Interpretation of arterial blood gases:Traditional versus Modern

Interpretation of Arterial Blood Gases

Traditional Vs Modern

By

Gamal Rabie Agmy , MD , FCCP Professor of Chest Diseases ,Assiut University

Handerson Hasselbach

Equation

• PH= pKa+ Log HCO3/PaCo2

• PH= Log HCO3/PaCo2

• PH= HCO3

• PH= HCO3

• PH= PaCo2

• PH= PaCo2

Co2+H2o=H2co3=Hco3- + H+

Hprotein=H+ + Protein-

• PH 7.35-7.45

• HCO3 22-26 mEq/L

• PaCo2 35-45 mmHg

• PaO2 97 mmHg

• BE/BD +2 to -2

• Haemoglobin 15 gram

• A-a O2 gradient N < 15

A-a O2 gradient OLDER PERSONS

– 2.5 +0.25*AGE

– eg. AGE 60, 2.5+15=17.5

– eg. AGE 80, 2.5+20=22.5

Normal Values

Definition of Respiratory Failure

Respiratory failure is a syndrome of

inadequate gas exchange due to

dysfunction of one or more essential

components of the respiratory system

Types of Respiratory Failure

Type 1 (Hypoxemic ): * PO2 < 60 mmHg on room air.

Type 2 (Hypercapnic / Ventilatory): *PCO2 > 50

mmHg

Type 3 (Peri-operative): *This is generally a subset of

type 1 failure but is sometimes considered

separately because it is so common.

Type 4 (Shock): * secondary to cardiovascular

instability.

FIO2

Ventilation without

perfusion (deadspace ventilation)

Diffusion abnormality

Perfusion without

ventilation (shunting)

Hypoventilation

Normal

Brainstem

Spinal cord

Nerve root Airway

Nerve

Neuromuscular junction

Respiratory muscle

Lung

Pleura

Chest wall

Sites at which disease may cause ventilatory disturbance

• Type 1:

A-Acute

B-Chronic

C-Acute on top of chronic

• Type 2: A-Acute

B-Chronic

C-Acute on top of chronic

Types of RF:

• Type 1:

PaO2<60 mm Hg

PaCo235-45 mmHg

• Type 2: PaO2<60 mm Hg

PaCo2>50 mmHg

Types of RF:

Type 1: • Acute:

Pao2<60mmHg HCO3 Normal

PaCo235-45mmHg PH > 7.45

• Chronic:

Pao2<60mmHg PaCo235-45mmHg

HCO3 <22mEq/L PH=7.40-7.45

• Acute on top of chronic:

Pao2<60mmHg HCO3 <22mEq/L

PaCo235-45mmHg PH > 7.45

Types of RF:

Type 2: • Acute:

Pao2<60mmHg HCO3 Normal

PaCo2>50 mmHg PH<7.35

• Chronic:

Pao2<60mmHg PaCo2 >50 mmHg

HCO3 >26 mEq/L PH=7.35-7.40

• Acute on top of chronic:

Pao2<60mmHg HCO3 >26 mEq/L

PaCo2 >50 mmHg PH<7.35

Types of RF:

Uncompensated

Partially compensated

Compensated

Uncompensated


Compensated

M acidosisM acidosis

Uncompensated

*PH < 7.35

*PaCo2 normal

*HCO3 < 22 ml eq/L


*PH< 7.35

*PaCo2 < 36 mmHg

*HCO3 < 22 ml eq/L

Compensated

PH 7.35-7.40

PaCo2 < 35 mmHg

HCO3 < 22 mleq/L

Uncompensated


Compensated

Uncompensated


Compensated

M alkalosisM alkalosis

Uncompensated

PH > 7.45

PaCo2 normal

HCO3 >26 ml Eq/L


PH >7.45

PaCo2 > 45 mmHg

HCO3 > 26 ml Eq/L

Compensated

PH 7.40-7.45

PaCo2 > 45 mmHg

HCO3 > 26 mlEq/L

Combined acidosis:

PH<7.35 PaCO2>50 HCO3<22

Combined alkalosis:

PH >7.45 PaCo2<35 HCO3>26

Blood Gas Analysis &

Acid-Base Disorders

Professor Gamal Rabie Agmy, MD, FCCP

Professor of Chest Diseases , Assiut

University

Blood Gas Analysis

Arterial blood

Sea level (101.3kPa, 760mmHg)

Quiet

Anti-coagulate blood

Inspire air (Whether O2 supply)

Why Order an ABG?

