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Blood Gas Analysis, Lecture notes of Biomedical Engineering

Describes Instrumentation in Blood Gas Analysis

Typology: Lecture notes

2015/2016

Uploaded on 06/14/2016

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Author(s): John G. Younger, M.D., 2009
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Download Blood Gas Analysis and more Lecture notes Biomedical Engineering in PDF only on Docsity!

Author(s): John G. Younger, M.D., 2009

License: Unless otherwise noted, this material is made available under the terms of the

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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