Acid-Base Balance Interactive Tutorial

Emily PhillipsMSN 621

Spring 2009E-mail:

emmalemmaRN@hotmail.comAll images imported from

Microsoft Clipart & Yahoo Image gallery

How to navigate this tutorial: To advance to the next slide click on the box To return to the previous slide click on

the box To return to the Main Menu: click the box Hover over underlined text for a

definition/explanation To return to the last slide viewed click on

the button Click the for additional information

Objectives: Define acid base balance/imbalance Explain the pathophysiology of organs

involved in acid base balance/imbalance Identify normal/abnormal and

compensated/uncompensated lab values

Explain symptoms related to acid base imbalances and compensated vs. uncompensated

Appropriate interventions and expected outcomes

Main Menu:

Acid-Base Pretest The Buffer Systems

ABG Interpretation& Case Studies

Acid-Base Review test

Diagnostic Lab Values

Metabolic Distubances

Respiratory Disturbances Acid-Base Compensation

Acid-Base Pretest: What is the normal

range for arterial blood pH?

7.38 – 7.46

7.40 – 7.52

7.35 – 7.45

Acid-Base Pretest: What 2 extracellular substances work together

to regulate pH?

Sodium bicarbonate& carbonic acid

Carbonic acid& bicarbonate

Acetic acid & carbonic acid

Acid-Base Pretest: Characterize an acid & a base based on the

choices below.

Acids release hydrogen (H+) ions& bases accept H+ ions.

Acids accept H+ ions & bases release H+ ions

Both acids & bases can release& accept H+ ions

Acid-Base Pretest: Buffering is a normal body mechanism

that occurs rapidly in response to acid-base disturbances in order to prevent changes in what?

HCO3-

H2CO3

H+

Acid-Base Pretest: What are the two systems in the body that

work to regulate pH in acid-base balance & which one works fastest?

The Respiratory & Renal systemsRenal

The Respiratory & Renal systemsRespiratory

The Renal & GI systemsRenal

Acid-Base Balance: Homeostasis of bodily fluids at a normal

arterial blood pH pH is regulated by extracellular

carbonic acid (H2CO3) and bicarbonate (HCO3-)

Acids are molecules that release hydrogen ions (H+)

A base is a molecule that accepts or combines with H+ ions

Acids and Bases can be strong or weak: A strong acid or base is one that

dissociates completely in a solution - HCl, NaOH, and H2SO4

A weak acid or base is one that dissociates partially in a solution-H2CO3, C3H6O3, and CH2O

The Body and pH: Homeostasis of pH is controlled through

extracellular & intracellular buffering systems

Respiratory: eliminate CO2

Renal: conserve HCO3- and eliminate H+

ions Electrolytes: composition of extracellular

(ECF) & intracellular fluids (ICF)- ECF is maintained at 7.40

Protein Buffer system

HCO3-

Buffersystem

K+ - H+

Exchange

Quick Review: Click the BoxesA donator of H+ ions An acceptor of H+

w/ pH <7.0 ions w/ pH >7.0

Regulated by EC Controlled by ECH2CO3 & HCO3

- & IC buffer systems

Eliminates CO2 Conserves HCO3-

Eliminates H+ ions

An Acid is: A Base is:

pH is:

Respiratory System:

pH is:

Renal System:

Respiratory Control Mechanisms:

Works within minutes to control pH; maximal in 12-24 hours

Only about 50-75% effective in returning pH to normal

Excess CO2 & H+ in the blood act directly on respiratory centers in the brain

CO2 readily crosses blood-brain barrier reacting w/ H2O to form H2CO3

H2CO3 splits into H+ & HCO3- & the H+

stimulates an increase or decrease in respirations

Renal Control Mechanisms: Don’t work as fast as the respiratory

system; function for days to restore pH to, or close to, normal

Regulate pH through excreting acidic or alkaline urine; excreting excess H+ & regenerating or reabsorbing HCO3

-

Excreting acidic urine decreases acid in the EC fluid & excreting alkaline urine removes baseH+ elimination