• Aids in establishing a diagnosis

• Helps guide treatment plan

• Aids in ventilator management

• Improvement in acid/base management allows for optimal function of medications

• Acid/base status may alter electrolyte levels critical to patient status/care

Clinical Significance

To evaluate oxygen status

To evaluate ventilation

To evaluate acid-base disorder

How to evaluate oxygen status?

PaO2:

Partial pressure of oxygen in Arterial

blood .

Normal: 95-98 mmHg (12.6-13 kPa)

Estimate formula of age:

PaO2=100mmHg-(age×0.33) ±5mmHg

Hypoxaemia

Mild: 80-60mmHg

Medorate: 60-40mmHg

Severe: <40mmHg

Respiratory Failure

PaO2<60mmHg respiratory failure

Notice: sea level, quiet, inspire air

rule off other causes (Congenital

cyanotic heart disease and abnormal

types of Hg)

Classification of Respiratory Failure

PaCO2: The carbon dioxide partial

pressure

of arterial blood

Normal: 35-45mmHg (4.7-6.0kPa)

mean: 40mmHg

Classification of Respiratory Failure

Type Ⅰ TypeⅡ

PaO2 (mmHg) <60 <60

PaCO2 (mmHg) ≤35-45 >50

Other Parameters

SaO2: Saturation of arterial blood

oxygen

Normal: 0.95-0.98

Significance: a parameter to evaluate

hypoxaemia, but not sensitive

ODC ( Dissociation curve of

oxygenated hemoglobin): “S” shape

SaO2%

PO2

Oxygen dissociation curve

PH 2,3DPG temperature CO2

ODC to right deviation

Oxygenated hemoglobin release oxygen

to tissue, prevent hypoxia of the tissue.

But absorbed oxygen of hemoglobin is

decreased from the alveoli.

Bohr effect: movement of ODC place is

induced by PH.

PA-aO2: Difference of alveoli-arterial

blood oxygenic partial pressure.

Normal: 15-20mmHg (<30mmHg in the

old)

Significance: a sensitive parameter in

gas exchange

PvO2: Partial pressure of oxygen in

mixed venous blood.

Normal: 35-45mmHg

mean: 40mmHg

Significance: Pa-vO2 is to reflect the

tissue absorbing oxygen.

CaO2: The content of the oxygen of the

arterial blood.

Normal: 19-21mmol/L

Significance: a comprehensive

parameter to evaluate arterial

oxygen.

Parameters in acid-basic disorder evaluation

PH: negative logarithm of

Hydrogen ion concentration.

Normal: 7.35-7.45

mean: 7.4

PH=Pka+log 〔HCO3

- 〕

0.03PaCO2

＝6.1+log 20

1

HCO3- (bicarbonate):

SB (standard bicarbonate)

AB (actual bicarbonate)

SB: the contents of HCO3- of serum of arterial

blood in 37℃, PaCO2 40mmHg, SaO2 100%.

Normal: 22-26 mmol/L

mean: 24mmol/L

AB: The contents of HCO3- in actual condition.

In normal person: AB=SB

AB and SB are parameters to reflect

metabolism, regulated by kidney.

Difference of AB-SB can reflect the

respiratory affection on serum HCO3- .

Respiratory acidosis: AB>SB

Respiratory alkalosis: AB<SB

Metabolic acidosis: AB＝SB<Normal

Metabolic alkalosis: AB=SB>Normal

Buffer bases（BB)：

is the total of buffer negative ion of blood.

BB: HCO3-

hemoglobin

plasma proteins

HPO42- (phosphate)

Normal: 45-55mmol/L

mean: 50mmol/L

Significance: Metabolic acidosis: BB

Metabolic alkalosis: BB

Bases excess （BE):

the acid or bases used to regulate blood PH 7.4 . ( in 38℃，PaCO2 40mmHg, SaO2 100%)

Normal: 0±2.3 mmol/L

Significance:

add acid: BE(+), BB

add base: BE(-), BB

Total plasma CO2 (T-CO2)：

total content of the CO2 .

