Evaluation and Analysis of Acid-Base Disorders

Ebadur RahmanEbadur RahmanFRCP (Edin),FASN, Specialty Certificate in Nephrology (UK)FRCP (Edin),FASN, Specialty Certificate in Nephrology (UK)

MRCP (UK), DIM (UK), DNeph (UK), MmedSciNephrology (UK).MRCP (UK), DIM (UK), DNeph (UK), MmedSciNephrology (UK).Consultant & clinical tutorConsultant & clinical tutorDepartment of NephrologyDepartment of Nephrology

Riyadh Armed Forces HospitalRiyadh Armed Forces Hospital

Acid-Base Analysis, What do You Need?

• Blood gas (pH, CO2)

• Serum chemistry (Na, Cl, HCO3)

• History

• CO2 = Acid– CO2 = pH (acidemia)

– CO2 = pH (alkalemia)

• HCO3 = Base– HCO3 = pH (alkalemia)

– HCO3 = pH (acidemia)

Acidemic vs. Alkalemic

• pH < 7.35 = Acidemic• pH > 7.45 = Alkalemic

Reference ranges and points

Parameter Reference range Reference point

pH 7.35-7.45 7.40

PCO2 33-44 mm Hg 40 mm Hg

PO2 75-105 mm Hg

HCO3- 22-28 mEq/L 24mEq/L

Anion gap 8-16 mEq/L 12 mEq/L

Osmolar gap <10 mOsm/L

Acid-Base Normals:

• pH= 7.40 (7.35 - 7.45)• PCO2 = 40 (35 - 45)

• HCO3 = 24 (22 - 26)

• Anion gap 12

ABG: 7.40 / 40 / 80 / 24

–pH –PaCO2

–PaO2

–HCO3

Delta ratio

Delta ratio Assessment

<0.4 Hyperchloraemic normal anion gap acidosis

0.4 – 0.8 Combined high AG and normal AG acidosisNote that the ratio is often <1 in acidosis associated with renal failure

1 - 2

Uncomplicated high-AG acidosisLactic acidosis: average value 1.6 DKA more likely to have a ratio closer to 1 due to urine ketone loss (if patient not dehydrated)

>2 Pre-existing increased [HCO3

-]:concurrent metabolic alkalosispre-existing compensated respiratory acidosis

𝛥 ratio = Anion gap/ [HCO𝛥 𝛥 3-] = (AG – 12)/(24 -

[HCO3-])

CompensationPrimary Disturbance pH HCO3

- PCO2 Compensation

Respiratory acidosis <7.35 Compensatory increase

Primary increase

Acute: 1-2 mEq/L increase in HCO3- for

every 10 mm Hg increase in PCO2

Chronic: 3-4 mEq/L increase in HCO3-

for every 10 mm Hg increase in PCO2

Respiratory alkalosis >7.45 Compensatory decrease

Primary decrease

Acute: 1-2 mEq/L decrease in HCO3- for

every 10 mm Hg decrease in PCO2

Chronic: 4-5 mEq/L decrease in HCO3-

for every 10 mm Hg decrease in PCO2

Metabolic acidosis <7.35 Primary decrease

Compensatory decrease

1.2 mm Hg decrease in PCO2 for every 1 mEq/L decrease in HCO3

-

Metabolic alkalosis >7.45 Primary increase Compensatory increase

0.6-0.75 mm Hg increase in PCO2 for every 1 mEq/L increase in HCO3

-

, PCO2 should not rise above 55 mm Hg in compensation

Four Primary Disorders:• PCO2 < 35 = respiratory alkalosis• PCO2 > 45 = respiratory acidosis• HCO3 < 22 = metabolic acidosis• HCO3 > 26 = metabolic alkalosis

– Can have mixed pictures with compensation– Can have up to 3 abnormality simultaneously (1

respiratory + 2 metabolic) – The direction of the pH will tell you which is

primary!

