ACID BASE DISORDER AND ARTERIAL BLOOD GAS

RESPIRATORY ACIDOSIS AND ALKALOSIS

AND METABOLIC ACIDOSIS AND

ALKALOSIS

Dr Shadab Kamal

• Define respiratory acidosis ?

Respiratory Acidosis

• ↑ in PaCO2> 45 mmHg and pH <7.35• due to inadequate alveolar ventilation.

• What are the cuases of acute respiratory acidosis?

• Alveolar hypoventilation• Increased CO2 production

Alveolar hypoventilation• Central nervous system depression• Neuromuscular disorders• Chest wall abnormalities• Pleural abnormalities• Airway obstruction• Parenchymal lung disease• Ventilator malfunction

Increased CO2 production• Large caloric loads• Malignant hyperthermia• Intensive shivering• Prolonged seizure activity• Thyroid storm• Extensive thermal injury (burns)

• What are clinical manifestations of respiratory acidosis?

Manifestations of Respiratory Acidosis

•Mainly NEUROMUSCULAR: CO2 narcosis

• Anxiety, Headache, Lethargy, Stupor, Focal Paresis, Tremors, Asterixis, Delirium, myoclonus, Seizures, Coma

• How to correct respiratory acidosis?

• By altering TV or f in mechanically ventilated patients.

• ↑Minute Ventilation will ↓ PaCO2.

• Recommended guidelines are to –Target the TV 5-8 mL/kg of (IBW)–Pplateau <30 cm H2O.

• What are the factors that affect PaCO2 during mechanical ventilation?

• VCO2, carbon dioxide production;

• VA, alveolarventilation; VE , minute ventilation; VD , dead space ventilation;

• VT, tidal volume;• TI, inspiratory time; TE,

expiratory time; • f, respiratory rate.

• Define Respiratory alkalosis ?

• PaCO2 <35 mm Hg and pH>7.45,• due to excessive alveolar ventilation.

• What are the causes of respiratory alkalosis?

Causes of Respiratory Alkalosis

CENTRAL RESPIRATORY STIMULATION

Structural Causes Non Structural Causes

Head trauma Pain

Brain tumor Anxiety

CVA Fever

Voluntary

PERIPHERAL RESPIRATORY STIMULATION • Hypoxemia Reflex Stimulation of Respiratory Center

via Peripheral Chemoreceptors• V/Q imbalance• Pulmonary Diffusion Defects• Hypotension• Pulmonary Shunt• High Altitude

INTRATHORACIC STRUCTURAL CAUSES:• ↓ movement of chest wall & diaphragm• ↓ compliance of lungs • Irritative lesions of conducting airways

OTHERS: • Heat exposure, Sepsis, Pregnency, Mechanical

ventilation

• What are menifestation of respiratory alkalosis?

Manifestations of Respiratory Alkalosis

• Mainly NEUROMUSCULAR: • Lightheadedness, Confusion, Decreased

intellectual function, • Paraesthesias (circumoral, extremities)• Muscle twitching, cramps, tetany,

Hyperreflexia• Syncope, Seizures

• How to correct Respiratory alkalosis in mechanically ventilated patient?

• We have to treat Hyperventilation

• For VCV, ↓ minute ventilation by ↓ f, and then ↓ VT if necessary .

• for PCV, decrease f first and then decrease inspiratory pressure, if necessary.

• Define metabolic acidosis?

• primary ↓ in HCO3 (< 22 mEq/L) and pH <7.35 .

• What are the pathophysiology of Metabolic acidosis?

Pathophysiology• HCO3 loss: Renal or GIT• Decreased renal acid excretion • Increased production of non-volatile acids

a. Ketoacids b. Lactate c. Poisons d. Exogenous acids

• What are the causes of high AG metabolic acidosis?

