abg by a taecher

Arterial blood gas analysis

THANKS TO:Dr. Chandramohan M,Intensivist,Bangalore

Decision to intubate• The decision to intubate should be primarily made on

clinical grounds and the figures in ABG values like PaO2<60 mmHg, PCO2>55mmHg etc should be used as a guideline only

Consider two scenarios:

• A 45 yr old patient with chronic neurological weakness with PCO2-60mmHg, PO2-58mmHg and with HR-80/min, BP- 130/80mmHg, RR-14/min, conscious, comfortable

• A 24yr old asthmatic with PCO2-60mmHg, PO2-58mmHg

and with HR-120/min,BP-100/70 mmHg,RR-40/min, in severe respiratory distress, drowsy

Which of the following cases would warrant immediate ventilatory support?

a. A 50-year-old man is comatose from drug overdose. PaCO2 is 51 mm Hg, PaO2 is 76 mm Hg, and pH is 7.31 while breathing room air.

b. A 29-year-old man is restless and in severe respiratory distress; he is breathing 42 times/min. PaCO2 is 33 mm Hg. pH is 7.42, and PaO2 is 47 mm Hg while breathing 60% oxygen through a face mask.

c. A 61-year-old woman who has severe emphysema is alert but is in moderate respiratory distress; her respiratory rate is 24/min. PaO2 is 75 mm Hg while breathing nasal oxygen at 2 L/min, PaCO2 is 59 mm Hg, and the pH is 7.34. Her chest x-ray is clear.

d. A 29-year-old woman is suffering from diabetic ketoacidosis. Her pH is 7.15, PaCO2 is 26 mm Hg and PaO2 is 110 mm Hg while breathing room air.

e. A 31-year-old drug addict responds briefly to the administration of Narcan (a narcotic antagonist) by opening her eyes and crying out and then lapses back into a state of semi-stupor. PaCO2 is 31 mm Hg. pH is 7.38, and PaO2 is 90 mm Hg while breathing nasal oxygen at 3 L/min.

Indications for Definitive Airway/Ventilatory support

Need for Airway Protection Need for Ventilation

Unconscious Apnea• Neuromuscular Paralysis• Unconscious

Severe Maxillofacial Injuries Inadequate Respiratory Effort• Tachypnea• Hypoxia• Hypercarbia• Cyanosis

Risk for aspiration• Bleeding• Vomiting

Severe head injury with need for controlling PaCO2 level

Risk for obstruction• Neck hematoma• Laryngeal, tracheal injury/burn• Stridor

• Any patient in cardiac arrest• Haemodynamic instability due to septic or cardiogenic shock

What is an ABG?

• The Components– pH / PaCO2 / PaO2 / HCO3 / O2sat / BE

• Desired Ranges– pH : 7.35 - 7.45– PaCO2 : 35-45 mmHg– PaO2 : 80-100 mmHg– HCO3 : 22-26 mEq/L– O2sat : 93-100%– Base Excess : +/-2 mEq/L

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

Approach to ABG Interpretation

Assessment of Acid-Base Status

Assessment of Oxygenation & ventilatory Status

There is an interrelationship, but less confusing if considered separately…..

The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes

Equation Physiologic Process

1. PaCO2 equation Alveolar ventilation2. Alveolar gas equation Oxygenation3. Oxygen content equation Oxygenation4. Henderson-Hasselbalch Acid-base balance equation

Assessment of Ventilatory Status….

Breathing pattern’s effect on PaCO2

Patient Vt f Ve Description A (400)(20) = 8.0L/min (slow/deep)

B (200)(40) = 8.0L/min (fast/shallow)

Patient Vt-Vd f Va A (400-150)(20) =5.0L/min (slow/deep)

B (200-150)(40) =2.0L/min (fast/shallow)

PaCO2 level is dependent on alveolar ventilation

Condition State of

PaCO2 in blood alveolar ventilation

> 45 mm Hg Hypercapnia Hypoventilation

35 - 45 mm Hg Eucapnia Normal ventilation

< 35 mm Hg Hypocapnia Hyperventilation

PaCO2 abnormalities…

• Assessment of Oxygenation….