& HCO3-conservation

Mechanisms of Acid-Base Balance: The ratio of HCO3

- base to the volatile H2CO3

determines pH Concentrations of volatile H2CO3 are regulated

by changing the rate & depth of respiration Plasma concentration of HCO3

- is regulated by the kidneys via 2 processes: reabsorption of filtered HCO3

- & generation of new HCO3-, or

elimination of H+ buffered by tubular systems to maintain a luminal pH of at least 4.5

Phosphate Buffer system

AmmoniaBuffer system

Acid-Base Balance Review test:

The kidneys regulate pH by excreting HCO3

- and retaining or regenerating H+

TRUE

FALSE

Acid-Base Review test: H2CO3 splits into HCO3

- & H+ & it is the H+ that stimulates either an increase or decrease in the rate & depth of respirations.

TRUE

FALSE

Acid-Base Review test: Plasma concentration of HCO3

- is controlled by the kidneys through reabsorption/regeneration of HCO3

-, or elimination of buffered H+ via the tubular systems.

TRUE

FALSE

Acid-Base Review test:

The ratio of H+ to HCO3- determines

pH.

TRUE

FALSE

Acid-Base Review test:

Secreted H+ couples with filtered HCO3-

& CO2 & H2O result.

TRUE

FALSE

Metabolic Disturbances: Alkalosis: elevated HCO3

- (>26 mEq/L) Causes include: Cl- depletion (vomiting,

prolonged nasogastric suctioning), Cushing’s syndrome, K+ deficiency, massive blood transfusions, ingestion of antacids, etc.

Acidosis: decreased HCO3- (<22 mEq/L)

Causes include: DKA, shock, sepsis, renal failure, diarrhea, salicylates (aspirin), etc.

Compensation is respiratory-related

Metabolic Alkalosis: Caused by an increase in pH (>7.45)

related to an excess in plasma HCO3-

Caused by a loss of H+ ions, net gain in HCO3

- , or loss of Cl- ions in excess of HCO3-

Most HCO3- comes from CO2 produced

during metabolic processes, reabsorption of filtered HCO3

-, or generation of new HCO3

- by the kidneys Proximal tubule reabsorbs 99.9% of

filtered HCO3-; excess is excreted in urine

Metabolic Alkalosis Manifestations: Signs & symptoms (s/sx) of volume

depletion or hypokalemia Compensatory hypoventilation,

hypoxemia & respiratory acidosis Neurological s/sx may include mental

confusion, hyperactive reflexes, tetany and carpopedal spasm

Severe alkalosis (>7.55) causes respiratory failure, dysrhthmias, seizures & coma

Treatment of Metabolic Alkalosis:

Correct the cause of the imbalance May include KCl supplementation for K+/Cl-

deficits Fluid replacement with 0.9 normal saline

or 0.45 normal saline for s/sx of volume depletion

Intubation & mechanical ventilation may be required in the presence of respiratory failure

Metabolic Acidosis: Primary deficit in base HCO3

- (<22 mEq/L) and pH (<7.35)

Caused by 1 of 4 mechanisms Increase in nonvolatile metabolic acids,

decreased acid secretion by kidneys, excessive loss of HCO3

-, or an increase in Cl-

Metabolic acids increase w/ an accumulation of lactic acid, overproduction of ketoacids, or drug/chemical anion ingestion

Metabolic Acidosis Manifestations: Hyperventialtion (to reduce CO2 levels),

& dyspnea Complaints of weakness, fatigue,

general malaise, or a dull headache Pt’s may also have anorexia, N/V, &

abdominal pain If the acidosis progresses, stupor, coma

& LOC may decline Skin is often warm & flush related to

sympathetic stimulation

Treatment of Metabolic Acidosis:

Treat the condition that first caused the imbalance

NaHCO3 infusion for HCO3- <22mEq/L

Restoration of fluids and treatment of electrolyte imbalances

Administration of supplemental O2 or mechanical ventilation should the respiratory system begin to fail

Quick Metabolic Review: Metabolic disturbances indicate an

excess/deficit in HCO3- (<22mEq/L or

>26mEq/L Reabsorption of filtered HCO3

- & generation of new HCO3

- occurs in the kidneys

Respiratory system is the compensatory mechanism

ALWAYS treat the primary disturbance

Respiratory Disturbances: Alkalosis: low PaCO2 (<35 mmHg)

Caused by HYPERventilation of any etiology (hypoxemia, anxiety, PE, pulmonary edema, pregnancy, excessive ventilation w/ mechanical ventilator, etc.)

