO3

Predicting pCO2 in metabolic alkalosis

•  In metabolic acidosis the expected pCO2 can be estimated from the HCO3

pCO2 should rise 0.7 for every increase in HCO3 of one, ±2

7.46 / 78 / 49 / 34 pH / pO2 / pCO2 / HCO3 Example:

–  HCO3 is 34-24 = 10 above normal, so pCO2 should be 7 over normal, 47±2

–  Actual pCO2 is 49, which is within the predicted range, indicating a simple metabolic alkalosis

Respiratory disorders

•  Metabolic compensation for respiratory acid-base disorders is slow.

•  So the predicted bicarbonate needs to be calculated for pre-compensation, called acute, and after compensation, called chronic.

–  Chronic compensation is complete so the pH will be closer to normal at the expense of increased alteration of serum bicarbonate.

Why is metabolic compensation slow?

•  The lungs ventilate 12 moles of acid per day as carbon dioxide

•  The kidneys excrete less than 0.1 mole of acid per day as ammonia, phosphate and free hydrogen ions

•  The high excretion capacity of the lungs relative to the kidneys means that metabolic disorders can be rapidly compensated by the lungs while respiratory disorders take a long time to be compensated for by the kidneys.

•  Example:

•  pCO2 is 38 above normal, so –  if the condition is acute the HCO3 should be 28±2 –  If the condition is chronic the HCO3 should be 35 ±2 –  Actual HCO3 is 30, which is within the predicted range,

for acute respiratory acidosis and outside of the range for chronic.

Respiratory acidosis For every increase in pCO2 of 10 mmHg the bicarbonate should increase:

•  1 mEq/L in acute • 3 mEq/L in chronic

7.19 / 78 / 78 / 30 pH / pO2 / pCO2 / HCO3

•  Example:

•  pCO2 is 15 below normal, so –  If the condition is acute the HCO3 should be decreased

by 3 or 21±2 –  If the condition is chronic the HCO3 should be

decreased by 6 or 18 ±2

Respiratory alkalosis For every decrease in pCO2 of 10 mmHg the bicarbonate should decrease:

•  2 mEq/L in acute • 4 mEq/L in chronic

7.44 / 78 / 25 / 17 pH / pO2 / pCO2 / HCO3

Summary of metabolic compensation for respiratory acid-base disorders



10:1 10:2

10:3 10:4 For every rise of 10 in the pCO2 the HCO3 will rise by 1 or 3

For every fall of 10 in pCO2 the HCO3 will fall by 2 or 4.

PCO2 : HCO3

Acute

Chronic


Metabolic acidosis

Metabolic alkalosis





Winter’s formula




Step 3: if you have metabolic acidosis, is there an anion gap?

What is the anion?

•  Metabolic acidosis is further evaluated by determining the anion associated with the increased H+ cation

It is either chloride Or it is not chloride

•  These can be differentiated by measuring the anion gap.

Non-Anion Gap Met Acid Anion Gap Met Acid

Anion gap

=

Calculating the anion gap

•  Anion gap = Na – (HCO3 + Cl)

•  Normal at St John is 12

–  Varies by hospital

–  Average anion gap in healthy controls is 6 ±3

•  Improving chloride assays have resulted in increased chloride levels and a decreased normal anion gap.

Other causes of a low anion gap

•  Increased chloride –  Hypertriglyceridemia –  Bromide –  Iodide

•  Decreased “Unmeasured anions” –  Albumin –  Phosphorous

•  Increased “Unmeasured cations” –  Hyperkalemia –  Hypercalcemia –  Hypermagnesemia –  Lithium –  Increased cationic paraproteins

•  IgG

Albumin Phos IgA

Chloride Bicarb

Sodium

Potassium Calcium Magnesium IgG

Normal anion

gap

7.38 / 212 / 27 / 16 •  Metabolic or Respiratory

Evaluate the ABG

•  Acidosis or Alkalosis •  Acidosis or Alkalosis

•  Metabolic or Respiratory

•  Anion gap or Non-Anion Gap •  Anion gap or Non-Anion Gap •  Predicted pCO2

  (16 x 1.5) + 8 ±2 =   30-34

•  Anion gap   144 – (110 + 16) =   18

•  Isolated metabolic acidosis?   No. There is concomitant

respiratory alkalosis.


144  110 3.4 16

The anion gap acidosis

•  Uremia (mild) •  Ingestions

–  Methanol –  Ethylene glycol

•  Ketoacidosis –  DKA –  Starvation –  Alcoholic

•  Sepsis

•  L-Lactic acidosis –  Salicylate intoxication –  Ischemia –  Cyanide intoxication

•  Nitroprusside

–  Malignancy –  Metformin –  Liver failure –  Thiamine deficiency

•  D-Lactic acidosis

•  Pyroglutamic acidosis

GOLDMARK •  The classic mnemonic, MUD PILES, sucks. �

The new mnemonic is GOLD MARK. Know it.