ASSESSMENT OF OXYGENATION

• How much oxygen is in the blood?PaO2 vs. SaO2 vs. CaO2

• Alveolar-arterial O2 tension difference

• PaO2/FIO2 ratio

Alveolar Gas Equation

• PAO2 = PIO2 - 1.2 (PaCO2)*

• Where PAO2 is the average alveolar PO2, and PIO2 is the partial pressure of inspired oxygen in the trachea

PIO2 = FIO2 (PB – 47 mm Hg)

• FIO2 is fraction of inspired oxygen and PB is the barometric pressure. 47 mm Hg is the water vapor pressure at normal body temperature.

* Note: This is the “abbreviated version” of the AG equation, suitable for most clinical purposes. In the longer version, the multiplication factor “1.2” declines with increasing FIO2, reaching zero when 100% oxygen is inhaled. In these exercises “1.2” is dropped when FIO2 is above 60%.

P(A-a)O2

• P(A-a)O2 is the alveolar-arterial difference in partial pressure of oxygen.

• PAO2 is always calculated based on FIO2, PaCO2, and barometric pressure.

• PaO2 is always measured on an arterial blood sample in a “blood gas machine.”

• Normal P(A-a)O2 ranges from @ 5 to 25 mm Hg breathing room air (it increases with age).

• A higher than normal P(A-a)O2 means the lungs are not transferring oxygen properly from alveoli into the pulmonary capillaries. Except for right to left cardiac shunts, an elevated P(A-a)O2 signifies some sort of problem within the lungs.

• PaO2 ….is the pressure exerted by dissolved oxygen, not a quantity of oxygen

• To quantify oxygen calculate CaO2

How much oxygen is in the blood?PaO2 vs. SaO2 vs. CaO2

OXYGEN PRESSURE: PaO2• Since PaO2 reflects only free oxygen molecules dissolved in

plasma and not those bound to Hb, PaO2 cannot tell us “how much” oxygen is in the blood;

OXYGEN SATURATION: SaO2• The percentage of all the available heme binding sites saturated

with oxygen is the Hb oxygen saturation (in arterial blood, the SaO2).

OXYGEN CONTENT: CaO2

• Only CaO2 (units ml O2/dl) tells us how much oxygen is in the blood; this is because CaO2 is the only value that incorporates the Hb content. Oxygen content can be measured directly or calculated by the oxygen content equation:

CaO2 = (Hb x 1.34 x SaO2) + (.003 x PaO2)

Oxygen delivery = Cardiac output x CaO2

Oxygen Dissociation Curve: SaO2 vs. PaO2

Also shown are CaO2 vs. PaO2 for two different hemoglobin contents: 15 gm% and 10 gm%. CaO2 units are ml O2/dl. P50 is the PaO2 at which SaO2 is 50%.

Point “X” is discussed on later slide.

CaO2CaO2CaO2

CO Does Not Affect PaO2 – Be Aware!

• Review the O2 dissociation curve shown on a previous slide. “X” represents the 2nd set of blood gases for a patient who presented to the ER with headache and dyspnea & h/o exposure to smoke in a closed room

• His first blood gases showed PaO2 80 mm Hg, PaCO2 38 mm Hg, pH 7.43. SaO2 on this first set was calculated from the O2-dissociation curve as 97%, and oxygenation was judged normal.

• He was sent out from the ER and returned a few hours later with mental confusion

• This time both SaO2 and COHb were measured (SaO2

shown by “X”): PaO2 79 mm Hg, PaCO2 31 mm Hg, pH 7.36, SaO2 53%, carboxyhemoglobin 46%.

• CO poisoning was missed on the first set of blood gases because SaO2 was not measured!

Which patient is more hypoxemic, and why?

• Patient A: pH 7.48, PaCO2 34 mm Hg, PaO2 85 mm Hg, SaO2 95%, Hemoglobin 7 gm%

• Patient B: pH 7.32, PaCO2 74 mm Hg, PaO2 59 mm Hg, SaO2 85%, Hemoglobin 15 gm%

• Patient A: Arterial oxygen content = .95 x 7 x 1.34 = 8.9 ml O2/dl• Patient B: Arterial oxygen content = .85 x 15 x 1.34 = 17.1 ml O2/dl• Patient A, with the higher PaO2 but the lower hemoglobin content,

is more hypoxemic.• In this problem the amount of oxygen molecules contributed by

the dissolved fraction is negligible and will not affect the answer.

