int12 acid-base disorders

7/28/2019 Int12 Acid-Base Disorders

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Acid-Base Disorders

Bashir Khassawneh

Hani Eid

08 / 11 / 2009


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Internal Medicine, lec#12 - Date: 8/11/2009

Dr. Bashir KhassawnehDone by: Hani Eid.

Acid-Base Disorders

Our body contains many enzymes which their function is essential to our life. Most of theseenzymes are pH-sensitive i.e. they won't function unless certain acidity is maintained.

So, to preserve the function of these pH-sensitive enzymes, the concentration ofhydrogen ions [H+] in both extracellular and intracellular compartments is tightly

controlled. Any small change in pH will impair the function of enzymes and may lead to

major cell dysfunction.

[H+]is determined by the balance between PaCO2 and serum HCO3 (bicarbonate). [H+] = 24 PaCO2 / [HCO3-]If [HCO3-] decreases [H+] increases

If PaCO2 increases [H+] increases

If PaCO2 increases [HCO3-] increases.If [HCO3-] decreases PaCO2 decreases.

If you remember, Henderson Hasselbalch equation:

pH = 6.1 + log ( [HCO3-] / 0.03PaCO2).-where 6.1 is the pKa for carbonic acid

(H2CO3) and 0.03, the factor which relates PaCO2 to the amount of CO2 dissolved in plasma.

The two primary organs that are essentially responsible for acid-base balance are thekidney and the lung.

The kidneys function to maintain the bicarbonate buffer system; kidney can retain theexisting bicarb and even generate new bicarb, raising the blood alkali. Kidney also can

actively excrete H+.

The lungs rapidly eliminate CO2 (which is essentially an acid). If CO2 accumulates in thebody, this will lead to acidosis. So the lungs and kidneys work in harmony in order to

maintain acid-base homeostasis.

Regarding [H+], the problem is that it presents in the body in a very minute amount (bynEq or nmol/L), this problem has been overcome by using the concept ofpH. So instead

of saying: "the normal [H+] in blood is 40nEq", we can say: "the normal physiological

blood pH is 7.40"!


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But, be careful! The pH is logarithm, so there is no direct relation, i.e. if [H+] increasesfrom 40nEq 62nEq, pH will drop by 0.2; on the other hand, if [H+] decreases from

40nEq 18nEq, pH will increase by 0.35!!

Normal Values:o [H+] = 40 nEq/Lo pH = 7.40 (7.35-7.45); 7.4 is the optimum pH but (7.35-7.45) is the

normal physiological range that the body accepts.o PaCO2 = 40 mm Hg (35-45)o HCO3 = 24 mEq/L (22-26)

An acid base disorder is a change in the normal value of extracellular pH that may resultwhen renal or respiratory function is abnormal, or when an acid or base load overwhelmsexcretory capacity.

Acidemia - Decrease in the blood pH below normal range of 7.35 -7.45.Alkalemia - Elevation in the blood pH above the normal range of 7.357.45.

Keep It Simple:PaCO2 = Acid

o PaCO2 = pH (Acidemia)o PaCO2 = pH (Alkalemia)

HCO3 = Baseo HCO3 = pH (Alkalemia)o HCO3 = pH (Acidemia)

Since PaCO2 is regulated by respiration, abnormalities that primarily alter thePaCO2 are referred to as respiratory acidosis (high PaCO2) and respiratory

alkalosis (low PaCO2).

In contrast, [HCO3-] is regulated primarily by renal processes. Abnormalitiesthat primarily alter the [HCO3-] are referred to as metabolic acidosis (low

[HCO3-]) and metabolic alkalosis (high [HCO3-]).

Simple acid base disorders: Disorders that are either metabolic or respiratory. Mixed acid base disorders: More than one acid base disturbance present. pH

may be normal or abnormal.


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In Numberso Acidosis: pH < 7.35

Respiratory: PaCO2 > 40 mmHg Metabolic: HCO3 < 24 mEq/L

o Alkalosis: pH > 7.45 Respiratory: PaCO2 < 40 mmHg Metabolic: HCO3 >24 mEq/L

Simple Acid-Base Disorders

This table was put in one of the OSCE questions and some arrows were omittedand the question was to fill the table with the suitable arrows!

