Acid Base Balance

Ph = 7.4 ↓

CO2 = 40

HCO3- = 24

↑↑ -hypercapnia

↑ [>24 & <30=24+6]

↓

↑↑

↑↑ [>30=24+6] --- renal absorption of bicarbonate. Need 24-36 hrs for maximum effect.

↑

↓↓ --hypocapnia

↓ [<24&>18=24-6]

Simply Chronic respiratory alkalosis. OR Acute respiratory alkalosis WITH Partially compensated metabolic acidosis Acute metabolic acidosis

↑

↓↓

↓↓ [<18=24-6 ] --- renal loss of bicarbonate

↓

No change

↓↓

Simply Chronic metabolic acidosis OR Acute metabolic acidosis WITH Partially compensated Resp. alkalosis OR Respiratory compensated metabolic acidosis.

↓

↓ - increases respiratory rate.

↓↓

Acute metabolic alkalosis

↑

No change

↑↑

Simply Chronic metabolic alkalosis OR Acute metabolic alkalosis WITH Partially compensated Resp. acidosis OR Respiratory compensated metabolic alkalosis

↑

↑- decreases respiratory rate.

↑↑

Acute respiratory acidosis OR Metabolic acidosis superimposed on chronic respiratory acidosis Simply Chronic respiratory acidosis OR Acute respiratory acidosis WITH Partially compensated metabolic alkalosis OR Metabolic alkalosis superimposed on acute respiratory acidosis Acute respiratory alkalosis

Acute respiratory acidosis Acute respiratory alkalosis Acute metabolic acidosis Acute metabolic alkalosis

Ph = 7.4 ↓

CO2 = 40

HCO3- = 24

↑↑

↑

↑

↓↓

↓

↓

No change

↓↓

↑

No change

↑↑

1. 2.

Acute respiratory acidosis -------HCO3- >24 & <30 [24+6] Chronic respiratory acidosis-----HCO3- >30 [24+6]

1.

Acute respiratory alkalosis ----- HCO3- <24 & >18 [24-6]

2.

Chronic respiratory alkalosis ---HCO3- <18 [24-6]

Combined Or Mixed Disorders : alkalosis + acidosis –if the PH is normal [7.35 – 7.45] , it must be a mixed disorder since there is no full compensation 2 acidosis (or) 2 alkalosis

a. b.

if PH is very low , it must be 2 acidosis if PH is very high , it must be 2 alkalosis So 1. 2. 3. 4.

1.

there are 4 Mixed/Combined Disorders: Mixed Respiratory acidosis with Metabolic Mixed Respiratory alkalosis with Metabolic Mixed Respiratory acidosis with Metabolic Mixed Respiratory alkalosis with Metabolic

alkalosis. acidosis. acidosis. alkalosis

[PH=7.38 ] [PH=7.42 ] [PH=7.2 ] [PH=7.6 ]

alkalosis + acidosis

a.

E.g: Adult salicylate intoxication in rheumatoid arthritis ---------Mixed respiratory alkalosis and Metabolic acidosis

i.

b. c. d.

e.

Salicylate is uncoupler of ETS-----increased O2 consumption----hypoxia------hyperventilation -------respiratory alkalosis -------as a compensation metabolic acidosis comes -----due to decreased ATP synthesis due to uncoupling neural tissue dies ------respiratory centre won’t works ------then respiratory acidosis comes -----so finally both respiratory and metabolic acidosis comes.

E.g: Child salicylate intoxication ---------Mixed respiratory alkalosis and Metabolic acidosis ------but this respiratory alkalosis goes away rapidly and land up in profound metabolic acidosis. E.g: endotoxic shock ----------- Mixed respiratory alkalosis and Metabolic acidosis Problem - PH = 7.42 , PCO2 = 22 , HCO3- = 14 Ans: Mixed respiratory alkalosis and metabolic acidosis . Steps in sloving this problem i. PH = 7.42 ---inference is alkalosis[because >7.4]

ii. iii.

PCO2 = 22 --- ↓ so this is respiratory alkalosis

iv.