Assessment of acid base balance…

Basics •Nano equivalent =1×10-9

•[H+]= 40 nEq/L (16 to 160 nEq/L) at pH-7.4•For every 0.3 pH change = [H+] double

Very fast 80% in ECF

Starts within minutes good response by 2hrs, complete by 12-24 hrs

Starts after few hrs complete by 5-7 days

Acid-base Balance Henderson-Hasselbalch Equation

[HCO3-]

pH = pK + log ------------- .03 [PaCO2]

For teaching purposes, the H-H equation can be shortened to its basic relationships:

HCO3-

pH ~ --------- PaCO2

Abnormal Values

pH < 7.35• Acidosis (metabolic

and/or respiratory)pH > 7.45• Alkalosis (metabolic

and/or respiratory)paCO2 > 45 mm Hg

• Respiratory acidosis (alveolar hypoventilation)

paCO2 < 35 mm Hg

• Respiratory alkalosis (alveolar hyperventilation)

HCO3 < 22 meq/L

• Metabolic acidosis HCO3 > 26 meq/L

• Metabolic alkalosis

SIMPLE ACID-BASE DISORDER

Simple acid-base disorder – a single primary process of acidosis or alkalosiswith or with out compensation

1.pH=7.2 PCO2 = 60 mmHg, HCO3-24mEq/L-no compensation

2.pH 7.36, PaCO2 53 ,HCO3 30-with compensation

Mixed Acid-base Disorders are Common

• In chronically ill respiratory patients, mixed disorders are probably more common than single disorders, e.g., RAc + MAlk, RAc + Mac, Ralk + MAlk.

• In renal failure (and other conditions) combined MAlk + MAc is also encountered.

Clues to a mixed disorder:• Normal pH with abnormal HCO3 or CO2 • PaCO2 and HCO3 move in opposite directions• pH changes in an opposite direction for a

known primary disorder

Characteristics of acid-base disorders

DISORDER PRIMARY RESPONSES

COMPENSATORY RESPONSE

Metabolic acidosis

[H+] PH HCO3- pCO2

Metabolic alkalosis

[H+] PH HCO3- pCO2

Respiratory acidosis

[H+] PH pCO2 HCO3-

Respiratory alkalosis

[H+]

PH pCO2 HCO3-

Compensation…The Rules..

The body always tries to normalize the pH so…• CO2 and HCO3 should rise and fall

together in simple disorders• Compensation never overcorrects the pH• Lack of compensation in an appropriate

time interval defines a 2nd disorder• Compensatory responses require

normally functioning lungs and kidneys

RENAL & RESPIRATORY COMPENSATIONS TO ACID-BASE DISTURBANCES

Disorder Compensatory responseMetabolic acidosis PCO2 1.2 mmHg per 1.0 meq/L HCO3

1. Compensation complete in 12-24hrs

2. Limit of CO2 is 10mmHg

Metabolic alkalosis PCO2 0.7 mmHg per 1.0 meq/L HCO3

-

1.Compensation is complete (pCO2 levels out) in 12-24 hours.2.The limit of compensation is a pCO2 of 60 mmHg

Respiratory acidosis [HCO3-]

Acute 1.0 meq/L per 10 mmHg Pco2

Chronic 3.5 meq/L per 10 mmHg Pco2

Respiratory alkalosis [HCO3-]

Acute 2.0 meq/L per 10 mmHg Pco2

Chronic 4.0 meq/L per 10 mmHg Pco2

Expected changes in pH for a 10-mm Hg change in PaCO2 resulting from either primary respiratory acidosis or respiratory alkalosis:

ACUTE CHRONIC

• Respiratory Acidosis

pH ↓ by 0.07 pH ↓ by 0.03

• Respiratory Alkalosis

pH ↑ by 0.08 pH ↑ by 0.03

Anion Gap

AG = [Na+] - [Cl- +HCO3-]• Elevated anion gap represents

metabolic acidosis• Normal value: 10 ± 2 mmol/L• Major unmeasured anions

– albumin– phosphates– sulfates– organic anions

Na+

Unmeasured cations Unmeasure

d anions

Cl-

HCO3-

‘Mind the gap’

cations = Anions

Anion gap = metabolic acidosis

Six steps for ABG ANALYSIS

• 1. The first step - Look at the pH - Label it.

pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L.

• ACIDOSIS

• 2. The second step look at -pCO2. Label it.

• pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L.

• IncreasedNormal pCO2 levels Normal pCO2 levels are 35-45mmHg. are 35-45mmHg. Below 35 is Below 35 is alkalotic, above 45 alkalotic, above 45 is acidic.is acidic.