Acidosis: elevated PaCO2 (>45 mmHg) Caused by HYPOventilation of any etiology

(sleep apnea, oversedation, head trauma, drug overdose, pneumothorax, etc.)

Compensation is metabolic-related

Respiratory Alkalosis: Characterized by an initial decrease in

plasma PaCO2 (<35 mmHg) or hypocapnia

Produces elevation of pH (>7.45) w/ a subsequent decrease in HCO3

- (<22 mEq/L)

Caused by hyperventilation or RR in excess of what is necessary to maintain normal PaCO2 levels

Respiratory Alkalosis Manifestations: S/sx are associated w/ hyperexcitiability

of the nervous system & decreases in cerebral blood flow

Increases protein binding of EC Ca+, reducing ionized Ca+ levels causing neuromuscular excitability

Lightheadedness, dizziness, tingling, numbness of fingers & toes, dyspnea, air hunger, palpitations & panic may result

Treatment of Respiratory Alkalosis:

Always treat the underlying/initial cause Supplemental O2 or mechanical

ventilation may be required Pt’s may require reassurance,

rebreathing into a paper bag (for hyperventilation) during symptomatic attacks, & attention/treatment of psychological stresses.

Respiratory Acidosis: Occurs w/ impairment in alveolar

ventilation causing increased PaCO2 (>45 mmHg), or hypercapnia, along w/ decreased pH (<7.35)

Associated w/ rapid rise in arterial PaCO2 w/ minimal increase in HCO3

- & large decreases in pH

Causes include decreased respiratory drive, lung disease, or disorders of CW/respiratory muscles

Respiratory Acidosis Manifestations: Elevated CO2 levels cause cerebral

vasodilation resulting in HA, blurred vision, irritability, muscle twitching & psychological disturbances

If acidosis is prolonged & severe, increased CSF pressure & papilledema may result

Impaired LOC, lethargy/coma, paralysis of extremities, warm/flushed skin, weakness & tachycardia may also result

Treatment of Respiratory Acidosis: Treatment is directed toward improving

ventilation; mechanical ventilation may be necessary

Treat the underlying cause Drug OD, lung disease, chest

trauma/injury, weakness of respiratory muscles, airway obstruction, etc.

Eliminate excess CO2

Quick Respiratory Review: Caused by either low or elevated PaCO2

levels (<35 or >45mmHg) Watch for HYPOventilation or

HYPERventilation; mechanical ventilation may be required

Kidneys will compensate by conserving HCO3

- & H+

REMEMBER to treat the primary disturbance/underlying cause of the imbalance

Compensatory Mechanisms: Adjust the pH toward a more normal

level w/ out correcting the underlying cause

Respiratory compensation by increasing/decreasing ventilation is rapid, but the stimulus is lost as pH returns toward normal

Kidney compensation by conservation of HCO3

- & H+ is more efficient, but takes longer to recruit

Metabolic Compensation: Results in pulmonary compensation

beginning rapidly but taking time to become maximal

Compensation for Metabolic Alkalosis: HYPOventilation (limited by degree of rise

in PaCO2) Compensation for Metabolic Acidosis:

HYPERventilation to decrease PaCO2 Begins in 1-2hrs, maximal in 12-24 hrs

Respiratory Compensation:

Results in renal compensation which takes days to become maximal

Compensation for Respiratory Alkalosis: Kidneys excrete HCO3

-

Compensation for Respiratory Acidosis: Kidneys excrete more acid Kidneys increase HCO3

- reabsorption

DIAGNOSTIC LAB VALUES & INTERPRETATION

Normal Arterial Blood Gas (ABG)Lab Values: Arterial pH: 7.35 – 7.45 HCO3

-: 22 – 26 mEq/L PaCO2: 35 – 45 mmHg TCO2: 23 – 27 mmol/L PaO2: 80 – 100 mmHg SaO2: 95% or greater (pulse ox) Base Excess: -2 to +2 Anion Gap: 7 – 14