Look at dirrection of arrows

• Metabolic all arrow same dirrection– Ph / HC03 / PC02= METABOLIC ACIDOSIS

– Ph / HC03 / PC02= METABOLIC ALKALOSIS

Look at dirrection of arrows

• RESPIRATORY arrow dirrection oposite to PH– Ph / HC03 / PC02= RESPIRATORY ALKALOSIS

– Ph / HC03 / PC02= RESPIRATORY ACIDOSIS

Simple Acid-Base Disorders

Example # 1:Blood gas: 7.50 / 29 / 23

• pH= 7.50 (7.35 - 7.45) ↑• PCO2 = 29 (35 - 45)↓

• HCO3 = 23 (22 - 26)↓↔

• Alkalemic

• Low PCO2 is the primary (respiratory alkalosis)

• No metabolic compensation = acute process

• Acute Respiratory Alkalosis• Acute: 1-2 mEq/L decrease in HCO3

- for every 10 mm Hg decrease in PCO2

A 65-year-old man with a history of emphysema comes to the physician with a 3-hour history of shortness of breath.

pH 7.18 ↓↓

PO2 61 mm Hg

PCO2 58 mm Hg

HCO3- 25 mEq/L ↑

Racid acute-History suggests hypoventilation

• supported by increased PCO2 and lower than anticipated PO2.

• Acute: 1-2 mEq/L increase in HCO3- for every 10

mm Hg increase in PCO2

• Chronic: 3-4 mEq/L increase in HCO3- for every 10

mm Hg increase in PCO2

• Respiratory acidosis (acute) incomplete/no renal compensation.

Example # 2:Blood gas: 7.25 / 60 / 26

• Acidemic

• pH= 7.25 (7.35 - 7.45) ↓• PCO2 = 60 (35 - 45)↑

• HCO3 = 24 (22 - 26)↔

• Anion gap 12↔

• Elevated CO2 is primary (respiratory acidosis)

• No metabolic compensation= acute process

• Acute Respiratory Acidosis

causes

• Central respiratory depression• Pneumonia • Chest wall deformity • Upper air way obstruction • Chest wall injury

Example # 3:Blood gas: 7.34 / 60 / 31

• Acidemic• Elevated CO2 is primary (respiratory acidosis)

• Metabolic compensation has occurred = chronic process

• Chronic Respiratory Acidosis with Metabolic Compensation**true metabolic compensation takes 3 days (72hrs)

Causes

• Copd• Neuromuscular disorder• Chronic cns depresion

Acute Respiratory Alkalosis

Example # 4:Blood gas: 7.50 / 48 / 36

• Alkalemic• Elevated HCO3 is primary (metabolic alkalosis)

• Respiratory compensation has occurred = acute /chronic ?

• Metabolic Alkalosis with Respiratory Compensation*

*Respiratory compensation takes only minutes

Metabolic Alkalosis with Respiratory Compensation

Example # 5:Blood gas: 7.20 / 21 / 8

• Acidemic• Low HCO3 Is primary (metabolic acidosis)

• Respiratory compensation is present

• Metabolic Acidosis with Respiratory Compensation

Anion Gap (AG):

• The calculated difference between the positively charged (cations) and negatively charged (anions) electrolytes in the body:

AG= Na+ - (Cl- + HCO3 -)

• Normal AG = 12 ± 2 (10 – 14)

Rule • Calculate the anion gap. If the anion gap is

20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentration

Principle: The body does not generate a large anion gap to compensate for a primary disorder (anion gap must be primary)

Rule• Calculate the excess anion gap (total anion gap – normal

anion gap) and add this value to the measured bicarbonate concentration: – if the sum is > than normal bicarbonate (> 30) there is an underlying

metabolic alkalosis – if the sum is less than normal bicarbonate (< 23) there is an

underlying nonanion gap metabolic acidosis

1. Excess AG = Total AG – Normal AG (12)2. Excess AG + measured HCO3

= > 30 or < 23?

Mixed Acid-Base Disorders

A 20-year-old man with a history of diabetes is brought to the emergency department with a 3-day history of feeling ill. He is non-adherent with his insulin. Urine ketones are 2+ and glucose is 4+.

pH 7.26 Na+ 136 mEq/L

PO2 110 mm Hg K+ 4.8 mEq/L

PCO2 19 mm Hg Cl- 101 mEq/L

HCO3- 8 mEq/L CO2, total 10 mEq/L

Glucose 343 mg/dLUrea 49 mg/dLCreatinine-1 mg/dL

.