• 3. The third step is to look at the HCO3- Label it. pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L

• INCREASEDA normal HCO3 level is 22-26 A normal HCO3 level is 22-26 mEq/L. If the HCO3 is below mEq/L. If the HCO3 is below 22, the patient is acidotic. If 22, the patient is acidotic. If the HCO3 is above 26, the the HCO3 is above 26, the patient is alkaloticpatient is alkalotic

4.Next match either the pCO2 or the HCO3 with the pH to determine the acid-base disorder.

• pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L• pH is on acidotic side & PCO2 is

increased. So it is respiratory acidosis

• 5. Fifth, does either the CO2 or HCO3 go in the opposite direction of the pH?

• pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L

• To find the primary and what is compensatory

• HCO3 is going in opposite direction of pH. So it is metabolic compensation

Is the compensation full or partial??

• Do the calculations…. pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of

27 mEq/L

• PCO2 is increased by =40• HCO3-=should be increased by 4 i.e. 24+4=28( for full compensation)

• 6. Calculate the anion gap if it is more there is Metabolic acidosis

AG = [Na+] - [Cl- +HCO3-AG = [Na+] - [Cl- +HCO3-]]

• Don’t forget to assess ventilation and oxygenation status ….

A patient’s in acute respiratory distress, ABG shows pH of 7.14, PaCO2 of 70 mm Hg, and HCO3- of 23 mEq/L. How would you describe the likely acid-base disorder(s)?

Example…

• Is the problem only acute respiratory acidosis or is there some additional process?

• For every 10-mm Hg rise in PaCO2 (before any renal compensation), pH falls about 0.07 units.

• Because this patient's pH is down by 0.26, or 0.05 more than expected for a 30-mm Hg increase in PaCO2, there must be an additional metabolic problem.

• Also note that with acute CO2 retention of this degree, the HCO3- should be elevated by 3 mEq/L.

• Decreased perfusion leading to mild lactic acidosis would explain the metabolic component.

Importance of History/Patients•pH 7.08•PCO2 80•HCO3-24mEq/l

•80KG Man post gastrectomy on CMV 6L of mv•Acidosis•No compensation •Expected HCO3- 24+4=28mEq/L

•pH 7.08•PCO2 80•HCO3-24mEq/l

•Diabetic + Chronic COPD•Baseline PCO2 -80•Stopped insulin few days back•DKA•Expected HCO3-=24+4×3.5=24+14=38

RESPIRATORY

ACIDOSIS METABOLIC

ACIDOSIS

Sample ABG

• pH 7.45• PaCO2 38• HCO3 23• Analysis:• Cause:??

Case 1

• Mr. A is a 60 year-old with pneumonia. He is admitted with dyspnea, fever, and chills. His blood gas is below:pH 7.28 CO2 56 PO2 70 HCO3 25 SaO2 89%

• What is your interpretation?• What interventions would be appropriate for Mr. A?• Mr. A has an uncompensated respiratory acidosis with

hypoxemia as a result of his pneumonia

Case 2

• Ms. B is a 24 year-old college student. She has acute GE and is complaining a of a 3 day history of watery diarrhea. A blood gas is obtained to assess her acid/base balance:pH 7.28 CO2 43 pO2 88 HCO3 20 SaO2 96%

• What is your interpretation?• What interventions would be appropriate for Ms. B?• Ms. B has an uncompensated metabolic acidosis. This

is due to excessive bicarbonate loss from her diarrhea.

Case 3• Mr. C is a 80 year-old nursing home resident admitted

with urosepsis. Over the last two hours he has developed shortness of breath and is becoming confused. His ABG shows the following results:pH 7.02 CO2 55 pO2 77 HCO3 14 SaO2 89%

• What is your interpretation?• What interventions would be appropriate for Mr. C?• Mr. C has a metabolic and respiratory acidosis with

hypoxemia. The metabolic acidosis is caused by his sepsis. The respiratory acidosis is secondary to respiratory failure.

Case 4

• 4. Mrs. D is a thin, elderly-looking 61 year-old COPD patient. She has an ABG done as part of her routine care in the pulmonary clinic. The results are as follows: pH 7.37 CO2 63 pO2 58 HCO3 35 SaO2 89%

• What is your interpretation?• What interventions would be appropriate for

Mrs. D?• Mrs. D has a fully-compensated respiratory

acidosis with hypoxemia

Case 5

• Ms. E is a 17 year-old with intractable vomiting. She has some electrolyte abnormalities, so a blood gas is obtained to assess her acid/base balance.pH 7.50 CO2 36 pO2 92 HCO3 27 SaO2 97%

• What is your interpretation?• What interventions would be appropriate for Ms. E?• Ms. E has an uncompensated metabolic alkalosis.

