6. IMPLICATIONS OF ABG
• LUNG FUNCTION
• OXYGENATION, VENTILATION, ACID BASE
STATUS
• EARLY DIAGNOSIS OF ARF
• DIAGNOSIS OF SECONDARY POLYCYTHEMIA
DUE
TO PULMONARY DISEASE
• HEART AND CIRCULATORY FUNCTION
• KIDNEY FUNCTION
• METABOLISM
• THE USE OF SOME MEDICATIONS
• QUALIFY PATIENTS FOR HOME OXYGEN USE
• DETECT EXPOSURE TO CARBON MONOXIDE
AND OTHER
CHEMICALS
12. EXCRETORY RATE OF CO2 = VA x Pa
CO2
Pa CO2 = 1 / VA
VA = ( VT - VD ) x f
INTERPRETATION OF PaCO2
PaCO2 > 45 mmHg = alveolar
hypoventilation = resp acidosis
PaCO2 < 35 mmHg = alveolar
hyperventilation = resp alkalosis
13.
14.
15.
16.
17. PaO2
Interpretation
> 100 mmHg hyperoxemia
90-100 normal
60-80 mild hypoxemia
40-60 moderate
hypoxemia
< 40 severe
hypoxemia
18. Oxygenation and external respiration
Causes of hypoxemia
Low FIO2
Hypoventilation
Diffusion defect
Ventilation perfusion mismatch
Dead space and shunting
Venous admixture
19. Oxygen transport and internal
respiration
Arterial oxygen content
Volume of dissolved oxygen
+
Volume of combined oxygen with
hemoglobin
-------------------------------------
Total oxygen content
--------------------------------------
20. volume of dissolved O2 = PaO2 x CsO2 = 0.3
vol%O2
volume of combined O2 = Hb x SaO2 x 1.34
= 19.7vol%
CaO2 = dissolved O2 + combined O2 = 20
vol%
CaO2 - CvO2 = 20 - 15.2 = 4.8 vol%
O2 consumption = Q x C(a - v) O2 = 250 mL
O2/min
total O2 transport = cardiac output x
CaO2 = 1000mLO2/min
31. Classification of laboratory metabolic
acid-base compensation
Classification BE HCO3
normal metabolic component 0 +/-2
24+/-2
metabolic acidosis < - 2 <
22
metabolic alkalosis > + 2
> 26
32. Stepwise approach to diagnosing acid-base
disorders
• Step1: Acidemic or alkalemic?
• Step2: Is the primary disturbance
respiratory or metabolic?
• Step3: For a respiratory disturbance,
determine whether it is acute or
chronic.
• Step4: For a metabolic acidosis,
determine whether an anion gap is
present.
• Step5: Determine whether other
metabolic disturbances coexist with
an anion gap acidosis.
• Step6: Assess the normal compensation
by the respiratory system for a
34. Step2: Is the primary disturbance
respiratory or metabolic?
A respiratory disturbance alters the arterial
PaCO2 (normal value 40, range 38-42). Go
to step 3.
A metabolic disturbance alters the serum
HCO3 (normal value 24, range 22-26)
• If HCO3 < 22, metabolic acidosis is
present. Go to step 4.
• If HCO3 > 26, metabolic alkalosis is
present, is respiratory compensation
adequate? Go to step 6.
35. Step3: For a respiratory disturbance, determine
whether it is acute or chronic.
• Ac resp acid: pH decrease = 0.08*(PaCO2-
40)/10
• Ch resp acid: pH decrease = 0.03*(PaCO2-
40)/10
• Ac resp alka: pH increase = 0.08*(40 -
PaCO2)/10
• Ch resp alka: pH increase = 0.017*(40 -
PaCO2)/10
36. Step4: For a metabolic acidosis, determine
whether an anion gap is present.
• Anion gap = Na - (Cl + HCO3)
• Anion gap metabolic acidosis, anion gap
> 12
• Normal or non anion gap acidosis, anion
gap </= 12
41. Step5: Determine whether other metabolic
disturbances coexist with an anion gap acidosis.
Corrected HCO3 = measured HCO3 +
(anion gap - 12)
If the corrected HCO3 varies significantly
above or below 24, then a mixed or more
complex metabolic disturbance exists.
To be more specific, if the corrected HCO3
is greater than 24, a metabolic alkalosis
coexists. If the corrected HCO3 is less
than 24 then a non anion gap acidosis
coexists.
42.
43.
44.
45.
46. Step6: Assess the normal compensation by the
respiratory system for a metabolic disturbance.
Winter’s Formula
Expected PaCO2 = (1.5*HCO3) +(8+/-2)
Winter’s Formula does not predict the resp
response to a metabolic alkalosis.
Two general rules
• a pt will increase PaCO2 above 40 but not greater
than 50-55 to compensate for a metabolic
alkalosis.
• a pt will be alkalemic if the PaCO2 is elevated to
compensate for a met alk ( If the patient is
acidemic,PH < 7.38, then an additional resp acid
is present).
47.
48.
49.
50.
51.
52.
53. Steps in evaluation and
classification of acid-base
compensation
• Evaluate for the presence of
compensation.
• Determine the probable primary
problem.
• Classify the degree of
compensation.
54. Alerts to mixed acid-base
disturbances
• If respiratory and metabolic parameters change
proportionately, pH remains unchanged.
• Both parameters are altered in fashion that
changes the pH in the same direction.
• Fails to compensate in the expected manner for
a primary disorder after sufficient time has
elapse.
• A metabolic alkalosis is accompanied by an
increase in the anion gap.
• Absent of compensation.
• Long standing pulmonary or renal disease.
• Excessive compensation.
• Respiratory assistance.
• Settings conducive to mixed disturbances.
• Triple disorders may also be encountered.