Macid high AG• History suggests diabetic ketoacidosis.• Metabolic acidosis with appropriate

respiratory compensation

DescriptionpH 7.26 Na+ 136 mEq/L

PO2 110 mm Hg K+ 4.8 mEq/L

PCO2 19 mm Hg Cl- 101 mEq/L

HCO3- 8 mEq/L Glucose 343 mg/dL

Urea 49 mg/dL Creatinine 1 mg/d

AG = 136-101-8=27 mEq/L L

1.2 mm Hg decrease in PCO2 for every 1 mEq/L decrease in HCO3-.

HCO3- decrease = 24-8 = 16 mEq/L

PCO2 decrease predicted = 1.2 x 16 = 19 mm Hg.

subtract from 40 mm Hg (reference point) = 21 mm Hg

High anaion gap metabolic acidosis

• Diabetic ketoacidosis• Lactic acidosis• Renal failure• Methanol • Ethanol • Alcohol

A 43-year-old man comes to the physician with a 3-day history of diarrhea. He has decreased skin turgor.

pH 7.31 Na+ 134 mEq/L

PO2 -- mm Hg K+ 2.9 mEq/L

PCO2 31 mm Hg Cl- 113 mEq/L

HCO3- 16 mEq/L Urea 74 mgl/dL

Creatinine 3.4 mmol/L

History is limited.

Metabolic acidosis with respiratory compensation.

Macid normal AGpH 7.31 Na+ 134 mEq/L

PO2 -- mm Hg K+ 2.9 mEq/L

PCO2 31 mm Hg Cl- 113 mEq/L

HCO3- 16 mEq/L Urea 74 mg/dL

Creatinine 3.4 mg/dLAG = 134-113-16=5 mEq/L

1.2 mm Hg decrease in PCO2 for every 1 mEq/L decrease in HCO3-.

HCO3- decrease = 24-16 = 8 mEq/L

PCO2 decrease predicted = 1.2 x 8 = 10 mm Hg.

subtract from 40 mm Hg (reference point) = 30 mm Hg

• Diarrhea • Uereteral divertion• Renal tubular acidosis

Remember the Rules1. Look at the pH: (< or > 7.40?) whichever caused the shift

(CO2 or HCO3) is the primary disorder2. Calculate the anion gap: if AG 20 there is a primary

metabolic acidosis (regardless of pH or HCO3)3. Calculate the excess anion gap, add it to HCO3:

Excess AG = Total AG – Normal AG (12) Excess AG + HCO3

= ?If sum > 30 there is an underlying metabolic alkalosisIf sum < 23 there is an underlying nonanion gap metabolic acidosis

Example # 1Blood gas: 7.50 / 20 / 15

Na= 140, Cl = 103

• Alkalemic• Low CO2 is primary (respiratory alkalosis)• Partial metabolic compensation for chronic condition?• AG = 22 (primary metabolic acidosis)• Excess AG (AG – 12) + HCO3

= 25 (no other primary abnormalities)

• Respiratory Alkalosis and Metabolic AcidosisThe patient ingested a large quantity of ASA and had both centrally mediated resp. alkalosis and anion gap met. Acidosis associated with salicylate overdose

Example # 2Blood gas: 7.40 / 40 / 24

Na= 145, Cl= 100• pH normal• AG = 21 (primary metabolic acidosis)• Excess AG (AG – 12) + HCO3

= 33 ( underlying metabolic alkalosis)

• Metabolic Acidosis and Metabolic AlkalosisThis patient had chronic renal failure (met. acidosis) and began vomiting (met. alkalosis) as his uremia worsened. The acute alkalosis of vomiting offset the chronic acidosis of renal failure = normal pH

Example # 3alcoholic ,Blood gas 7.50 / 20 / 15

Na= 145, Cl = 100• Alkalemic• Low CO2 is primary (respiratory alkalosis)• AG = 30 (primary metabolic acidosis)• Excess AG (AG – 12) + HCO3

= 33 (underlying metabolic alkalosis)

• Respiratory alkalosis, Metabolic Acidosis and Metabolic AlkalosisThis patient had a history of vomiting (met. alkalosis), Alcohol intake= (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)

How Many Primary Abnormalities Can Exist in One Patient?