For example, in metabolic acidosis [HCO3-] decreases from HendersonHasselbalch equation PaCO2 decreases as compensation. (compensation concept will be d iscussed).

In order to interpret and analyze acid-base status, we have to perform ABG(Arterial Blood Gases) which is the only way to measure blood pH so far. Also it

gives PaCO2 level.

Furthermore, we do serum level of certain electrolytes (Na+, K+, Cl-, andHCO3-). [HCO3-] can be measured from ABG (using Henderson Hasselbalch

equation) but won't be that accurate; it should be done on serum chemistry.

Compensation Concept-Acid Base disorders are associated with defense mechanisms referred to as

compensatory responses that function to reduce the effects of the particular disorder on

the pH. They do not restore the pH back to a normal value. This can only be done

with correction of the underlying cause.


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-In each of these disorders, compensatory renal or respiratory responses act to minimize

the change in H+ concentration by minimizing the alteration in the PaCO2/[HCO3-] ratio.

Lungs compensate for metabolic disorders and kidneys compensate forrespiratory disorders.

For example, if a patient has severe respiratory acidosis, kidney will manage byretaining HCO3- and vice versa.

Respiratory Compensation-Compensates for metabolic disorders.

-Prompt response.

In metabolic acidosiso Hyperventilation - PaCO2

-In metabolic acidosis, [HCO3-] level will decrease.

-The body responds to this by trying to restore the PaCO2 / [HCO3-] ratio.

-This is done by reducing the PaCO2.

- The drop in arterial pH stimulates both the central and peripheral

chemoreceptors controlling respiration, resulting in an increase in alveolar

ventilation.

- To estimate the expected PaCO2 range based on respiratory compensation,

one can use this formula:

PaCO2 = ((1.5 [HCO3-]) + 8) 2-if PaCO2 is outside this range, then there are two (mixed) disorders are

going on at the same time.

In metabolic alkalosiso Hypoventilation - PaCO2

-In metabolic alkalosis, there is an elevation in [HCO3-] above the

normal range [H+]pH.

-The body responds by trying to increase the PaCO2 to match the

increase in [HCO3-] and thus maintain the PaCO2/[HCO3] ratio.

-Elevation in PaCO2 is accomplished by lowering alveolar ventilation.

-the formula:

PaCO2= 0.9 HCO3

-for example, if plasma [HCO3-] has been raised from 24 44 mEq/L

then HCO3 = 20.

PaCO2 = 0.9 HCO3 = 0.9 20 = 18. Therefore, PaCO2 elevation

should be 18 PaCO2 measured should be 40 + 18 = 58 mEq/L.


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Metabolic Compensation-Compensation for respiratory disorders

-Done by the kidney.

-Slow response

-Starts in 6-12 hours.

-Steady state in few days.

-Metabolic compensation is a bit different; the kidney takes time in order to start

regulation (not immediately as the lungs).

-If a patient develops acute metabolic acidosis (for e.g. from lactic acidosis

caused by septic shock most commonly), lungs immediately manage by

hyperventilation; whereas, if one undergoes hyperventilation, kidneys will take

some hours to manage!

-As a result, there is no metabolic compensation in the acute phase.

-Compensation happens in chronic cases, for e.g. COPD patient always has high

PaCO2 (let's say 55 mEq/L) so [HCO3-] is expected to be high also. In respiratory acidosis

Stimulates HCO3 reabsorption -- HCO3

In respiratory alkalosis

Inhibits HCO3 reabsorption -- HCO3

Chronic Respiratory Acidosis:o HCO3= 0.35 PaCO2o So approximately HCO3 changes by one third (1/3) of what

PaCO2 changes. E.g. if PaCO2 is found to be 55 mEq/L

PaCO2= 15 mEq/L HCO3 = 5 mEq/L. so we expect from

the kidney to retain HCO3 and HCO3 will elevate to be 25+5=

29 mEq/L.

o This indicates that the disorder is simple and the kidney hascompensated for it.