Observe here clearly that PH =7.42 which is 0.02 above than 7.4 & near normal to 7.4 ( 7.35 – 7.45 is normal range according to goljan ) ( see clearly in other problems that in a partially compensated , PH is either < 7.35 or > 7.45 ---but never between 7.35 – 7.45 , if present it is mixed disorder with acidosis + alkalosis ) -----so this is not Chronic respiratory alkalosis because compensation ( chronic ) never brings the PH to normal & compensation is always outside the normal range ( partially compensated) So this is mixed respiratory alkalosis and Metabolic acidosis because PH is above 7.4 and come to 7.42 .

v.

HCO3- = 14 ---↓↓ so this is Chronic respiratory alkalosis ---but this is NOT the answer

2. 2 acidosis (or) 2 alkalosis If the CO2 and HCO3- change in opposite directions , it is a combined disturbance . It is either a combined respiratory and metabolic acidosis or a combined respiratory and metabolic alkalosis Combined Respiratory And Metabolic Alkalosis.----------if the CO2 is depressed and HCO3- elevated Combined Respiratory And Metabolic Acidosis.------------if the CO2 is elevated and HCO3- depressed

i.

E.g: PCO2=55mmHg , HCO3- =20 mEq/L Ans: Combined respiratory and metabolic acidosis.— 1. PH = these type of problems they don’t give PH 2. PCO2=55mmHg ----inference is either a. Acute respiratory acidosis b. Chronic respiratory acidosis c. Chronic metabolic alkalosis

3. ii. Respiratory acidosis (compensation) Chronic Metabolic acidosis Final blood gas

HCO3- =20 mEq/L ---↓ ---but in All Acute respiratory acidosis or Chronic respiratory acidosis or Chronic metabolic alkalosis the HCO3 has to be ↑ ------but here it is ↓ -----so this is Combined Respiratory And Metabolic Acidosis

E.g: Cardiorespiratory arrest with no breathing (acute respiratory acidosis) and cardic standstill(metabolic acidosis from lactic acidosis ) Note : arrows represent the degree of magnitude PH PCO2 HCO3↓ ↑ ↑↑(primary) ↓

↓(compensation)

↓↓(primary)

↓↓(extreme academia)

↑

↓

Steps to follow to solve the problem : 1. first see PH

2.

a. b. c.

PH low – it is acidosis [ E.g: 7.3 ,7.2 etc ------- PH is < 7.35]

d.

PH is very low , it must be “mixed 2 acidosis” [ E.g: 7.08 & PCO2 =49 , HCO3- = 14 –--- combined respiratory and metabolic acidosis---some times PH is NOT given in the problems. See the point 2. c.i above .

e.

PH is very high , it must be “mixed 2 alkalosis” ---some times PH is NOT given in the problems. See the point 2. c.i above .

PH high – it is alkalosis [ E.g: 7.6 ,7.51 etc -------- PH is > 7.45] PH is normal – “mixed alkalosis + acidosis” [ E.g: between “7.35 – 7.45” – “0.05 less or more than 7.4” … like 7.42 … it must be a mixed disorder since there is NO full compensation

next see CO2 [after coming to the conclusion that it is acidosis when PH is low ]

a.

If PCO2 high (↑↑).----------------It is Respiratory acidosis i. Then see HCO3-

1.

HCO3- is ↑ [>24 & <30=24+6 ] ---Acute Respiratory acidosis

2. 3.

HCO3- is ↑↑ [>30=24+6] ---Chronic Respiratory acidosis HCO3- is ↓ -------- It is Combined Respiratory And Metabolic Acidosis OR Respiratory acidosis WITH Metabolic acidosis. E.g: PH =7.23 ; PCO2=54 ;

HCO3- = 10

b.

If PCO2 Not elevated ---------it is Acute metabolic acidosis

i. c. 3.

There is NO need to see HCO3- which is ↓↓.

If PCO2 Decreased ------------it is Chronic metabolic acidosis or Respiratory Compensated metabolic acidosis [ HCO3- is ↓↓ ]

i. There is NO need to see HCO3- which is ↓↓. Think vise versa the step 2 for Respiratory alkalosis if PCO2 low .

Problems : 1. PH 7.3 , PCO2 = 30 , HCO3- = 14

2.

3.

a. b.

PH 7.3 – inference is acidosis

a. b. c.

