Acid-Base Physiology 2012. Objectives Understand normal mechanisms and regulation of acid-base...

Acid-Base Physiology2012

Objectives Understand normal mechanisms and

regulation of acid-base balance Interpret blood gases Understand the effects of acidosis

and alkalosis Evaluate and manage acidosis and

alkalosis

Normal Physiology Acid-base balance maintained by normal

pulmonary excretion of CO2 and renal excretion of acid

Organic buffers: HCO3-, HPO4, protein anions, carbonate 90% of bicarb is reabsorbed by kidney

Renal excretion – H+ combines with urinary titratable acids (phosphates) or ammonia to form ammonium

Henderson-Hasselbach equation:pH = 6.1 + log (HCO3 ÷ (0.03 x PCO2))

Compensatory MechanismspH is determined by ratio of HCO3 and PCO2

Body responds to changes in pH by attempting to normalize the pH Buffering Respiratory – alterations in paCO2 Renal – alterations in bicarbonate

excretion

Compensatory Mechanisms Compensated metabolic acidosis:

1.2 mmHg in pCO2 for every 1 meq/L in HCO3 Compensated metabolic alkalosis:

0.7 mmHg in pCO2 for every 1 meq/L in HCO3 Compensated respiratory acidosis:

Acute- 1 meq/L for every 10 mmHg in pCO2 Chronic- 3.5 meq/L for every 10 mmHg in

pCO2 Compensated respiratory alkalosis:

Acute- 2 meq/L for every 10 mmHg in pCO2 Chronic- 4 meq/L for every 10 mmHg in pCO2

Blood Gas Interpretation General guidelines:

1) Is it acidosis or alkalosis?Acidosis – pH < 7.38Alkalosis – pH > 7.42

2) Is it primary respiratory or metabolic?Evaluate paCO2 and bicarbonate

3) Is there compensation?Calculations from previous slides

Blood Gas Interpretation 4) If respiratory disturbance, is it acute or

chronic? Respiratory acidosis:

Acute decrease in pH = 0.08 x (paCO2-40)/10 Chronic decrease in pH = 0.03 x (paCO2-40)/10

Respiratory alkalosis Acute increase in pH = 0.08 x (40-paCO2)/10 Chronic increase in pH = 0.03 x (40-paCO2)/10

5) If metabolic disturbance, is there an anion gap?

Check serum Na, Cl, HCO3

Case #1 pH 7.16, pCO2 70, HCO3 24 Acidosis or alkalosis? Respiratory or metabolic? Compensated? Clinical Scenario?



Case #1 pH 7.16, pCO2 70, HCO3 24 Acidosis or alkalosis? Respiratory or metabolic? Compensated?

No … likely acute Clinical Scenario?


2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6

Acute Respiratory Acidosis Respiratory pathophysiology – airway

obstruction, severe pneumonia, chest trauma, pneumothorax

Acute drug intoxication (narcotics, sedatives)

Residual neuromuscular blockade CNS disease (head trauma, decreased

consciousness)

Bicarbonate is often normal, kidneys have not had time to compensate

Case #2 pH 7.6, pCO2 23, HCO3 22 Acidosis or alkalosis? Respiratory or metabolic? Compensated? Clinical scenario?




No … likely acute Clinical scenario?


4 mo mechanically ventilated pt who was bagged during transport to CT scan by an overeager intern

Respiratory Alkalosis Pain, anxiety Hypoxemia Interstitial lung

disease Severe congestive

heart failure (pulmonary edema)

Pulmonary emboli Drugs – salicylates,

methylxanthines, nicotine

Sepsis, fever Hepatic failure –

encephalopathy Pregnancy Thyrotoxicosis CNS hemorrhage Overagressive

mechanical ventilation





1.2 mmHg in pCO2 for every 1 meq/L in HCO3

Compensated metabolic alkalosis: 0.7 mmHg in pCO2 for every 1 meq/L in HCO3

Compensated respiratory acidosis: Acute- 1 meq/L for every 10 mmHg in pCO2 Chronic- 3.5 meq/L for every 10 mmHg in pCO2

Compensated respiratory alkalosis: Acute- 2 meq/L for every 10 mmHg in pCO2 Chronic- 4 meq/L for every 10 mmHg in pCO2


Yes … 1.2 mmHg decrease in pCO2 for every 1 meq/L decrease in HCO3

Clinical scenario?


10 yo dev delayed pt admitted with diarrhea, med list reveals mom has been giving Miralax every 4 hours

Metabolic Acidosis Anion gap

Metabolic Acidosis Anion gap

Lactic acidosis DKA Toxic ingestions (salicylates, ethylene

glycol, ethanol, isopropyl alcohol, paraldehyde, methanol)

Renal failure – uremia

Metabolic Acidosis Nonanion gap

Metabolic Acidosis Nonanion gap

RTA Diarrhea Hypoaldosteronism Potassium-sparing diuretics Pancreatic loss of bicarbonate Ureteral diversion Carbonic anhydrase inhibitors Acid administration (ArgCl, NaCl)







Compensated respiratory acidosis: Acute- 1 meq/L for every 10 mmHg in pCO2 Chronic- 3.5 meq/L for every 10 mmHg in

pCO2 Compensated respiratory alkalosis:



Yes … 0.7 mmHg increase in pCO2 for every 1 meq/L increase in HCO3

Clinical scenario?