Causes of High AG Metabolic Acidosis

• Starvation

• Ketoacidosis: Diabetic, Alcoholic• Lactic Acidosis

• Toxicity: Methanol, Ethylene Glycol, Propylene Glycol, Paraldehyde, Salicylates.

• Renal Failure

• What are the causes of Non AG metabolic acidosis?

Causes of Non AG Metabolic Acidosis

HCO3 loss: GIT:- Diarrhoea, Pancreatic or biliary drainage, ureterosigmoidostomyRenal:- Proximal (type 2) RTA, Ketoacidosis (during therapy)

Impaired renal acid excretion:Distal (type 1) RTA, Hyperkalemia (type 4) RTA Hypoaldosteronism, Renal Failure

Misc:Hyperalimentation , Cholestyramine, HCl therapy

• What are the menifestation of metabolic acidosis?

Manifestations of Metabolic Acidosis

CARDIOVASCULAR:- • Impaired cardiac contractility, • ↑ pulmonary vascular resistance,• ↓ in CO, BP & hepatic and renal Blood flow,

Sensitization to reentrant arrhythmias & reduced threshold of VF,

• Attenuation of cardiovascular responsiveness to catecholamines

RESPIRATORY:-• Hyperventilation• strength of respiratory muscles & muscle

fatigue• DyspneaCEREBRAL:-• Inhibition of metabolism • Mental status changes (somnolence & coma)

METABOLIC:• Increased metabolic demands• Insulin resistance• Inhibition of anaerobic glycolysis• Reduction in ATP synthesis• Hyperkalemia(secondary to cellular shifts)• Increased protein degradation

• What are the management of Metabolic acidosis?

General measures:-• Correct Any respiratory component of the

acidemia.• PaCO2 to be maintained (≈30) to partially

return pH to normal.• If pH < 7.20*, NaHCO3 (usually a 7.5%

solution), may be necessary.

The amount of NaHCO3 given :• empirically as a fixed dose (1 mEq/kg) or • derived from the base excess and the calculated

bicarbonate space NaHCO3 = Base Deficit × 30% × body weight in Kg• Only 50% of the calculated dose is given, after

which another ABG is measured”

• Serial ABG are mandatory to avoid complications(e.g, overshoot alkalosis and sodium overload) and to guide further therapy.

• pH>7.25 is sufficient to overcome the adverse physiological effects of the acidemia.

• Profound or refractory acidemia may require acute hemodialysis

• NaHCO3 in treating cardiac arrest and low flow states is not recommended - Paradoxical intracellular acidosis - particularly when CO2 elimination is impaired

• Alternate buffers that do not produce CO2 , such as Carbicarb or tromethamine (THAM) .

Specific therapy of Metabolic acidosis

• DKA: Replacement of the existing fluid deficit (as a result of a hyperglycemic osmotic diuresis) as well as insulin, potassium, phosphate, and magnesium.

• Lactic Acidosis:- restoring adequate oxygenation and tissue perfusion.

• Salicylate toxicity:- Alkalinization of the urine with NaHCO3 to a pH >7.0 increases its elimination

• Define Metabolic alkalosis?

• Primary ↑ in plasma HCO3>26mEq/L and pH >7.45.

• What are the pathophysiology of metabolic Alkalosis?

Pathophysiology

• HCO3- gain • H+ loss either from Renal or from GIT• H+ shift from ICF to ECF• Contraction of volume/chloride depletion

• What are the causes of metabolic alkalosis?

Chloride-sensitive• Gastrointestinal:- Vomiting, Gastric drainage,

diarrhea• Renal:- Diuretics• Sweat:- Cystic fibrosisChloride-resistant• Primary hyperaldosteronism• Edematous disorders (secondary)• Cushing’s syndrome• Severe hypokalemia

Miscellaneous• Massive blood transfusion• Acetate-containing colloid solutions• Alkaline administration with renal insufficiency• Alkali therapy • Combined antacid and cation-exchange resin therapy• Hypercalcemia Milk-alkali syndrome Bone metastases• Sodium penicillins• Glucose feeding after starvation

• What are the menifestation of metabolic alkalosis?

