Ventilation

Question 1

Why do we use PCO2 to assess ventilation?

Answer 1

Because CO2 is much (20x) more soluble in blood than O2, so measuring CO2 is usually a close approximation of ventilatory efficacy.

Question 2

What is the calculation for minute ventilation?

Answer 2

Minute ventilation = tidal volume x resp rate

Question 3

What is the difference between hypoxia and hypoxemia?

Answer 3

• Hypoxemia - decreased O2 dissolved in the blood (plasma)
• Hypoxia - decreased O2 delivery to the tissues

Question 4

Why do we use PaO2 as the parameter to measure gas exchange and not PCO2?

Answer 4

because issues with oxygen delivery will occur first if gas exchange is not performed adequately

Question 5

Intrapulmonary and intrapleural pressure do what upon expiration vs. inspiration?

Answer 5

• Expiration: both increase
• Inspiration: both decrease (due to contraction of diaphragm and intercostal muscles)

Question 6

What are the 5 causes of hypoxemia? (Ranked from most to least common)

Answer 6

1. Ventilation-perfusion mismatch
2. Low FiO2
3. Hypoventilation
4. Anatomic shunts
5. Diffusion barriers

Question 7

What is the most common cause of hypoxemia in anesthetized patients?

Answer 7

V-Q mismatch

Question 8

What would cause a high V/Q ratio?

Answer 8

If blood flow is decreased; your PaO2 will equal the FiO2 and PaCO2 will be 0 mmHg

• This is called dead space ventilation

Question 9

What would cause a low V/Q ratio?

Answer 9

Ventilation is decreased, typically by atelectasis; PaO2 = PvO2 and PaCO2 = PvCO2

• Physiologic shunt - allow deoxygenated blood to enter systemic circulation and is effectively a R—> L shunt

Question 10

T or F: oxygen insufflation can slow the adverse effects of hypoventilation/apnea

Answer 10

true

Question 11

How do anatomic shunts (such as a reverse PDA) differ from physiologic shunts?

Answer 11

Physiologic shunts return to normal once patient recovers from anesthesia, anatomic shunts will not resolve without anatomic correction

Question 12

What might be a cause for diffusion impairment?

Answer 12

pulmonary fibrosis, due to the thickening of the blood-gas barrier

Question 13

What are the 4 types of hypoxia?

Answer 13

1. Circulatory
2. Anemic
3. Cytotoxic/histotoxic
4. Hypoxemic

Question 14

Describe circulatory hypoxia

Answer 14

Due to cardiac arrest or extremely low cardiac output

• Tissue oxygen delivery cannot occur b/c the cardiac pump/vascular system is not functional/severely compromised

Question 15

Describe anemic hypoxia

Answer 15

tissue oxygen delivery is insufficient b/c of low Hb content (particularly oxyhemoglobin)

Question 16

Describe cytotoxic/histotoxic hypoxia

Answer 16

Certain toxicities, such as cyanide, make the cells unable to take up or use oxygen for normal metabolic processes

Question 17

Describe hypoxemic hypoxia

Answer 17

the patient has decreased tissue oxygen delivery due to the presence of hypoxemia (any of 5 reasons)

Question 18

What are the indications for controlled ventilation?

Answer 18

1. Hypoventilation
2. Intrathoracic surgery/disease
3. Intracranial disease
4. Patients under neuromuscular blockade

Question 19

What is the formula for compliance?

Answer 19

compliance = change in volume/change in pressure

Question 20

Why is it important to ventilate patients with intracranial disease?

Answer 20

B/c increases in CO2 can cause acute cerebral vasodilation —> increased ICP; should ventilate to keep ETCO2 at low normal concentrations

Question 21

What is the difference between assisted and controlled ventilation?

Answer 21

• Assisted: periodic positive pressure breaths; helps alleviate mild hypoventilation and may slow atelectasis
• Controlled: intermittent positive pressure ventilation; may be done using a reservoir bag or a ventilator

Question 22

What value should the peak inspiratory pressure (PIP) never exceed?

Answer 22

20 cmH2O

Question 23

What is the inspiratory:expiratory ratio on a ventilator usually set to?

Answer 23

1:2, because normal ventilation has an expiratory pause

Question 24

How do synchronized intermittent mandatory ventilators (SIMV) differ from your normal one?

Answer 24

The ventilator will give a breath once the patient makes an effort, or if they take too long, it will initiate one on its own

Question 25

How does positive pressure ventilation affect the patient physiologically?

Answer 25

the ventilator is “pushing” gas into the lungs, thus it can actually decrease BP by compressing the heart and impairing cardiac preload during diastole, as well as by compressing the great veins and decreasing venous return * the higher the tidal volume, the more dramatic this effect may be

Question 26

In normal small animal patients, the ETCO2 is approximately how much less than the PaCO2? Why might the difference between them increase?

Answer 26

5 mmHg less; if there is a large amount of dead space

Question 27

What causes a right shift of the oxyhemoglobin dissociation curve (reduced affinity)?

Answer 27

* Increased temp * Increased 2-3 DPG * Increased [H+] (acidemia/acidosis)

Question 28

What causes a left shift of the oxyhemoglobin dissociation curve (increased affinity)?

Answer 28

* Decreased temp * Decreased 2-3 DPG * Decreased [H+] (alkalosis/alkalemia) * CO

Question 29

What are the 5 classifications of ventilators?

Answer 29

1. Power 2. Cycling 3. Major Control Variable 4. Driving Mechanism 5. Bellows Orientation

Question 30

How do MRI-compatible ventilators differ from the norm?

Answer 30

They are typically pneumatically powered, since the magnetic field interferes with most electronics; unlike most ventilators, which are plugged into electronic outlets

Question 31

What type of cycling (when each breath is initiated) are most anesthesia ventilators?

Answer 31

Time-cycled - operator sets time variables that determine the RR

Question 32

Major control variable is what determines when the breath is complete. Most anesthesia ventilators are volume-controlled...what does this mean?

Answer 32

The tidal volume is set and the patient receives that volume regardless of what the pressure is * it is better to start low with the TV to prevent an excessive peak inspiratory pressure

Question 33

One type of ventilator is not volume-controlled, but rather is pressure controlled. Which one is this, and when might you use it?

Answer 33

The Bird ventilator; the patient receives a breath up to a certain pressure * would use in a colic horse

Question 34

What is the driving mechanism of the majority of ventilators?

Answer 34

Pneumatically-driven

Question 35

Which ventilator is the exception to the pneumatically-driven rule?

Answer 35

The Hallowell Taphonius large animal anesthesia machine, which is piston-driven, so there is no need for driving gas in the machine

Question 36

Why are ascending bellows preferred to descending in ventilators?

Answer 36

Because it is easier to detect a leak, as the bellows will fall if there is one present