Ventilation Flashcards
Why do we use PCO2 to assess ventilation?
Because CO2 is much (20x) more soluble in blood than O2, so measuring CO2 is usually a close approximation of ventilatory efficacy.
What is the calculation for minute ventilation?
Minute ventilation = tidal volume x resp rate
What is the difference between hypoxia and hypoxemia?
- Hypoxemia - decreased O2 dissolved in the blood (plasma)
- Hypoxia - decreased O2 delivery to the tissues
Why do we use PaO2 as the parameter to measure gas exchange and not PCO2?
because issues with oxygen delivery will occur first if gas exchange is not performed adequately
Intrapulmonary and intrapleural pressure do what upon expiration vs. inspiration?
- Expiration: both increase
- Inspiration: both decrease (due to contraction of diaphragm and intercostal muscles)
What are the 5 causes of hypoxemia? (Ranked from most to least common)
- Ventilation-perfusion mismatch
- Low FiO2
- Hypoventilation
- Anatomic shunts
- Diffusion barriers
What is the most common cause of hypoxemia in anesthetized patients?
V-Q mismatch
What would cause a high V/Q ratio?
If blood flow is decreased; your PaO2 will equal the FiO2 and PaCO2 will be 0 mmHg
- This is called dead space ventilation
What would cause a low V/Q ratio?
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
T or F: oxygen insufflation can slow the adverse effects of hypoventilation/apnea
true
How do anatomic shunts (such as a reverse PDA) differ from physiologic shunts?
Physiologic shunts return to normal once patient recovers from anesthesia, anatomic shunts will not resolve without anatomic correction
What might be a cause for diffusion impairment?
pulmonary fibrosis, due to the thickening of the blood-gas barrier
What are the 4 types of hypoxia?
- Circulatory
- Anemic
- Cytotoxic/histotoxic
- Hypoxemic
Describe circulatory hypoxia
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
Describe anemic hypoxia
tissue oxygen delivery is insufficient b/c of low Hb content (particularly oxyhemoglobin)
Describe cytotoxic/histotoxic hypoxia
Certain toxicities, such as cyanide, make the cells unable to take up or use oxygen for normal metabolic processes
Describe hypoxemic hypoxia
the patient has decreased tissue oxygen delivery due to the presence of hypoxemia (any of 5 reasons)
What are the indications for controlled ventilation?
- Hypoventilation
- Intrathoracic surgery/disease
- Intracranial disease
- Patients under neuromuscular blockade
What is the formula for compliance?
compliance = change in volume/change in pressure
Why is it important to ventilate patients with intracranial disease?
B/c increases in CO2 can cause acute cerebral vasodilation —> increased ICP; should ventilate to keep ETCO2 at low normal concentrations
What is the difference between assisted and controlled ventilation?
- 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
What value should the peak inspiratory pressure (PIP) never exceed?
20 cmH2O
What is the inspiratory:expiratory ratio on a ventilator usually set to?
1:2, because normal ventilation has an expiratory pause
How do synchronized intermittent mandatory ventilators (SIMV) differ from your normal one?
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
How does positive pressure ventilation affect the patient physiologically?
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
In normal small animal patients, the ETCO2 is approximately how much less than the PaCO2? Why might the difference between them increase?
5 mmHg less; if there is a large amount of dead space
What causes a right shift of the oxyhemoglobin dissociation curve (reduced affinity)?
- Increased temp
- Increased 2-3 DPG
- Increased [H+] (acidemia/acidosis)
What causes a left shift of the oxyhemoglobin dissociation curve (increased affinity)?
- Decreased temp
- Decreased 2-3 DPG
- Decreased [H+] (alkalosis/alkalemia)
- CO
What are the 5 classifications of ventilators?
- Power
- Cycling
- Major Control Variable
- Driving Mechanism
- Bellows Orientation
How do MRI-compatible ventilators differ from the norm?
They are typically pneumatically powered, since the magnetic field interferes with most electronics; unlike most ventilators, which are plugged into electronic outlets
What type of cycling (when each breath is initiated) are most anesthesia ventilators?
Time-cycled - operator sets time variables that determine the RR
Major control variable is what determines when the breath is complete. Most anesthesia ventilators are volume-controlled…what does this mean?
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
One type of ventilator is not volume-controlled, but rather is pressure controlled. Which one is this, and when might you use it?
The Bird ventilator; the patient receives a breath up to a certain pressure
- would use in a colic horse
What is the driving mechanism of the majority of ventilators?
Pneumatically-driven
Which ventilator is the exception to the pneumatically-driven rule?
The Hallowell Taphonius large animal anesthesia machine, which is piston-driven, so there is no need for driving gas in the machine
Why are ascending bellows preferred to descending in ventilators?
Because it is easier to detect a leak, as the bellows will fall if there is one present