Manifestations of Metabolic Alkalosis

•CardiovascularArteriolar constrictionReduction in Coronary BF/ Anginal thresholdPredisposition to refractory SV & V arrhythmias

(especially if pH > 7.6)

•Respiratory - Hypoventilation (Compensatory) Hypercapnia / Hypoxemia

•MetabolicStimulation of anaerobic glycolysis & organic

acid productionReduction plasma ionized Calcium concHypokalemia (secondary to cellular shifts)Hypomagnesemia & Hypophosphatemia

•CerebralReduction in Cerebral BF mental status

changes (stupor, lethargy & delirium) N-M irritability (related to low ionized plasma

Ca) Tetany , Hyperreflexia , Seizures

• What are the management of metabolic alkalosis?

• Correction of metabolic alkalosis is never complete until the underlying disorder is treated.

• On controlled ventilation, any respiratory component causing alkalemia should be corrected by ↓ MV to normalize PaCO2 .

• TOC for Cl-sensitive metabolic alkalosis -administration of iv saline (NaCl) and potassium (KCl).

• H2 -blocker therapy is useful when excessive loss of gastric fluid is a factor.

• Acetazolamide may also be useful in edematous patients.

• Alkalosis associated with primary ↑ in mineralocorticoid activity readily responds to aldosterone antagonists (spironolactone).

• When pH > 7.60, treatment with: iv HCl(0.1 mol/L) , NH4Cl(0.1 mol/L) , arginine

hydrochloride, Or hemodialysis.

How to diagnose ACID-BASE disoder?

First: Initial clinical assessment based on clinical

details

• From history, examination and investigations make a clinical

decision as to what is the most likely acid-base disorder

• Be aware that in some situations the history may be

inadequate, misleading or the range of possible diagnosis is

large

• Mixed disorders are always very difficult

Structured approach to diagnosis

Second: Acid – Base Diagnosis

Perform a systematic evaluation of the blood gas and

other results and make an acid-base diagnosis

Finally: Clinical Diagnosis

Synthesize the information to make an overall clinical

diagnosis

Terms• ACIDS–Acidemia–Acidosis • Respiratory CO2

•Metabolic HCO3

• BASES–Alkalemia–Alkalosis• Respiratory CO2

•Metabolic HCO3

When you read an ABG which parameter you read first?

• Oxygenation

• Look at the PaO2, SaO2 and FiO2 of patient • Predicted minimum –PaO2 = FiO2 x 5, –COPD = FiO2 X 3

• Normal PaO2/FiO2 ratio:105mmHg/0.21=500, • < 300 = ARDS(ALI included in it;Berlin criteria)

Adequacy of Oxygen

• First look at the pH – any deviation from normal • ed pH < 7.35, Acidemia, then Uncompensated disorder • ed pH > 7.45, Alkalemia, then Uncompensated disorder • pH (7.35 – 7.45) – Normal, then there must be either no

disorder or a compensated disorder

Classify pH as normal, acidemia and alkalemia

Grading ACIDAEMIA ALKALAEMIAMild 7.30-7.34 7.46-7.50Moderate 7.20-7.29 7.51-7.54Severe <7.2 >7.55Incompatible with life

<6.8 >7.8

• Respiratory – if pH and PaCO2 move in Opposite direction in relation to normal values. (OR)

• Metabolic – if pH and PaCO2 move in Same direction. (SM)

• Primary Respiratory acidosis – ↓ pH & ↑ PaCO2

• Primary Respiratory alkalosis - ↑ pH & ↓ PaCO2

• Primary Metabolic acidosis – ↓ pH and ↓ PaCO2

• Primary Metabolic alkalosis –↑ pH and ↑ PaCO2

Analyze: disorder is Respiratory or Metabolic

• With time body tries to bring pH towards normal.• Lungs and kidneys are primary buffer response systems.• Primary Respiratory acidosis – ↓ pH & ↑ PaCO2 ↑ HCO3