Normal: HCO3- >95%

Logistics

• When to order an arterial line -- – Need for continuous BP monitoring

– Need for multiple ABGs

– COP measurement by thermodilution method

• Where to place -- the options – Radial

– Femoral

– Brachial

– Dorsalis Pedis

– Axillary

Acid Base Balance

• The body produces acids daily

– 15,000 mmol CO2

– 50-100 mEq Nonvolatile acids

• The lungs and kidneys attempt to maintain balance

Acid Base Balance

• Assessment of status via bicarbonate-carbon dioxide buffer system

– CO2 + H2O <--> H2CO3 <--> HCO3- + H+

– ph = 6.10 + log ([HCO3] / [0.03 x PCO2])

The Terms

• ACIDS

– Acidemia

– Acidosis

• Respiratory

CO2

• Metabolic

HCO3

• BASES

– Alkalemia

– Alkalosis

• Respiratory

CO2

• Metabolic

HCO3

Respiratory Acidosis

• ph, CO2, Ventilation

• Causes

– CNS depression

– Pleural disease

– COPD/ARDS

– Musculoskeletal disorders

– Compensation for metabolic alkalosis

Respiratory Acidosis

• Acute vs Chronic

– Acute - little kidney involvement. Buffering via titration via Hb for example • pH by 0.08 for 10mmHg in CO2

– Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges hypochloremia) • pH by 0.03 for 10mmHg in CO2

Respiratory Alkalosis

• pH, CO2, Ventilation

• CO2 HCO3 (Cl to balance charges hyperchloremia)

• Causes – Intracerebral hemorrhage

– Salicylate and Progesterone drug usage

– Anxiety lung compliance

– Cirrhosis of the liver

– Sepsis

Respiratory Alkalosis

• Acute vs. Chronic

– Acute - HCO3 by 2 mEq/L for every 10mmHg in PCO2

– Chronic - Ratio increases to 4 mEq/L of HCO3 for every 10mmHg in PCO2

– Decreased bicarb reabsorption and decreased ammonium excretion to normalize pH

Metabolic Acidosis

• pH, HCO3

• 12-24 hours for complete activation of respiratory compensation

• PCO2 by 1.2mmHg for every 1 mEq/L HCO3

• The degree of compensation is assessed via the Winter’s Formula

PCO2 = 1.5(HCO3) +8 2

The Causes

• Metabolic Gap Acidosis – M - Methanol

– U - Uremia

– D - DKA

– P - Paraldehyde

– I - INH

– L - Lactic Acidosis

– E - Ehylene Glycol

– S - Salicylate

• Non Gap Metabolic

Acidosis

– Hyperalimentation

– Acetazolamide

– RTA (Calculate urine

anion gap)

– Diarrhea

– Pancreatic Fistula

Metabolic Alkalosis

• pH, HCO3

• PCO2 by 0.7 for every 1mEq/L in HCO3

• Causes

– Vomiting

– Diuretics

– Chronic diarrhea

– Hypokalemia

– Renal Failure

Mixed Acid-Base Disorders

• Patients may have two or more acid-

base disorders at one time

• Delta Gap

Delta HCO3 = HCO3 + Change in anion gap

Delta HCO3 >24 = metabolic alkalosis

The Steps

• Start with the pH

• Note the PCO2

• Calculate anion gap

• Determine compensation

Sample Problem #1

• An ill-appearing alcoholic male presents

with nausea and vomiting.

– ABG – 7.25 / 34 / 85 / 16

– Na- 137 / K- 3.8 / Cl- 90 / HCO3- 16

Sample Problem #1

• Anion Gap = 137 - (90 +16) =31

anion gap metabolic acidosis

• Winters Formula = 1.5(16) + 8 2

= 32 2

compensated

• Delta Gap =( 31 – 12) + 16 = 35

metabolic alkalosis

Sample Problem #2

• 22 year old female presents for

attempted overdose. She has taken an

unknown amount of Midol containing

aspirin, cinnamedrine, and caffeine. On

exam she is experiencing respiratory

distress.

Sample Problem #2

• ABG - 7.47 / 19 / 123 / 14

• Na- 145 / K- 3.6 / Cl- 109 / HCO3- 14

• ASA level - 38.2 mg/dL

Sample Problem #2

• Anion Gap = 145 - (109 + 14) = 22


• Winters Formula = 1.5 (14) + 8 2

= 29 2

uncompensated

• Delta Gap = 22 - 12 = 10

10 + 14 = 24

no metabolic alkalosis

Sample Problem #3

• 47 year old male experienced crush

injury at construction site.