Acid-Base pH and HCO3-

Arterial pH of ECF is 7.40 Acidemia: blood pH < 7.35 (increase in H+) Alkalemia: blood pH >7.45 (decrease in H+)

If HCO3- levels are the primary disturbance,

the problem is metabolic Acidosis: loss of nonvolatile acid & gain of

HCO3-

Alkalosis: excess H+ (kidneys unable to excrete) & HCO3

- loss exceeds capacity of kidneys to regenerate

Acid-Base PCO2, TCO2 & PO2

If PCO2 is the primary disturbance, the problem is respiratory; it’s a reflection of alveolar ventilation (lungs) PCO2 increase: hypoventilation present PCO2 decrease: hyperventilation present

TCO2 refers to total CO2 content in the blood, including CO2 present in HCO3

-

>70% of CO2 in the blood is in the form of HCO3

- PO2 also important in assessing respiratory

function

Base Excess or Deficit:

Measures the level of all buffering systems in the body – hemoglobin, protein, phosphate & HCO3

-

The amount of fixed acid or base that must be added to a blood sample to reach a pH of 7.40

It’s a measurement of HCO3- excess or

deficit

Anion Gap: The difference between plasma

concentration of Na+ & the sum of measured anions (Cl- & HCO3

-) Representative of the concentration of

unmeasured anions (phosphates, sulfates, organic acids & proteins)

Anion gap of urine can also be measured via the cations Na+ & K+, & the anion Cl- to give an estimate of NH4

+

excretion

Anion Gap The anion gap is increased in conditions

such as lactic acidosis, and DKA that result from elevated levels of metabolic acids (metabolic acidosis) A low anion gap occurs in conditions that

cause a fall in unmeasured anions (primarily albumin) OR a rise in unmeasured cations

A rise in unmeasured cations is seen in hyperkalemia, hypercalcemia, hyper-magnesemia, lithium intoxication or multiple myeloma

Sodium Chloride-Bicarbonate Exchange System and pH: The reabsorption of Na+ by the kidneys

requires an accompanying anion- 2 major anions in ECF are Cl- and HCO3

-

One way the kidneys regulate pH of ECF is by conserving or eliminating HCO3

- ions in which a shuffle of anions is often necessary

Cl- is the most abundant in the ECF & can substitute for HCO3

- when such a shift is needed.

Acid-Base Interpretation Practice: Please use the following key to interpret

the following ABG readings. Click on the blue boxes to reveal the

answers Use the button to return to the key at

any time Or use the “Back to Key” button at the

bottom left of the screen

Acid-Base w/o Compensation:

Parameters: pH PaCO2 HCO3-

Metabolic Alkalosis

Normal

MetabolicAcidosis

Normal

RespiratoryAlkalosis

Normal

RespiratoryAcidosis

Normal

Interpretation Practice: pH: 7.31 Right! PaCO2: 48 Try Again HCO3

-: 24 Try Again

pH: 7.47 Try Again PaCO2 : 45 Right! HCO3

- : 33 Try Again

Back to Key

Resp. Acidosis

Resp. Alkalosis

Metabolic Acidosis

Resp. Alkalosis

Metabolic Alkalosis

Metabolic Acidosis

Interpretation Practice:

pH: 7.20 Try Again PaCO2: 36 Try Again HCO3

-: 14 Right!

pH: 7.50 Try Again PaCO2 : 29 Right! HCO3

- -: 22 Try Again

Metabolic Alkalosis

Resp. Acidosis

Metabolic Acidosis

Metabolic Alkalosis

Resp. Alkalosis

Resp. Acidosis

Back to Key

Acid-Base Fully Compensated:Parameters: pH PaCO2 HCO3

-

Metabolic Alkalosis

Normal>7.40

MetabolicAcidosis

Normal<7.40

RespiratoryAlkalosis

Normal>7.40

RespiratoryAcidosis

Normal<7.40

Interpretation Practice: pH: 7.36 Try Again PaCO2: 56 Try Again HCO3

-: 31.4 Right!

pH: 7.43 Right! PaCO2 : 32 Try Again HCO3: 21 Try Again

Compensated Resp. Alkalosis

Compensated Metabolic Acidosis

Compensated Resp. Acidosis

Compensated Resp. Alkalosis

Compensated Metabolic Alkalosis

Compensated Metabolic Acidosis

Back to Key

Acid-Base Partially Compensated:

Parameters: pH PaCO2 HCO3-

Metabolic Alkalosis

MetabolicAcidosisRespiratoryAlkalosisRespiratoryAcidosis

Interpretation Practice:

pH: 7.47 Right! PaCO2: 49 Try Again HCO3

-: 33.1 Try Again

pH: 7.33 Try Again PaCO2 : 31 Try Again HCO3

- : 16 Right!