Case 6

• Mr. F is a 18 year-old comatose, quadriplegic patient who has the following ABG done as part of a medical workup:pH 7.44 CO2 22 pO2 96 HCO3 16 SaO2 98%


Mr. F ?• His blood gas shows a fully-compensated

respiratory alkalosis

Case 7• A 76 yrs, female admitted with right sided weakness, visual

disturbance, slurred speech. Started on NG feeding but has large vomit 24 hrs later. She initially appears well later she became agitated, distressed, pyrexial

• PEx: RS-basal coarse crepts present, CNS-confused, other findings same as before

• PR-100/min, RR- 29/min, BP-110/70 mmHg, O2%-92% with supplementary O2 ( reservoir mask + O2)

FIO2 .60 Na+

136 mEq/L

pH 7.4 K+ 3.8 mEq/L

PaCO2 33 mm Hg Cl- 99 mEq/L

PaO2 65 mm Hg lactate 1.5 mmol/L

SaO2 92%

HCO3- 21 mEq/L

%COHb 2.1%

Hb 13 gm%

How would you characterize her state of oxygenation, ventilation, and acid-base balance? What is the likely diagnosis??

• Type I respiratory impairment, mild respiratory alkalosis balanced by metabolic acidosis

• Aspiration pneumonia

Case 8

• Mrs. H is found pulseless and not breathing this morning. After a couple minutes of CPR she responds with a pulse and starts breathing on her own. A blood gas is obtained:pH 6.89 CO2 70 pO2 42 HCO3 13 SaO2 50%


Mrs. H?• Mrs. H has a severe metabolic and respiratory

acidosis with hypoxemia

Case 9

• Mr. X is in respiratory distress. He has a history of Type-I diabetes mellitus and is now febrile. His ABG shows:pH 7.00 CO2 59 pO2 86 HCO3 14 SaO2 91%


Mr. X?• Mr. X has a metabolic and respiratory acidosis

with hypoxemia.

Case 10

• Ms. Y was admitted for a drug overdose. She is being mechanically ventilated and a blood gas is obtained to assess her for weaning. The results are as follows:pH 7.54 CO2 19 pO2 100 HCO3 16 SaO2 98%

• What is your interpretation?• What interventions would be appropriate for Ms. Y?• Mrs. Y is being overventilated which caused a

partially-compensated respiratory alkalosis

Case 11

A 46-year-old man has been in the hospital for two days with pneumonia. He was recovering but has just become diaphoretic, dyspneic, and hypotensive. He is breathing oxygen through a nasal cannula at 3 l/min.

pH 7.41

PaCO2 20 mm Hg

%COHb 1.0%

PaO2 80 mm Hg

SaO2 95%

Hb 13.3 gm%

HCO3- 12 mEq/L

CaO2 17.2 ml O2/dl

How would you characterize his state of oxygenation,ventilation, and acid-base balance?

• Normal pH with very low bicarbonate and PaCO2 indicates combined respiratory alkalosis and metabolic acidosis.

Case 12• A 59 yrs, male, chronic alcoholic, h/o severe upper

abdominal pain x 3 days, breathlessness present, excessive alcohol consumption for past few weeks

• PEx: looks sick, shortness of breath present, epigastric tenderness present, CXR- few b/l scattered opacites

• PR-120/min, RR- 28/min, BP-75/60 mmHg, O2%-98%• On 8 l/min O2 with mask & reservoir bag

Hb 11 gm% Na+ 142 mEq/L

pH 7.31 K+ 3.9 mEq/L


PaO2 81 mm Hg lactate 4 mmol/L

SaO2 98% iCa+ 0.8 mmol/L

HCO3- 15 mEq/L

BE -12 mEq/L

How would you characterize his state of oxygenation, ventilation, and acid-base balance? What is the probable diagnosis and

furhter action ??