• Three primary abnormalities is the max • One respiratory -because a person cannot

simultaneously hyper and hypoventilate• One patient can have both a metabolic

acidosis and a metabolic alkalosis – usually one chronic and one acute

Example # 4Blood gas: 7.10 / 50 / 15

Na= 145, Cl= 100• Acidemic• High CO2 and low HCO3

- both primary (respiratory acidosis and metabolic acidosis)

• AG = 30 (metabolic acidosis is anion gap type)• Excess AG + HCO3

= 33 (underlying metabolic alkalosis)

• Respiratory Acidosis, Metabolic Acidosis and Metabolic AlkalosisThis is an obtunded patient (resp. acidosis) with a history of emesis (metabolic alkalosis) and lab findings c/w diabetic ketoacidosis (metabolic acidosis w/ gap)

Example # 5Blood gas: 7.15 / 15 / 5

Na= 140, Cl= 110• Acidemic• Low HCO3

- primary (metabolic acidosis)• AG= 25 (metabolic acidosis is anion gap type)• Excess AG + HCO3 = 18 (underlying nonanion gap metabolic

acidosis)

• Anion Gap and Nonanion gap Metabolic AcidosisDiabetic ketoacidosis was present (anion gap met. acidosis). Patient also had a hyperchloremic nonanion gap met. acidosis secondary to failure to regenerate bicarbonate from ketoacids lost in the urine.

• 40 year male admitted to surgical unit due to infected diabetic foot.other co-morbidities hypertension and dyslipidemia were reasonabally controlled.He was febrile with temperature 38.5c and blood pressure 130/80.His laboratory investigations showed.

• WBC.12 Hb.9.6. Platelets.822 while renal profile showed. Serum Urea 8mmol/L • Serum creatinine 76 umol/L, Serum Na 142 mmol/L serum K 6.5 mmol/L.

ABGs.PH.7.4 Hco3 24, K 4.9 mmol/L.What is most likely diagnosis.• A.Type IV Renal tubular acidosis• B.Pseudohperkalemia• C.Type II renal tubular acidosis• D.Type I renal tubular acidosis• E.Addison’s disease.

• Answer:B. Type IV renal tubular acidosis is common in diabetic patients with acidosis and hyperkalemia, While others two types I &II have hypokalemia and acidosis.Patient did not have acidosis or features of Addisons disease.Postassium in ABGs was within normal limits which suggested patient had pseudohyperkalemia secondary to thrombocytosis.

• 16 years female was referred for further investigations for short stature and stone disease.she was behind her milestones.Laboratory investigations showed.MSU, PH 7 otherwise bland sediment.Renal profile.Na 137mmol/L K 2.9 mmol/L Chloride 115 mmolL, Serum urea and Creatinine normal.Serum HCO3 10.what is the diagnosis.

• A.Type 1 renal tubular acidosis• B.Type 2 renal tubular acidosis• C.Type 4 renal tubulare acidosis• D.High anion gap metabolic acidosis• E.Metabolic alkalosis.

• Answer: A. Anion gap for this patient is 12 while the patient having low bicarbonate suggesting metabolic acidosis.It is normal anion gap metabolic acidosis.As there is history of nephrolithiasis and patient is hypokalemia which favours the diagnosis of type 1 renal tubular acidosis.Urinary PH is 7 which is high and these patient can not acidify urine and maintain urinary PH always above 5.5 due to defect in the

secretion of H ion in the distal convoluted tubules.

• 60 years old male was treated ifosfamide based chemotherapy for soft tissue sarcoma.He was found persistant hypokalemia with low serum bicarbonate level on laboratory investigations.His current laboratory investigations are as following.Renal profile.Serum Urea 7 mmol/L, Serum creatinine 95 umol/L, Serum Na 140mmol/L and Serum K 3 mmol/L, Serum chloride 113mmol/L.ABG’s.PH.7.4,HCO3,18 and PCO2.47.What kind of acid base disorder happened.

• A..Metabolic alkalosis• B..Type 1 renal tubular acidosis• C..Type 2 renal tubular acidosis• D..Type 4 renal tubular acidosis• E..Respiratory acidosis.

• Answer: C. Ifosfamide chemotherapy is effective in treating soft tissue cancer and it causes proximal renal tubular defect leading to proximal renal tubular acidosis with hyperchloremic,hypokalemic,normal anion gap metabolic acidosis.Here anion gap is 9.