Partially Compensated Metabolic Alkalosis

Partially Compensated Resp. Alkalosis

Partially Compensated Metabolic Acidosis

Partially Compensated Metabolic Alkalosis

Partially Compensated Resp. Acidosis

Partially Compensated Metabolic Acidosis

Back to Key

Case Study 1: Mrs. D is admitted to the ICU. She has

missed her last 3 dialysis treatments. Her ABG reveals the following: pH: 7.32 Low, WNL = 7.35-7.45 PaCO2: 32 Low, WNL = 35-45mmHg HCO3

-: 18 Low, WNL = 22-26mEq/L

Assess the pH, PaCO2 & HCO3-. Are the

values high, low or WNL?

The pH is:

The PaCO2 is:

The HCO3- is:

Case Study 1 Continued:

What is Mrs. D’s acid-base imbalance?

Right! Try Again

Remember the difference between full & partial compensation. Go back & use the appropriate key if necessary.

Partially Compensated Metabolic Acidosis

Fully Compensated Resp. Acidosis

Case Study 2: Mr. M is a pt w/ chronic COPD. He is

admitted to your unit pre-operatively. His admission lab work is as follows: pH: 7.35 WNL = 7.35-7.45 PaCO2: 52 High, WNL = 35-45mmHg HCO3

-: 50 High, WNL = 22-26mEq/L

Assess the above labs. Are they abnormal or WNL?

The pH is:

The PaCO2 is:

The HCO3- is:

Case Study 2 Continued:

What is Mr. M’s acid-base disturbance?

Try AgainRight!

Think about appropriate interventions- if the problem is metabolic, the respiratory system compensates & vice versa

Fully Compensated Metabolic Acidosis

Fully Compensated Resp. Acidosis

Case Study 3: Miss L is a 32 year old female admitted w/

decreased LOC after c/o the “worst HA of her life.” She is lethargic, but arouseable; diagnosed w/ a SAH. Her ABG reads: pH: 7.48 High; WNL = 7.35-7.45 PaCO2: 32 Low; WNL = 35-45mmHg HCO3

-: 25 High; WNL = 22-26mEq/L What is the significance of her ABG

values?

The pH is:

The PaCO2 is:

The HCO3- is:

Case Study 3 Continued:

What is Miss L’s imbalance?

Right! Try Again

Great Job! You’ve reached the end of the tutorial & I hope you found it helpful. Thank you!

Resp. Alkalosis

Metabolic Alkalosis

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Porth, C.M. (2005). Pathophysiology Concepts of Altered Health States (7th ed.). Philadelphia: Lippincott Williams & Wilkins.

http://en.wikipedia.org/wiki/Dissociation_(chemistry). Retrieved 3/6/09.

http://www.clt.astate.edu/mgilmore/pathophysiology/Acid and Base.ppt#1. Retrieved 3/6/09.

http://www.uhmc.sunysb.edu/internalmed/nephro/webpages/Part_E.htm.Retrieved 3/6/09.

http://medical-dictionary.thefreedictionary.com/Volatile+acid. Retrieved 3/6/09.

REFERENCEShttp://wiki.answers.com/Q/How_does_the_phosphate_buffer_system_help_in_maintaining_the_ph_of_our_body

. Retrieved 3/10/09.

Alspach, J.G. (1998). American Association of Critical-Care Nurses Core Curriculum for Critical Care Nursing (5th ed.). Philadelphia: Saunders.

http://medical-dictionary.thefreedictionary.com. Retrieved 4/14/09.

Acid-Base Balance & Oxygenation Power Point. (2007). Milwaukee: Froedtert Lutheran Memorial Hospital Critical Care Class.