• Type I respiratory impairment, severe metabolic acidosis with partial compensation most likely due to acute pancreatitis

• A 55 yr, female c/o sudden onset of breathlessness & left sided chest pain, underwent knee replacement operation 4 days back, no relevant past medical history

• PEx: slight shortness of breath, CVS/RS-NAD, no clinical evidence of DVT, CXR-NAD, ECG-only tachycardia

• PR-98/min, RR- 20/min, BP-150/90 mmHg, temp-36.6 degrees,O2%-99%

Case 13

FIO2 .21 Na+

136 mEq/L

pH 7.43 K+ 3.8 mEq/L



SaO2 99%

HCO3- 25.8 mEq/L

%COHb 2.1%

Hb 10 gm%

How would you characterize his state of oxygenation, ventilation, and acid-base balance? Does she require any further work up??

• Normal gas exchange, normal acid base status• PA02=(.21 x 713) + (37 x 1.2), PaO2 91

mm Hg• P(A-a)02= 106-91=15 mmHg• Patient is high risk for PE• Normal ABG never excludes it and she

requires V/Q scan or CTPA

Case 14• A 36 yr, male with alprazolam tab consumption x 3,

slightly drowsy but easily arousable, no relevant past medical history

• PEx: RS/CVS-NAD• PR-80/min, RR- 14/min, BP-110/70 mmHg, temp-36.6 degrees, SpO2%-99%

FIO2 .21 Na+

138 mEq/L

pH 7.37 K+ 3.8 mEq/L



SaO2 74%

HCO3- 24 mEq/L

%COHb 2.1%

Hb 13 gm%

How would you characterize his state of oxygenation, ventilation, and acid-base balance? What is the likely explanation for low PaO2??

• Appearance of severe type I respiratory failure, normal acid base status

• There is marked discrepancy between SaO2 by pulse oximeter and that calculated by ABG

• It is a venous sample & repeat ABG should be done

Case 15• A 70 yrs, male, comes to casualty with shortness of

breath and excessive tiredness, h/o chronic PR bleeding present

• Past medical history-k/c/o HTN, IHD• PEx: RS/CVS-NAD• PR-110/min, RR- 23/min, BP-140/90 mmHg, O2%-99% on air

FIO2 .21 Na+

138 mEq/L

pH 7.49 K+ 3.8 mEq/L



SaO2 99%

HCO3- 22 mEq/L

%COHb 2.1%

Hb 6.7 gm%

How would you characterize his state of oxygenation, ventilation, and acid-base balance? What is the likely explanation for breathlessness??

• Hyperventilation- no impairment of oxygenation but hypoxemia due to anaemia

• Uncompensated respiratory alkalosis• Patient has severe anaemia due to iron

deficiency/chronic rectal bleeding

Case 16• A 21yrs,female, known asthmatic, with 6hr h/o

worsening breathlessness & wheeze, no relief from salbutamol inhaler

• PEx: tachypnoeic, using accessory muscles, just managing to speak in full sentences

• PR-115/min, RR- 30/min, BP-110/80 mmHg, O2%-96% on air FIO2 .21 Na+

138 mEq/L

pH 7.38 K+ 3.8 mEq/L



SaO2 96%

HCO3- 24 mEq/L

Hb 12 gm%

How would you characterize her state of oxygenation, ventilation, and acid-base balance? Would you be concerned?? If so why??

• Mild type I respiratory impairment• PaCO2-high end of the normal range• Life threatening attack

Treat the patient not the ABG!!!

• “ABG’’ should supplement clinical judgment not substitute it”

• Treat the underlying clinical condition(s); this will usually suffice to correct most acid-base disorders.

• If there is concern that acidemia or alkalemia is life-threatening, aim toward correcting pH into the range of 7.30 - 7.52 ([H+] = 50-30 nM/L).

• THANK YOU FOR PATIENT HEARING…

A patient is admitted to the ICU with the following lab values:

BLOOD GASESpH: 7.40PCO2: 38HCO3: 24PO2: 72ELECTROLYTES, BUN & CREATININENa: 149K: 3.8Cl: 100CO2: 24BUN: 110Creatinine: 8.7What is(are) the acid-base disorder(s)?(in this case venous CO2=arterial HCO3-)

Step 1: Anion gapAG = Na+ - (Cl + CO2)= 149 - (100 + 24) = 25This high an AG indicates an anion gap metabolic

acidosis.Step 2: Delta anion gapcalculated AG= 25 mEq/Lnormal AG = 12 mEq/L25 - 12 = 13 mEq/L; this is the excess or delta anion gapStep 3: Delta serum CO2 = normal CO2 - measured CO2