• Primary Respiratory alkalosis - ↑ pH & ↓ PaCO2 ↓ HCO3

• Primary Metabolic acidosis – ↓ pH and ↓ PaCO2 ↓ HCO3

• Primary Metabolic alkalosis –↑ pH and ↑ PaCO2 ↑ HCO3

• HCO3 & PaCO2 will always move towards same direction

Evaluate compensation and correlate pH, PCO2, HCO3

• If diagnosed Primary respiratory acidosis/alkalosis – • Is there appropriate renal compensation – Acute or Chronic

• Acute Resp Acidosis: 10 mm Hg ↑ PaCO2 above 40 = ↑HCO3 by 1 mmol/L

• Acute Resp Alkalosis: 10 mm Hg ↓ PaCO2 below 40 = ↓ HCO3 by 2 mmol/L

• Chronic Resp Acidosis: 10 mm Hg ↑ PaCO2 above 40 = ↑HCO3 by 3 mmol/L

• Chronic Resp Alkalosis: 10 mm Hg ↓ PaCO2 below 40 = ↓HCO3 by 4 mmol/L OR

• Rise of PaCO2 by 20 mmHg = fall of pH by 0.1

• Fall of PaCO2 by 10 mmHg = rise of pH by 0.1

Calculate actual compensation seen and match with expected

• If diagnosed Primary Metabolic acidosis/alkalosis • Look for the expected change in PaCO2 –

• If PaCO2 is Higher, or Lower than that expected – then• With a primary metabolic disorder a superimposed primary

respiratory disorder is presentExpected change in PaCO2

• Metabolic acidosis - Expected Compensation in PaCO2 = 1.5 x HCO3+ 8 (±2)

• Metabolic alkalosis - Expected compensation in PaCO2 = 0.7 × HCO3+ 21 (±2)

Calculate actual compensation seen and match with expected

• If pH is normal – And PaCO2 – is High or Low

• Normal pH with High PaCO2 indicates a Mixed Respiratory

Acidosis & Metabolic Alkalosis

• Normal pH with Low PaCO2 indicates a Mixed Respiratory

Alkalosis & Metabolic Acidosis

• Therefore, if the PaCO2 is 70 mm Hg and the pH is 7.40, the

compensatory change in pH is more than expected indicating that

there is metabolic alkalosis in addition to the compensation .

Find out if the disorder is mixed

• If Metabolic Acidosis is diagnosed – Check Anion Gap• To assess the associated metabolic disorder and also

explain the cause of metabolic acidosis

• Δ AG/ Δ HCO3 < 0.1 - Combined high AG acidosis + non AG acidosis

• Δ AG/ Δ HCO3 = 1 - Anion Gap acidosis, DKA due to urinary ketone loss

• Δ AG/ Δ HCO3 >1.5 Metabolic acidosis and alkalosis

Unmask hidden disorders

RESPIRATORY ACIDOSIS:• Primary change is ↑ in PaCO2 leading to a ↓ in pH

• For each 10 mm Hg ↑ in PaCO2, pH ↓ by 0.05

RESPIRATORY ALKALOSIS:

• Primary change is ↓ in PaCO2 leading to ↑ in pH

• For each 10 mm Hg ↓ in PaCO2 pH ↑ by 0.1

Points to remember

METABOLIC ACIDOSIS:

• Primary change is ↓ in HCO3 or ↑ H+ leading to ↓ pH

• For ↓ in HCO3 of 7 – 7.5 mEq/ L, pH ↓ by 0.1

METABOLIC ALKALOSIS:

• Primary change is ↑ HCO3 or ↓ H+ leading to ↑ pH

• For ↑ in HCO3 of 7-7.5 mEq/L - pH ↑ by 0.1

Points to remember