• ABG - 7.3 / 32 / 96 / 15

• Na- 135 / K-5 / Cl- 98 / HCO3- 15 /

BUN- 38 / Cr- 1.7

• CK- 42, 346

Sample Problem #3

• Anion Gap = 135 - (98 + 15) = 22



= 30 2

compensated

• Delta Gap = 22 - 10 = 12

12 + 15 = 27

mild metabolic alkalosis

Sample Problem #4

• 1 month old male presents with

projectile emesis x 2 days.

• ABG - 7.49 / 40 / 98 / 30

• Na- 140 / K- 2.9 / Cl- 92 / HCO3- 32

Sample Problem #4

• Metabolic Alkalosis, hypochloremic


= 53 2

uncompensated

SVCC Respiratory Care

Programs

ABG Analysis, Introduction

• pH, PaCO2, PaO2 are measured directly

by special electrodes contained in a

device made for that purpose

• Other indirect measurements can be

made or calculated from the above

measurements i.e., HCO3-, O2 Sat.


Programs

QA in Blood Gas Analysis

• ABG lab must be able to assure

accurate and reliable results

• The above is accomplished by applying

protocols in 3 areas:

- pre-analytic error

- calibration

- quality control


Programs

Pre-analytic Error • All factors that cause variance in lab results

prior to the sample arriving in the ABG lab.

• 4 factors assoc. with signif. P. E. are:

- air bubbles in sample

- time delay (iced sample with more than

60 min. or uniced with more

than 10 min.)

- blood clots in sample

- small sample size where excessive

anticaogulation is suspect


Programs

Calibration

• Purpose is assure consistency

• Def.: the systemic standardization of the

graduation of a quantitative measuring

instrument

• Calibrating standards for blood gas analyzers

should simulate the physical properties of

blood and meet manuf. specs.

• When 2 standards are used ---> 2-point

calibration, performed after 50 blood gases or

at least every 8 hours


Programs

Calibration (cont’d)

• A “one-point calibration” is an

adjustment of the electronic response of

an electrode to a single standard and is

performed more freq. than a 2 pt. cal.,

ideally prior to each sample analysis


Programs

Quality Control

• Refers to a system that documents the

accuracy and reliability of the blood gas

measurements and is essential to assure

accuracy in the blood gas lab

• Media available as blood gas controls

include:

- aqueous buffers

- glycerin soltn.

- human/animal serum and blood

- artificial blood

• A QC system must ID problems and specify

corrective action, document. of accept. oper.


Programs

QC (cont’d)

• Documentation of QC is usu. on Levy-

Jennings Chart which shows measured

results on the y axis versus time of

measurement on the x axis

• SD is used to summarize a mass of data: the

difference between a number in a data set

and the mean of the data set is called a

deviation. A deviation shows how much a

number varies from the mean


Programs

QC (cont’d)

• A properly functioning electrode that

repeatedly analyzes a known value will

produce results within a rel. small range, e.g.,

a PaCO2 electrode that analyzes a 40 mmHg

standard 100 times will produce results where

2/3 of the measurements are 39 - 41 mmHg

and nearly all measurements fall in 38 - 42

range

• 95% of the control measurements should fall

within 2 SD


Programs

QC (cont’d)

• Random errors indicates a value outside of 2

SD of the mean: a single random error has

minor signif., but if number increased the

machine and techniques must be evaluated

• Systematic errors is recurrent measurable

deviation from the mean

• Causes of systematic errors:

- contaminated standard

- variations in electrode temp.

- inconsistent introduction of standard


Programs

QC (cont’d)

• Causes of systematic error (cont’d)

- inconsistent calibration

technique - change in QC

standard storage or prep. - electrode

problems, e.g., protein contamin.,

membrane malfunction,

contamin. electrolyte, or electrical

problems


Programs

QC Levels

• Level 1 simulates a patient

hypoventilating

• Level 2 simulates a patient with normal

ventilatory status

• Level 3 simulates a patient

hyperventilating

HCO3- (bicarbonate):

SB (standard bicarbonate)

AB (actual bicarbonate)

SB: the contents of HCO3- of serum of arterial

blood in 37-38℃, PaCO2 40mmHg, SaO2 100%.

Normal: 22-27mmol/L

mean: 24mmol/L

AB: The contents of HCO3- in actual condition.

In normal person: AB=SB

AB and SB are parameters to reflect

metabolism, regulated by kidney.

Difference of AB-SB can reflect the

respiratory affection on serum HCO3- .

