Respiratory Gas Exchange Monitors

Question 1

What is the primary method to confirm bilateral lung ventilation?

Answer 1

Chest auscultation.

Even if end tidal CO2 detection is the primary mechanism to exclude esophageal intubation.

Question 2

Precordial Stehoscope (Wenger Chestpiece)

Answer 2

is a heavy, bell shaped piece of metal placed over the chest or suprasternal notch.

The child size works well for mot patients.

The bell is connected to the anesthesiologist by extension tubing.

Question 3

Esophageal Stehoscope

Answer 3

is a soft plastic catheter with balloon-covered distal openings.

quality of of breath and heart sounds is much better than with precordial.

limited to intubated patients

Question 4

Pulse Oximeter Principles

Answer 4

combine oximetry and plethysmography to non invasively measure oxygen saturation in arterial blood.

Question 5

Lambert-Beer Law

Answer 5

oxygenated and reduced hemoglobin differ in their absorption of red and infrared light

Question 6

Oxyhemoglobin (HbO2) absorption

Answer 6

absorbs more INFRARED light (940 nm)

Question 7

Deoxyhemoglobin absorption

Answer 7

absorbs more RED light (660 nm) and thus appears BLUE

Question 8

Understanding the pulse ox readings

Answer 8

the greater the ratio of red/infrared absorption, the LOWER the arterial saturation

Question 9

Plethysmography

Answer 9

arterial pulsations are identified by this, allowing corrections for light absorption by non pulsating venous blood and tissue

Question 10

Carbon Monoxide Poison

Answer 10

because carboxyhemoglobin (COHb) and (oxyhemoglobin) HbO2 absorb light at 660 nm identically, pulse oximeters that compare only two wavelengths of light will register falsely HIGH reading in patients with carbon monoxide poison.

Question 11

Methemoglobinemia

Answer 11

Methemoglobin has the SAME absorption coefficient at both red and infrared wavelengths.

The resulting 1:1 absorption ratio corresponds to a saturation reading of 85%.

Thus, methemoglobinemia cause a FALSELY LOW saturation reading when SaO2 is actually greater than 85% and a falsely high reading if SaO2 is actually less than 85%

Question 12

Low SpO2 readings

Answer 12

most pulse oximeters are INACCURATE at LOW SpO2, and all demonstrate a delay between changes in SaO2 and SpO2

Question 13

Pulse Oximetry Artifacts

Answer 13

excessive ambient light, motion, methylene blue dye, venous pulsations in dependent limb, low perfusion (low CO, profound anemia, hypothermia, increased SVR), malpositioned sensor, leakage of light from light emitting diode to the photodiode, bypassing the arterial bed (optical shunting)

Question 14

Capnography

Answer 14

valuable monitor of the pulmonary, cardiovascular, and anesthetic breathing systems

Question 15

How do Capnographs work?

Answer 15

rely on the absorption of infrared light by CO2.

As with oximetry, absorption of infrared light by CO2 is governed by the Beer-Lambert Law

Question 16

What do capnographs indicate?

Answer 16

They rapidly and reliably indicate ESOPHAGEAL intubation.

The do not reliably detect bronchial intubation.

Air from the stomach may have CO2, but it should be washed out within a few breaths.

Question 17

Malignant Hyperthermia and End Tidal CO2

Answer 17

the increased metabolic rate caused by MH causes a marker rise in ETCO2

Question 18

Alveolar Dead Space

Answer 18

alveoli that are VENTILATED but NOT perfused

any significant reduction in lung perfusion: air embolism, decreased CO, or decreased BP increases alveolar dead space, dilutes expired CO2, and lessens ETCO2.