Chronic Respiratory Alkalosis:o HCO3= 0.5 PaCO2o In case of chronic respiratory alkalosis, the compensation will

be more efficient than acidosis.


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Case Study- 1:A 39 year old woman was admitted with a history of generalized weakness,

dyspnea, continuous nausea and diarrhea. Bowel motions were frequent and

watery.

ABG: pH 7.29, PaCO2 25.6, PaO2 98

Na+=125, K

+=2.8, Cl

-=101, HCO3=14

Obviously, she has gastroenteritis. In diarrhea, stool is rich in alkali HCO3

loss metabolic acidosis.

Now>> Let's learn a stepwise approach to interpreting the ABG.1-First of all, we look at the pH.Is there an acid base disorder present?

pH is 7.29 low there is acidosis.

2-Then, we look at PaCO2, HCO3-.

-What is the acid base process (alkalosis vs acidosis) leading to the abnormal pH?-Are both values normal or abnormal?

-look at PaCO2. Is it abnormal? If so, is the change goes on with the pH? If yes,

this is a respiratory problem. If no, this is a compensatory change in PaCO2.

-In simple acid base disorders, both values are abnormal and direction of the

abnormal change is the same for both parameters.

-One abnormal value will be the initial change and the other will be the

compensatory response.

3-Then, We distinguish the initial change from the compensatory response.

- The initial change will be the abnormal value that correlates with the abnormal pH.

- If Alkalosis, then PaCO2 low or HCO3-high

- If Acidosis, then PaCO2 high or HCO3-low.

Once the initial change is identified, then the other abnormal parameter is thecompensatory response if the direction of the change is the same. If not, suspect a

mixed disorder.

Once the initial chemical change and the compensatory response aredistinguished, then identify the specific disorder. See table below.- If PaCO2 is the initial chemical change, then process is respiratory.

- If HCO3- is the initial chemical change, then process is metabolic.

Acid Base Disorder Initial Chemical Change Compensatory Response

Respiratory Acidosis PaCO2 HCO3-

Respiratory Alkalosis PaCO2 HCO3-

Metabolic Acidosis HCO3- PaCO2

Metabolic Alkalosis HCO3- PaCO2


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-So in our case, PaCO2is low (the acidosis isnt respiratory and the PaCO2

decrease is compensatory). [HCO3-] is low (it is metabolic acidosis and this is

the initial change).

4-to know whether the degree ofcompensation is adequate, we calculate the

estimated PaCO2 by applying compensation rules.

-If calculated compensation is:-Within the expected range simple acid/base disorder-Less or more than expected 2 acid/base disorder(mixed disorders)

back to our case: PaCO2 = [1.5 HCO3 + 8] 2

PaCO2 = [1.5 12] + 8 2

PaCO2 = 25 2.

PaCO2 = 25.6, so it is a case of simple metabolic acidosis that the lungs were able to

compensate for.-BUT, what if the PaCO2 (in the case study) was 50mEq/L??it will be a mixed disorder!

-If a patient has diabetic ketoacidosis (DKA) and metabolic acidosis he will

hyperventilate to wash out CO2, if CO2 was normal or high, then there is another

disorder (mixed case).

-No need to talk in details about respiratory acidosis and alkalosis as they are lung

disorders; we have to talk about metabolic acidosis and metabolic alkalosis.

Metabolic Acidosis Metabolic acidosis is grouped according to the anion gap (AG).What is anion gap? It is the difference between the sum of the measured cationsand measured anions in the plasma or serum calculated as follows:

{[Na +]([Cl-] + [HCO3-])}.

The word 'gap' is misleading, because in our body anions must equal cationsotherwise the body won't be neutral!

So why is there a 'gap'? There are hidden anions like the negatively charged plasma

proteins that are unmeasured; this anion gap estimates the unmeasured anions.