Respiratory acidosis: AB>SB

Respiratory alkalosis: AB<SB

Metabolic acidosis: AB＝SB<Normal

Metabolic alkalosis: AB=SB>Normal

8 Sequential Rules:

• Rule #1

– Must know the pH; pH determines whether the

primary disorder is an acidosis or an alkalosis

• Rule #2

– Must know the PaCO2 and serum HCO3-

• Rule #3

– Must be able to establish that the available data

(pH, PaCO2, and HCO3-) are consistent

Are the data consistent?

• The Henderson Equation:

3

2

24HCO

PaCOH

Convert [H+] to pH:

• Subtract calculated [H+] from 80; this gives

the last two digits of a pH beginning with 7

– example: calculated [H+] of 24 converts to pH

of (80-24)~7.56

– example: calculated [H+] of 53 converts to pH

of (80-53)~7.27

• Refer to table 1 in handout for more precise

conversion, or if calculated [H+] exceeds 80

Relationship between [H+] & pH

Relationship between [H+] & pH

pH [H+] pH [H

+]

7.80

7.75

16

18

7.30

7.25

50

56

7.70

7.65

20

22

7.20

7.15

63

71

7.60

7.55

25

28

7.10

7.00

79

89

7.50

7.45

32

35

6.95

6.90

100

112

7.40

7.35

40

45

6.85

6.80

141

159

Simple Acid-Base Disorders:

T ype of D isorder pH PaC O 2 [H C O 3]

M etabolic A cidosis

M etabolic A lkalosis

A cute R espiratory A cidosis

C hronic R espiratory A cidosis

A cute R espiratory A lkalosis

C hronic R espiratory A lkalosis


• The compensatory variable always changes

in the SAME DIRECTION as the

primarily deranged variable

• Compensation is always more pronounced

in CHRONIC RESPIRATORY disorders

than in acute respiratory disorders

8 Sequential Rules:

• Rule #4:

– must know if compensation is appropriate

– compensation never overshoots

• Must have known “rules of thumb” to

interpret appropriateness of compensation

Rules of Compensation:

• Metabolic Acidosis

– PaCO2 should fall by 1 to 1.5 mm Hg x the fall

in plasma [HCO3]

• Metabolic Alkalosis

– PaCO2 should rise by .25 to 1 mm Hg x the rise

in plasma [HCO3]


• Acute Respiratory Acidosis

– Plasma [HCO3] should rise by ~1mmole/l for

each 10 mm Hg increment in PaCO2

• Chronic Respiratory Acidosis

– Plasma [HCO3] should rise by ~4mmoles/l for



• Acute Respiratory Alkalosis

– Plasma [HCO3] should fall by ~1-3 mmole/l for

each 10 mm Hg decrement in PaCO2, usually

not to less than 18 mmoles/l

• Chronic Respiratory Alkalosis




Case #1:

• A 24 years old with chronic renal failure

presents to ER with history of increasing

azotemia, weakness, and lethargy. Exam

reveals the patient to be modestly

hypertensive, and tachypneic. Labs reveal

BUN=100 mg, and Creatinine=8 mg.

Case #1:

• Steps 1&2: must know pH, PaCO2, HCO3

• pH=7.37, PaCO2=22, and HCO3=12

• Step 3: are the available data consistent?

3

2

24HCO

PaCOH

Case #1:

• [H+]=44, equates to pH~7.36; data are thus

consistent

• What is the primary disorder?

• “_________Acidosis”

• Which variable (PaCO2, HCO3) is deranged

in a direction consistent with acidosis?

• Primary disorder is “Metabolic Acidosis”

Is compensation appropriate?

• HCO3 is decreased by 12 mmoles/l

• PaCO2 should decrease by 1 to 1.5 times the

fall in HCO3; expect PaCO2 to decrease by

12-18 mm Hg or be between 22-28 mm Hg

• Since PaCO2 is 22 mm Hg, compensation is

appropriate, and the data are consistent with

a simple metabolic acidosis with respiratory

compensation

8 Sequential Rules:

• Rule #5:

– If the data are consistent with a simple disorder,

it does not guarantee that a simple disorder

exists; need to examine the patient’s history

• Rule #6:

– When compensatory responses do not lie within

the accepted range, by definition a combined

disorder exists.

Case #2:

• A 16 year old male with sickle cell anemia,

hemochromatosis, & subsequent cirrhosis,

presents with a several day history of

emesis. At presentation to the pedes ER, he

is hypotensive, orthostatic, and confused.