5 yo s/p appendectomy with NG tube left to suction on 7CH for 5 days

Metabolic Alkalosis Chloride-responsive (urine Cl < 10

meq/L) Gastric acid loss (vomiting, NG suction) Contraction alkalosis (often due to loop or

thiazide diuretics) Posthypercapnia syndrome

Metabolic Alkalosis Chloride-resistant

Mineralocorticoid excess Renal chloride wasting (Bartter

syndrome) Exogenous alkali (milk-alkali syndrome,

massive blood transfusion) Hypokalemia

Case #5 pH 7.30, pCO2 89, HCO3 42 Acidosis or alkalosis? Respiratory or metabolic? Compensated? Clincial scenario?






Compensated respiratory acidosis: Acute- 1 meq/L for every 10 mmHg in

pCO2 Chronic- 3.5 meq/L for every 10 mmHg

in pCO2 Compensated respiratory alkalosis:



Yes … 3.5 meq/L increase in HCO3 for every 10 mmHg increase in CO2

Clincial scenario?


35 yo CF patient on the Peds floor with end-stage lung disease

Chronic Respiratory Acidosis Chronic lung diseases (BPD, CF) Neuromuscular disorders Severe restrictive lung disease Severe obesity



Blood Gas Interpretation 4) If respiratory acidosis or alkalosis, is it

acute or chronic? Respiratory acidosis:

Acute decrease in pH = 0.08 x (paCO2-40)/10

Chronic decrease in pH = 0.03 x (paCO2-40)/10

Respiratory alkalosisAcute increase in pH = 0.08 x

(40-paCO2)/10Chronic increase in pH = 0.03 x (40-

paCO2)/10



No … Combined acidosis

Clinical scenario?


2 mo found down at home, hypoperfusion leading to lactic acidosis, hypoventilation leading to respiratory acidosis

Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis? Respiratory or metabolic? Compensated? Clinical scenario?



Case # 7 pH 7.46, pCO2 24, HCO3 16 Acidosis or alkalosis? Respiratory or metabolic? Compensated?

Well … 2 meq/L decrease in HCO3 for every 10 mmHg decrease in CO2

Combined respiratory alkalosis & metabolic acidosis

Clinical scenario?


Anxious 5 yo who is hyperventilating and has a history of RTA





No … 0.7 mmHg increase for every 1 meq/L predicts pCO2 of 48

Combined metabolic alkalosis and respiratory acidosis

Clinical scenario?


1 yo with vomiting for 3 days who presents to the ED with lethargy and decreased arousal, hypoventilating

Physiologic Effects of Acidosis Shifts the oxygen-hemoglobin

dissociation curve to the right Decreased affinity for O2

Physiologic Effects of Acidosis Pulmonary effects – vasoconstriction

decreases pulmonary blood flow Cardiac effects – depressed

contractility Neurologic effects – increased cerebral

blood flow, increased ICP Extracellular shift of K+

hyperkalemia Sympathetic overactivity, resistance to

catecholamines

Physiologic Effects of Alkalosis Shifts the oxygen-hemoglobin

dissociation curve to the left Stronger bond between Hb and O2 Decreased O2 delivery to tissues

Physiologic Effects of Alkalosis Cardiac arrhythmias Lungs – vasodilation increases

pulmonary blood flow Neurologic effects – headache,

seizures, altered mental status Decreased cerebral blood flow from

vasoconstriction

Decreased levels of ionized Ca++ Intracellular shift of potassium

severe hypokalemia

Management - Respiratory Acidosis Treat the underlying disorder Assist or increase ventilation

Secure airway if necessary Increase tidal volume or respiratory rate if

mechanically ventilated Noninvasive ventilation Bronchodilators

Reverse sedative medications

Management – Respiratory Alkalosis Treat the underlying disorder Decrease ventilation

Decrease respiratory rate Decrease tidal volume

Sedation and pain control Reassurance for anxious patients

Management - Metabolic Disorders Acidosis

Treat the underlying disorder Consider bicarb administration depending on

etiology Dialysis in the setting of renal failure

Alkalosis Treat the underlying disorder Chloride-responsive: replete chloride (NaCl, KCl,

ArgCl) Carbonic-anydrase inhibitors if diuresis needed

Conclusion The body has compensatory mechanisms to

maintain acid-base balance. Blood gases should be interpreted in a

systematic way to determine the etiology of the acid-base disturbance.

Acidosis causes pulmonary vasoconstriction, cardiac depression, arterial vasodilation, & decreased O2 affinity.

Alkalosis causes pulmonary vasodilation, arterial vasoconstriction, & increased O2 affinity.

Management of acid-base disorders primarily involves treatment of the underlying disorder.

Everyone always has slides of their kids …

QUESTIONS?

References www.uptodate.com www.emedicine.com Morganroth ML. Six steps to acid-base

analysis: clinical applications. The Journal of Critical Illness. 1990;5:460-69.

Morganroth ML. An analytic approach to diagnosing acid-base disorders. The Journal of Critical Illness. 1990;5:138-50.

http://www.uptodate.com/

http://www.emedicine.com/

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