Download Blood Gas Analysis and more Lecture notes Biomedical Engineering in PDF only on Docsity!
Author(s): John G. Younger, M.D., 2009
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Things You Can Know Without Performing
a Blood Gas Analysis
- (^) Historical
- (^) Is the patient having difficulty breathing?
- (^) Is the patient having a change in symptoms over time?
- Physical
- (^) Is the patient working to breathe?
- Is the patient wheezing?
- (^) Does the patient have rales?
- (^) Noninvasive Measurement
- Is the patient sufficiently oxygenated (SaO 2 )?
Pulse Oximetry ‘The 5th Vital Sign’
- (^) Non-invasive
- (^) Instantaneous
- (^) Ubiquitous
- (^) SaO 2 < 95% the usual cutoff for normal versus ‘abnormal’
- (^) Limitations:
- (^) Patient must have pulse
- (^) Detects only significant decreases in PO 2
- Does not comment on content (^) Roger W. Stevens (Wikipedia)
Most Dyspneic Patients Don’t Require ABG
Analysis
- (^) When cause of dyspnea is established
- (^) Asthma, CHF, restrictive lung disease, etc.
- (^) When cause of dyspnea is suspected and patient is not especially ill
- (^) E.g., a child with new-onset wheezing in January
- (^) When dyspnea is so severe as to warrant immediate mechanical ventilation
- (^) The decision to intubate and mechanically ventilate is almost always one based on clinical, not laboratory, grounds
Limitations of ABGs
- (^) ABGs measure gas partial pressures (tensions)
- Remember: PO 2 is not the same as content! A severely anemic patient
may have an oxygen content reduced by half while maintaining perfectly
acceptable gas exchange and therefore maintaining pO 2
- (^) Technical issues
- (^) They hurt
- (^) Sampling from a vein by mistake
- (^) Finding an arterial pulse can be difficult in very hypotensive patients
- (^) Complications such as arterial thrombosis are possible, but awfully rare
ABGs: What You Get
- Arterial PO 2
- (^) Arterial PCO 2
- (^) Arterial pH
- (^) Some electrolytes (e.g., Na+, K+, Ca++)
- (^) Lactate
• [HCO 3 - ]
- SaO 2
- (^) Other assorted calculated results
Measured.
The real
meat of the
sample
An Organized Approach to ABG
Interpretation
- (^) Determining oxygenation abnormalities
- (^) Determining acid-base status and evaluating adequacy of ventilation
Oxygenation: Two Key Questions Addressed with an ABG
Is the patient hypoxic?
Is the hypoxia due to:
Hypoventilation One of the 3 other causes Or a combination of both
Importantly, an ABG alone cannot differentiate diffusion block, V/Q inequality, and shunt!
Looking at the PO 2 versus Calculating the A-a gradient
- (^) In a comfortable patient breathing room air, glancing at the PO 2 will
allow a cursory interpretation of oxygenation
- (^) However, most ABGs are performed in sick patients
- (^) Supplemental oxygen may be present
- (^) Importantly, the patient may be compensating for an oxygenation defect
by hyperventilating, hiding the abnormal oxygenation
Why do details like RQ matter?
- (^) ABGs occasionally used as dichotomous results, prompting changes in management
Possible Pulmonary Embolism
A-a Gradient Normal A-a Gradient Abnormal
No Worries Further Work-up
Evaluating Oxygenation with ABGs
Check Aa Gradient
No Yes
Is the patient hypoxic?
Hypoventilation
Normal Elevated
Check Aa Gradient
No defect Compensated Defect. i.e., patient is hyper ventilating or on supplemental O 2
Other Defect
Normal
Elevated
No Yes
Evaluation of Acid-Base Status:
Is the patient acidemic or alkalemic?
What is the pH?
< 7.38 >7.
Acidemic Alkalemic
Evaluation of Acid-Base Status: Is the disorder respiratory or
metabolic?
If acidemic (pH < 7.38)
What is the PCO 2?
40 mmHg < 40 mmHg
Respiratory acidosis Metabolic acidosis