• What acid-base disorders might be

anticipated based on the above information?

Case #3:

• 16 yo male with sickle cell anemia, hemo-

chromatosis, & subsequent cirrhosis, and

several days of emesis. In the pedes ER, he

is hypotensive, orthostatic, and confused.

• Emesis-loss of H+ (HCl)-metabolic alkalosis

• Orthostatic hypotension-?lactic acidosis

• SCD-decreased O2 delivery-?lactic acidosis

• Cirrhosis-decreased lactate metabolism

Case #3:

• What baseline information is available?

• pH=7.55, PaCO2=66

• „lytes: Na+=166, K+=3.0, Cl-=90, HCO3=56

• Are the data internally consistent?

3

2

24HCO

PaCOH

Case #3:

• [H+]~28, equates to pH~7.55; consistent

• What is the primary abnormality?

• “_________ Alkalosis”

• PaCO2ed, HCO3 ed, therefore…….

• “Metabolic Alkalosis” presumed due to

emesis

• Is compensation appropriate?

Case #3:

• Metabolic Alkalosis

– PaCO2 should rise by .25 to 1 mm Hg x the rise

in plasma [HCO3]

• HCO3 ed by 32; PaCO2 should by 8-32

• PaCO2 ed by 26, so compensation appears

appropriate

• What about multiple risk factors for lactic

acidosis?

Case #3:

• Could there be a concealed lactic acidosis?

• What is the anion gap?

• Na+- (Cl- + HCO3), normally 12-14

• Anion gap here is 166 - (90 + 56) = 20

• ed anion gap implies metabolic acidosis

• Combined metabolic alkalosis &

metabolic acidosis therefore present

8 Sequential Rules:

• Rule #7: Always calculate the anion gap

• Often it is the only sign of an occult

metabolic acidosis

– acidotic patients partially treated with HCO3

– acidotic patients with emesis

• May be the only sign of metabolic acidosis

“concealed” by concomitant acid-base

disorders

Causes of Anion Gap Acidosis:

• Endogenous acidosis

– Uremia (uncleared organic acids)

– Ketoacidosis, Lactic acidosis (increased organic

acid production), Rhabdomyolosis

• Exogenous acidosis

– ingestions: salicylate, iron; paraldehyde use

• Other Ingestions:

– Methanol toxicity, Ethylene Glycol toxicity

Causes of normal Anion Gap Acidosis:

• Diarrhea

• Isotonic saline infusion

• Renal tubular acidosis

• Acetazolamide

• Ureterocolic shunt

Anion Gap:

• Based on the concept of electroneutrality; the

assumption that the sum of all available cations=

the sum of all available anions. Restated as:

• Na+ + Unmeasured Cations (UC) = Cl- + HCO3 +

Unmeasure Anions (UA); conventionally restated:

• Na+-(Cl-+HCO3)=UA-UC=Anion Gap=12 to 14

Anion Gap:

• Na+-(Cl-+HCO3)=UA-UC

• Serum albumin contributes ~1/2 of the total anion

equivalency of the “UA” pool. Assuming normal

electrolytes, a 1gm/dl decline in serum albumin

decreases the anion gap factitiously by 3 mEq/L.

Therefore an anion gap of 12 mEq/L is corrected

to 17-18 mEq/L when the serum albumin is half of

normal; this is an important correction factor in

settings of chronic illness or malnourished patients

Case #3:

• A 3 year old is brought to the pedes ER at

~3am, stuporous and tachypneic. History is

remarkable for his parents having cleaned

out their medicine cabinet earlier that day.

An ABG and electrolytes have been

accidentally drawn by the nurse.

Case #4:

• Available data: pH=7.53, PaCO2=12;

Na+=140, K+=3.0, Cl-=106, HCO3=10

• Are the data internally consistent?

3

224HCO

PaCOH

Case #4:

• [H+]~29, so pH~7.51; data consistent

• What is the primary disturbance?

• “__________ Alkalosis”


in a direction consistent with alkalosis?

• ed PaCO2, ed HCO3; so “Respiratory

Alkalosis”

Case #4:


• Acute respiratory alkalosis




• PaCO2 ed by ~30 mm Hg; HCO3 should

fall by 3-9 mmole/l; HCO3 is too great, so

superimposed metabolic acidosis

Case #4:


• 140 - (106 + 10) = 24; elevated anion gap

consistent with metabolic acidosis

• What is the differential diagnosis?

