Pulmonology

Question 1

EVALUATION OF PULMONARY GAS EXCHANGE

Question 2

Pulse Oximetry

Answer 2

Noninvasive and indirect measurement of arterial O2 saturation (SaO2) estimated by light absorption characteristics of oxygenated and deoxygenated hemoglobin in peripheral blood.

2. Limitations:

a. Measures oxygen saturation, not O2 delivery to tissues

b. Insensitive to hyperoxia. See eFig. 25.1 for oxyhemoglobin dissociation curve.

c. Artificially increased by: Carboxyhemoglobin levels >1% to 2%
d. Artificially decreased by: Intravenous dyes, opaque nail polish, and methemoglobin levels>1%

e. Unreliable when pulse signal is poor: Hypothermia, hypovolemia,
shock, edema, movement artifact

Question 3

Capnography

Answer 3

1. Measures CO2 concentration of expired gas by infrared or mass spectroscopy
2. End-tidal CO2 (ETCO2) correlates with PaCO2 (usually within 5 mmHg in healthy subjects).
3. Used to evaluate proper placement of an endotracheal tube, to monitor ventilation in mechanically ventilated patients, to assess effectiveness of CPR, and during polysomnography

Question 4

Blood gases

Answer 4

1. Arterial blood gas (ABG): Most accurate way to assess oxygenation (PaO2), ventilation (PaCO2), and acid–base status (pH and HCO3 ). See Chapter 28 for normal mean values.
2. Venous blood gas (VBG): PvCO2 averages 6 to 8 mmHg higher than PaCO2; venous pH is slightly lower than arterial pH.
3. Capillary blood gas (CBG): Correlation with ABG is generally best for pH, moderate for PCO2, and worst for PO2.

Question 5

Analysis of Acid–Base Disturbances

Answer 5

Determine primary disturbance (metabolic versus respiratory), then assess for mixed disorder by calculating expected compensatory response.

Question 6

What does pulse oximetry measure?

Answer 6

Arterial oxygen saturation (Sa02)

Question 7

How is arterial oxygen saturation estimated in pulse oximetry?

Answer 7

By analyzing the light absorption characteristics of oxygenated and
deoxygenated hemoglobin in peripheral blood

Question 8

What are the limitations of pulse oximetry?

Answer 8

a. It measures oxygen saturation, not oxygen delivery to tissues.
b. It is insensitive to hyperoxia (high oxygen levels).
c. It can be artificially increased by carboxyhemoglobin levels
>1% to 2%.
d. It can be artificially decreased by intravenous dyes, opaque nail polish, and methemoglobin levels
>1%.
e. It becomes unreliable when the pulse signal is poor due to factors like hypothermia, hypovolemia, shock, edema, or movement artifact.