Normal AG= (9 - 12 mEq/L), if AG is > 20, we call it wide AG. Wide Anion Gap Metabolic Acidosis: (Usually caused by serious problems).

-Causes of wide anion gap metabolic acidosis is best remembered by the popular

mnemonic MUDPILES or KULT:M = Methanol

U = Uremia (renal failure).

D = DKA and ketoacidosis

P = Paraldehyde

I = Iron & IsoniazidINH

L = Lactic acidosis (usually due to sepsis).

E = Ethanol and Ethylene GlycolS = Salycilate

K= Ketoacidosis (DKA, alcoholic ketoacidosis,

starvation)

U = Uremia (Renal Failure)

L =Lactic acidosis

T = Toxins (Ethylene glycol, methanol,

paraldehyde, salicylate)


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Non (normal) Anion Gap Metabolic Acidosis-Hyperalimentation (over IV feeding).

-Acetazolamide, amphotericin.

-RTARenal Tubular Acidosis.-Ureteral Diversions.-Diarrhea.-Pancreatic fistula.-Saline resuscitation.

-The biggest group of them is RTA.

-Many patients have been diagnosed just according to the ABG.

BACK to Case-study-1

Calculate the AG.

AG = Na(Cl+HCO3)

AG = 125(101+14) = 10It is a normal anion gap metabolic acidosis.

Metabolic Alkalosis Characterized by

o Primary in HCO3 concentrationo Compensatory in PaCO2

Classified according to urinary chlorideo Chloride responsive: when Cl is high in urine. Usually, the cause is

vomiting in this group.o Chloride resistant.

Metabolic Alkalosis -Chloride Responsive:o Urine Cl > 20 mEq/L.o Volume Contraction:

Nasogastric suctioning Vomiting Diuretics

o Post Hypercapnia, when PaCO2 is high HCO3 increases tocompensate, suddenly the hypercapnia is relieved by hyperventilation or

HCO3. What will remain? HCO3 alone! Because it takes time to go back

to normal.o Hypokalemia.o Hypomagnesemia.

these 2 disorders are missed in medicine.

o Penicillin.o Diuresis.


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Metabolic Alkalosis -Chloride Unresponsive:o Urine Cl- < 20 mEq/L.

Caused by:o Mineralcorticoid excesso Exogenous steroids (this is the biggest group).o Alkali Ingestiono Licoriceo Too much wineo Tobacco chewerso Bartters Syndrome

Cl- responsive if you give them normal saline infusion they will get benefit,whereas Cl- unresponsive they won't.

Case Study- 2:A 78 year old lady presented with at a 1 week history of abdominal pain and vomiting.ABG: pH 7.49, PaCO2 52, PaO2 78

Na 137, K 2.2, CL 91, HCO3 38Urine CI 43 mEq/L.

pH > 7.49 alkalosis. PaCO2 = 52 high it is not respiratory alkalosis, this is compensatory. HCO3- = 38 high metabolic alkalosis! Compensation: PaCO2= 0.9 HCO3 0.9 X 14 =~ 12.6 PaCO2 =~ 52. Result: this is simple compensated metabolic alkalosis. The cause: Vomiting and hypokalemia. It is Cl- responsive. treatment: normal saline. Mixed Acid-Base Disorders

o Any combination of acid-base disorder can occur, except for respiratoryacidosis and respiratory alkalosis.

o pH may be within normal range (7.35-7.45).In one of the 6

thyear OSCE questions pH was 7.40m most of the student got

confused and thought it was a normal pH. But actually the PaCO2 was high

and HCO3 was low and it was a mixed disorder where pH may be normal.

o Remember that we cannot get over-compensation; it is impossible for apatient with DKA to hyperventilate until pH = 7.42! This never happens!

o Simultaneous co-existence of disorders 2 respiratory + 2 metabolic Acute on top of chronic


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E.g1: we may see 2 respiratory problems one is acute and anotheris chronic like COPD patient with CO2 retention, suddenly he felt

tired and CO2 retention has been exacerbated.