• Combined (true) respiratory alkalosis and

metabolic acidosis seen in sepsis, or

salicylate intoxication

Case #5:

• A 5 year old with Bartter‟s Syndrome is

brought to clinic, where she collapses. She

has recently been febrile, but history is

otherwise unremarkable. An ABG and

serum electrolytes are obtained: pH=6.9,

PaCO2=81; Na+=142, K+=2.8, Cl-=87,

HCO3=16

Case #5:

• Are the data consistent?

• [H+]=122, pH~6.9; data are consistent

3

224HCO

PaCOH

Case #5:


• “_________ Acidosis”


in a direction consistent with acidosis?

• Both; pick most abnormal value--

• “Respiratory Acidosis”


Case #5:

• Acute Respiratory Acidosis

– Plasma [HCO3] should rise by ~1mmole/l for


• Since HCO3 is inappropriately depressed,

compensation is not appropriate, and there

is a concomitant metabolic acidosis as well


• AG=39, confirms metabolic acidosis

Case #5:

• Combined Respiratory Acidosis and

Metabolic Acidosis; are there other

disorders present?

• What about the dx of Bartter‟s Syndrome?

• Bartter‟s Syndrome characterized by

hypokalemic metabolic alkalosis

• Does this patient have a concealed

metabolic alkalosis?

Case #5:

• Anion gap is 39, or 25-27 greater than

normal

• Typically, increases in anion gap correlate

with decreases in HCO3

• Assuming a 1:1 relationship, as anion gap

increases by 25, HCO3 should fall by 25

• Starting HCO3 must have been 16 + 25 = 41

Case #5:

• Therefore, starting HCO3 was ~41 mmol/l,

consistent with expected chronic metabolic

alkalosis. This metabolic alkalosis was

“concealed” by the supervening profound

metabolic and respiratory acidoses

associated with her arrest event.

• Final diagnosis: Metabolic alkalosis,

metabolic acidosis, & respiratory acidosis

8 Sequential Rules; Rule #8

• Rule #8: Mixed Acid-Base Disorders

• Coexistant metabolic acidosis and

metabolic alkalosis may occur. Always

check the change in the anion gap vs.

decrement in bicarbonate to rule out a

concealed metabolic disorder.

Case #6:

• A 3 year old toddler is brought to the ER at

3 am after being found unarousable on his

bedroom floor, with urinary incontinence.

EMS monitoring at the scene revealed sinus

bradycardia. One amp of D50W and 5 mg

of naloxone were given IV without

response. Vital signs are stable; respiratory

effort is regular, but tachypneic. He is

acyanotic.

Case #6:

• Initial lab studies (lytes, ABG & urine tox

screen) are sent. Initial dextrostick is >800.

• Initial available data are:

• Na+=154, K=5.6, Cl=106, HCO3=5,

BUN=6 creatinine=1.7, glucose=804,

PO4=12.3, Ca++=9.8, NH4=160, serum

osms=517

• pH=6.80, PaCO2=33, PaO2=298

Case #6:


• ________ Acidosis

• Metabolic Acidosis


• No; PaCO2 level is inappropriately high

• Are other disorders present?

• Respiratory acidosis (due to evolving coma)

Case #6:

• What is our differential thus far?

– Anion gap vs. non-anion gap metabolic acidosis

– DKA, lactic acidosis, renal failure, ingestion

• The urine tox screen comes back negative

– What does urine tox screen actually screen for?

• The patient‟s IV falls out. He then has a

seizure, is incontinent of urine, and fills the

specimen bag you placed on ER arrival.

Case #6:

• What is the calculated serum osmolality,

and does an osmolal gap exist?

• 2(Na) + BUN/2.8 + Glucose/18

– Calculated=355, Measured=517

• What is the most likely diagnosis?

• How can this be confirmed definitively?

– Review of urinalysis

– Serum ethylene glycol level

Case #6:

Anion gap metabolic acidosis

Osmolal gap

Methanol, ethylene glycol

ethyl alcohol, isopropyl alcohol

Gap-Gap

*AG excess/HCO3 deficit=

(Measured AG-12)/ (24-measured HCO3)

*Ratio <1 in presence of high AG acidosis means coexistance of normal AG metabolic acidosis

*Ratio >1 in presence of high AG acidosis means coexistance of metabolic alkalosis

Interpretation of arterial blood gases:Traditional versus Modern

Health & Medicine

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