•  G Glycols

•  O Oxoproline: Pyroglutamic

•  L L-lactic acidosis

•  D D-Lactic acidosis

•  M Methanol

•  A Aspirin

•  R Renal failure

•  K Ketoacidosis

AN Mehta, JB Emmett , M Emmett, Lancet, 372, 9642, p 892, 2008


Metabolic acidosis

Metabolic alkalosis







Winter’s formula




Step 4: if you have an AGMA, is there an osmolar gap?

Osmolar gap •  In the presence of a large anion gap (>20-25) of undetermined etiology

you must rule out a toxic alcohol. –  Methanol –  Ethylene Glycol

•  The low molecular weight of the alcohols means that modest ingestions have a relatively large impact on the serum osmolality –  Few grams equals many milimoles

•  Their presence can be detected by comparing the measured osmolality (which includes the alcohol) to a calculated osmolality (which does not account for the alcohol).

•  If the measured osmolality is significantly more (>10) than the calculated osmolaility you have an osmolar gap.

€

Calculated osmolality = (2 ×Na) +BUN2.8

+Glucose

18+

Ethanol4.6

7.16 / 212 / 22 / 8 •  Metabolic or Respiratory

Question 4: evaluate the ABG



•  Anion gap or �Non-Anion Gap

•  Anion gap or �Non-Anion Gap

•  Predicted pCO2

  (8 x 1.5) + 8 ±2 =   18-22

•  Anion gap   142 – (110 + 8) =   24

•  Isolated metabolic acidosis?   Yes. There is no concomitant

respiratory disorder.


142 110 46 5.4 8 2.2 Serum Osmolality: 312

•  Osmolar gap   Calc Osmolality

  (2 x 142) + 46/2.8 + 88/18 =   284 + 16 + 5 = 305

  Osmolality Gap   312 – 305 = 7

•  Osmolar gap or �Non-Osmolar Gap

•  Osmolar gap or �Non-Osmolar Gap

88

Osmolar gap is not specific

•  Elevated osmolar gap will be found with: –  Ethylene glycol

–  Methanol –  Isopropyl alcohol

–  Ketoacidosis

–  Lactic acidosis

–  Mannitol infusion

–  Hypertriglyceridemia


Metabolic acidosis

Metabolic alkalosis






Determine the bicarbonate before


Pre-existing met. alkalosis Pre-existing NAGMA No pre-existing acid-base disorders

Winter’s formula




Step 5: if you have an AGMA, determine what the bicarbonate was before the anion gap

•  If you have an anion gap metabolic acidosis the anion gap should increase by one for every one that the bicarbonate falls.

The acid-base time machine

=

•  Assume that the loss of bicarbonate due to addition of an anion is roughly 1:1

•  So for every increase in the anion gap of one the bicarbonate should drop by one

HCO3 before = HCO3

now + (AGcurrent – AGnormal)

The acid-base time machine

∆ HCO3 = ∆ Anion Gap

HCO3 before – HCO3

now = AGcurrent – AGnormal

12)

7.14 / 212 / 18 / 6


Evaluate:



•  Anion gap or Non-Anion Gap •  Anion gap or Non-Anion Gap

•  Predicted pCO2

  (8 x 1.5) + 8 ±2 =   18-22

•  Anion gap   134 – (104 + 8) =   22

•  Isolated metabolic acidosis?   Yes.


134 104 3.4 8

•  Additional metabolic disorder?

•  Bicarbonate prior to anion gap   HCO3 + (AG – 12) = HCO3 before   8 + (22 – 12) =   18

  Yes.

  Non-anion gap metabolic acidosis

Most common error in acid-base

Personal observation

AE

•  66 yo white male •  PMHx DM, paraplegia 2° MVA •  Klebsiella urosepsis induced ARF

•  Blood Cxrs + for Klebsiella

•  8/16/04 139 107 31 5.4 20 1.2

•  8/26/04 138 104 38 4.4 21 1.9

•  8/28/04 137 108 53 3.8 16 2.9 –  Start oral bicarbonate

•  8/29/04 139 111 56 3.9 14 2.8 –  Start bicarbonate gtt

•  8/30/04 137 104 62 3.5 22 3.0

•  7.52 / 31 / 46 / 25

alkalosis Respiratory Predicted HCO3: Acute: 23 Chronic: 21

acid base 2013

Documents