PH 7.22 – inference is acidosis

a. b. c.

PH 7.22 – inference is acidosis

PCO2 = 30 --- ↓ --so Respiratory Compensated metabolic acidosis or Chronic metabolic acidosis PH 7.22, PCO2 = 69 , HCO3- = 27 PCO2 = 69 --- ↑↑ --so this is respiratory acidosis

HCO3- =27 is ↑ --- Acute Respiratory acidosis PH 7.33, PCO2 = 68 , HCO3- = 34 PCO2 = 69 --- ↑↑ --so this is respiratory acidosis HCO3- is ↑↑

--- so this is Chronic Respiratory acidosis

Principles : 1. for every change of 10mmHg in PCO2 ( up or down ) there is an opposite change of 0.08 in PH ( ie as CO2 increases , PH decreases and as CO2 decreases , PH increases) 2. for every increase of 10mmHg in PCO2 there is a 3.5mEq/l increase in plasma HCO3- concentration. 3. PH 7.4 = ratio of HCO3-/PCO2 ~ 20/1

4.

the body does not overcompensate . for example , patients do not “ overbreathe into respiratory alkalosis” as respiratory compensation for metabolic acidosis . correction is made to just short of normal. Compensation brings arterial Ph close to but NOT usually into the normal range of 7.35-7.45

Ph PCO2 HCO3A 7.37 65 37 B 7.22 60 26 C 7.35 60 32 Clinical correlations : • Chronic bronchitis plus diarrhea suggests combined chronic respiratory acidosis and metabolic acidosis OR B • Marked obesity suggests chronic hypercapnia or chronic respiratory acidosis OR C • Severe acute asthma suggests acute respiratory acidosis OR B • Chronic bronchitis plus diuretics suggests chronic hypercapnia with superimposed metabolic alkalosis. The metabolic alkalosis in this cases is on the basis of volume contraction from the use of the diuretics OR A.

Salicylate Toxicity Salicylate overdose causes a high anion gap metabolic acidosis in both children and adults. Adults commonly develop a respiratory alkalosis (due to respiratory centre stimulation) as well - this is uncommon in children. * Absorption: Salicylates are readily absorbed in the unionised form from the small intestine. * Metabolism: The major route of biotransformation is conjugation with glycine in the liver. * Excretion: The amount of drug excreted unchanged in the urine is small but can be markedly increased if urine is alkaline. High levels of salicylate are toxic because the drug uncouples oxidative phosphorylation as well as inhibiting some enzymes in the cell. Salicylates directly stimulate the respiratory center to cause hyperventilation (respiratory alkalosis) which is dose-dependent. This stimulation is much more pronounced in adults than in children. Metabolic acidosis is the most serious acid-base disorder and is due to increased production of endogenous acids rather than the salicylate itself. Plasma salicylate levels rarely exceed a maximum of about 5 mmol/l and the decrement in the [HCO3] is significantly higher than this in these severe cases. Acidosis is much more pronounced in infants as compared to adults, which is the reverse of the situation with the hyperventilation. In adults, respiratory alkalosis usually predominates. The particular organic acid anions involved in the acidosis of salicylate intoxication have not been identified. Ketoacidosis may also occur in children who are ill and fasted (ie starvation ketosis). The combination of metabolic acidosis and respiratory alkalosis can be a difficult situation to diagnose from the blood gases. The problem relates to whether the hyperventilation is primary (ie respiratory alkalosis) or is compensatory for the metabolic acidosis. Clinical Presentation The presentation in severe overdose is a comatose patient with marked hyperventilation and possibly convulsions. Small children usually have a

fever. The diagnosis of overdose or over-ingestion is usually easily made from the history. Clinicians should have a high index of suspicion in children with a metabolic acidosis particularly if ketoacidosis, lactic acidosis and renal failure have been excluded. Another clue is that salicylates greatly increase urinary uric acid excretion and plasma urate level is usually very low. Urine can be screened with a ferric chloride test for salicylates. Treatment Points in Salicylate Toxicity Remove salicylates from the body. Gastric lavage - only if time from ingestion is short. Forced alkaline diuresis will increase urinary excretion. Haemodialysis is more effective but rarely necessary in view of the efficacy of forced alkaline diuresis.