=27 (average normal venous CO2) - 24 = 3 mEq/LStep 4: Bicarbonate Gap = delta AG - delta CO2 = 13 - 3

= 10 mEq/L This means the measured bicarbonate is 10 mEq/L

higher than expected from the excess AG, indicating (in this case) a metabolic alkalosis. Thus this patient, with normal pH and PaCO2, has BOTH metabolic acidosis and metabolic alkalosis. The patient was both uremic (causing metabolic acidosis) and had been vomiting (metabolic alkalosis)

pH < 7.35Acidosis

pH > 7.35Alkalosis

pCO2 > 40Respiratory

HCO3 < 24Metabolic

pCO2 < 40Respiratory

HCO3 > 24

Metabolic

PaCO2 ↑10→HCO3 ↑4

PaCO2 ↑10→HCO3 ↑1

PaCO2 ↓10→HCO3 ↓4

PaCO2 ↓10→HCO3 ↓2

PaCO2 ↑7

→HCO3 ↑10

Urine Cl < 10 Cl ResponsiveAnion Gap < 12

Non-Anion GapAnion Gap > 12

Anion Gap

Urine Cl > 10 Cl Unresponsive

Interpreting ABGs

Osm Gap > 10Methanol

Ethylene Glycol

Osmolar Gap < 10Ketoacidosis

Lactic acidosis Uremia

Aspirin/salicylate tox

DiarrheaRenal tubular acidosis

AcetazolamideTotal parenteral nutrition

Ureteral diversionPancreas transplant

CNS depressantsNeuromuscular disorder

Thoracic cage abnormalitiesObstructive lung disease

Obesity/hypoventilation syndromeMyxedema coma

Anxiety/painSepsis

CNS (stroke)Aspirin OD

Chronic liver diseasePulmonary embolism

PregnancyHyperthyroidism

Loss of body fluids:Vomiting

Nasogastric suctioningDiuretic use

Excess body fluids:Exogenous steroidsCushing’s syndromeHyperaldosteronism Bartter’s syndrome

=Na - (Cl+HCO3)

Acute

Chronic

PaCO2 ↓12→HCO3 ↓10

Compensation:If: ΔPCO2/ΔHCO3 =CO2/HCO3ratioThen it is comp.

Acute

Chronic

(2xNa) + (Glu/18) + (BUN/2.8) = calculated serum osmoles

HCO3 loss Extra H+

P(A-a)O2: Test Your Understanding - Answers

• a) PAO2 = .40 (760 - 47) - 1.2 (50) = 225 mm Hg; P(A-a)O2 = 225 - 150 = 75 mm Hg

The P(A-a)O2 is elevated but actually within the expected range for supplemental oxygen at 40%, so the patient may or may not have a defect in gas exchange.

• b) PAO2 = .28 (713) - 1.2 (75) = 200 - 90 = 110 mm Hg; P(A-a)O2 = 110 - 95 = 15 mm Hg

Despite severe hypoventilation, there is no evidence here for lung disease. Hypercapnia is most likely a result of disease elsewhere in the respiratory system, either the central nervous system or chest bellows.

• c) PAO2 = .21 (713) - 1.2 (15) = 150 - 18 = 132 mm Hg; P(A-a)O2 = 132 - 120 = 12 mm Hg

Hyperventilation can easily raise PaO2 above 100 mm Hg when the lungs are normal, as in this case.

d) PAO2 = .80 (713) - 40 = 530 mm Hg (Note that the factor 1.2 is dropped since FIO2 is above 60%)

P(A-a)O2 = 530 - 350 = 180 mm Hg

P(A-a)O2 is increased. Despite a very high PaO2, the lungs are not transferring oxygen normally.

e) PAO2 = .21 (713) - 1.2 (72) = 150 - 86 = 64 mm Hg; P(A-a)O2 = 64 - 80 = -16 mm Hg

A negative P(A-a)O2 is incompatible with life (unless it is a transient unsteady state, such as sudden fall in FIO2 -- not the case here). In this example, negative P(A-a)O2 can be explained by any of the following: incorrect FIO2, incorrect blood gas measurement, or a reporting or transcription error.

P(A-a)O2: Test Your Understanding - Answers

PaCO2 and Alveolar Ventilation: Q & A

1. What is the PaCO2 of a patient with respiratory rate 24/min, tidal volume 300 ml, dead space volume 150 ml, CO2 production 300 ml/min? The patient shows some evidence of respiratory distress.