E.g2: a patient suffering from metabolic alkalosis from diuresisand suddenly he experienced vomitingacute metabolic

alkalosis.o The pH determines which is primary:

pH < 7.40primary disorder is Acidosis pH > 7.40primary disorder is Alkalosis

Case Study- 3:A 21 year old MS is brought to the ER at ~3 am, stuporous and tachypneic. History is

remarkable for failing the respiratory module in 2nd year. An ABG and electrolytes

have been drawn by the ER nurse.

ABG: pH=7.43, PaCO2=18

Na+=143, K+=3.8, Cl-=106, HCO3=12

pH =7.43 within normal range, NO diagnosis till now.

PaCO2 = 18 ! Very low respiratory alkalosis.HCO3 = 12. low; Compensatory ? Mixed?

-In acute cases we expect to have hyperventilation but we dont expect a

big change in HCO3 as the change seen here, so these are 2 mixeddisorders (metabolic acidosis & respiratory alkalosis)

We have metabolic acidosis so we have to calculate the anion gap.

AG = 143(106+12) = 25 wide AG metabolic acidosis.

Compensation: PaCO2 = 1.5 12 +8 = 26 2 mixed!

This is a case ofwide AG metabolic acidosis & respiratory alkalosis.

The cause was Aspirin Overdose! Aspirin stimulates respiratory centre.If you dont know this case in advance you will never diagnose it! Stuporous

patient who has failed an exam they will think about psychiatric disorders and

start giving him valium!!

Primary Respiratory Disorders- Acute vs. Chronic:o ACUTE

PaCO2= 10 pH=0.08 (~0.1)o CHRONIC

PaCO2= 10 pH=0.03Or.. In acute respiratory acidosis, pH = 0.008 PaCO2

In chronic respiratory acidosis, pH = 0.003 PaCO2.


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-In acute disorders, because there is no compensation, any change in PaCO2

will make a big change in pH. Whereas in chronic disorders, because of renal

adaptation of [HCO3-], a similar change of PaCO2 can't change the pH as much

as acute case can do. Case Study- 4:A 55 year old woman presented to the ER with dyspnea and wheezes. She is heavy

smoker. An ABG and electrolytes have been drawn by the ER nurse.

ABG: pH=7.33, PaCO2=65Na+=144, K+=4.2, Cl-=104, HCO3=32pH =7.33 acidosis.PaCO2 = 65! Respiratory acidosis.

HCO3 high (compensatory).

Compensation : HCO3= 1/3 PaCO2

PaCO2 = 6540 = 25

HCO3=1/3 25 =~ 8 8 + 24 = 32AG = 144(104+32) = 8 normal.

Chronic compensated respiratory acidosis.

What is the significance of knowing whether the case is acute or chronic?

When a patient comes to the ER with respiratory acidosis we have to assess his case ifit is acute or chronic, because if I know that his case is chronic I will be sure that

nothing new has happened to him, I can give him nebulizers and steroids then send

him home.

Note:

In acute respiratory acidosis, pH = 0.008 PaCO2

In chronic respiratory acidosis, pH = 0.003 PaCO2.

acute_______________________________________chronic

Finally, I would like to dedicate this lecture to my great group A5! You weremy source of inspiration =)

To.. Laith 3anani, Samer Kuleib, A7mad Salem, Saif Abu3orabi, Suleiman

Ghunaim, Mohammad Kurd, Bara2 Da7leh, M7moud Bader, Ra2ed Jaradat,

Bara2a Rashan, Malak abu 3eisheh, Hind Sarayrah, Reham Nawwaf, Olga

Khawaldeh, Areej Salameen.. Thank you all =D

By Hani Eid.

PaCO2=25

7.33

www.sawa2006.com


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Correction for Lec#7

In page number 4:

The mistake:

Renal artery occlusion: (complete obstruction of renal artery) It is caused mostly due

to EMBOLISM from the heart, and we suspect this condition if the pt has AF

(this what the dr say and I think he meant ARF

[acute renal failure]).

The correction: the dr said AF which means atrial fibrillation

Sry for this mistake

int12 acid-base disorders

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