First, you must calculate the alveolar ventilation.

Since minute ventilation is 24 x 300 or 7.2 L/min, and dead space ventilation is 24 x 150 or 3.6 L/min, alveolar ventilation is 3.6 L/min. Then

300 ml/min x .863 PaCO2 = -----------------------

3.6 L/min

PaCO2 = 71.9 mm Hg

Alveolar Gas EquationPAO2 = PIO2 - 1.2 (PaCO2)

where PIO2 = FIO2 (PB – 47 mm Hg)

Except in a temporary unsteady state, alveolar PO2 (PAO2) is always higher than arterial PO2 (PaO2). As a result, whenever PAO2 decreases, PaO2 also decreases. Thus, from the AG equation:

• If FIO2 and PB are constant, then as PaCO2 increases both PAO2 and PaO2 will decrease (hypercapnia causes hypoxemia).

• If FIO2 decreases and PB and PaCO2 are constant, both PAO2 and PaO2 will decrease (suffocation causes hypoxemia).

• If PB decreases (e.g., with altitude), and PaCO2 and FIO2 are constant, both PAO2 and PaO2 will decrease (mountain climbing leads to hypoxemia).

Alveolar Gas Equation: Q & A

1. What is the PAO2 at sea level in the following circumstances? (Barometric pressure = 760 mm Hg)

a) FIO2 = 1.00, PaCO2 = 30 mm Hgb) FIO2 = .21, PaCO2 = 50 mm Hgc) FIO2 = .40, PaCO2 = 30 mm Hg

To calculate PAO2 the PaCO2 must be subtracted from the PIO2. Again, the barometric pressure is 760 mm Hg since the values are obtained at sea level. In part a, the PaCO2 of 30 mm Hg is not multiplied by 1.2 since the FIO2 is 1.00. In parts b and c, PaCO2 is multiplied by the factor 1.2.

a) PAO2 = 1.00 (713) - 30 = 683 mm Hg b) PAO2 = .21 (713) - 1.2 (50) = 90 mm Hgc) PAO2 = .40 (713) - 1.2 (30) = 249 mm Hg

Ventilation-perfusion Imbalance

• A normal amount of ventilation-perfusion (V-Q) imbalance accounts for the normal P(A-a)O2.

• By far the most common cause of low PaO2 is an abnormal degree of ventilation-perfusion imbalance within the hundreds of millions of alveolar-capillary units. Virtually all lung disease lowers PaO2 via V-Q imbalance, e.g., asthma, pneumonia, atelectasis, pulmonary edema, COPD.

• Diffusion barrier is seldom a major cause of low PaO2 (it can lead to a low PaO2 during exercise).

Respiratory and Renal Compensatory Mechanisms for ACID-BASE

Disturbance

Quantification of Dead space

VD

VVT

=25-40% is normal

In MV pts till 55% is normal

More than 60% is abnormal

dead space

Volume not taking part in gas

exchange=dead space

Effective alveolar ventilation

=MV-VD

PCO2 vs. Alveolar Ventilation

The relationship is shown for metabolic CO2 production rates of 200 ml/min and 300 ml/min (curved lines).

A fixed decrease in VA(x-axis)

in the hypercapnic patient will result in a greater rise in PaCO2

(y-axis) than the same VA change when PaCO2 is low or normal.

This graph also shows that if VA is fixed, an ↑ in CO2 production will result in an ↓ in PaCO2.

Problems: Oxygenation

• Room Air, PaO2 = 45, PaCO2 =30– PAO2 = 150 – 1.2(30) = 114 mm Hg– P(A-a)O2 = 114 - 45 = 69 elevated

• Now on 100% O2, PaO2 = 65, PaCO2= 32– minimal elevation in PaO2– shunt major cause of hypoxemia

Problems: Oxygenation

• Room Air, PaO2 = 45, PaCO2 = 45 – PAO2 = 150 - 1.2(45) = 96– P(A-a)O2 = 96 - 45 = 51

• Now on 100% O2, PaO2 =555, PaCO2 = 48– PAO2 = 1.0(760 - 47) - 1.2(48) = 655– P(A-a)O2 = 655 - 555 = 100– Dramatic increase in PaO2– V/Q mismatch major cause of hypoxemia

Respiratory/Metabolic

abg by a taecher

Technology