AMP 31: Mechanical Ventilation (old)

Question 1

What are the two main types of respiratory failure?

Answer 1

oxygenation failure and ventilation failure

Question 2

What does oxygenation depend on?

Answer 2

surface area available for gas exchange and preservation of the gas exchange barrier

Question 3

What does ventilation depend on?

Answer 3

Fresh gas flow through alveoli removing CO2

Question 4

What are the three main indications for mechanical ventilation?

Answer 4

1. severe hypoxemia despite O2 therapy
2. severe hypoventilation despite therapy
3. excessive respiratory effort

Question 5

What is the definition of “severe hypoxemia”?

Answer 5

cyanosis, PaO2 < 60 mm Hg, or SpO2 < 90%

Question 6

What is the marker of “severe hypoventilation”?

Answer 6

PCO2 > 60 mm Hg

Question 7

What does primarily control PCO2?

Answer 7

minute ventilation

Question 8

What does “minute ventilation” mean and what are its determinants?

Answer 8

amount of fresh gas flow to alveoli in a minute, determined by respiratory rate and tidal volume

Question 9

What are the three main ventilator breath patterns used?

Answer 9

• Assist control ventilation (A/C)
• Synchronized intermittent mandatory ventilation (SIMV)
• Continuous spontaneous ventilation

Question 10

Describe A/C ventialtion

Answer 10

Assist/Control Ventilation

• all breaths are mandatory breaths (generated by the machine)
• minimum respiratory rate is set but patient can trigger additional breaths which are then generated by the machine, i.e., patient can increase the RR but all breaths are full ventilator breaths

Question 11

What determines peak airway pressure in volume controled ventilation?

Answer 11

• tidal volume chosen
• compliance of the respiratory system

Question 12

What determines the tidal volume in pressure controlled ventilation?

Answer 12

• level of airway pressure chosen
• compliance of the respiratory system

Question 13

Describe SIMV

Answer 13

Synchronized Intermittent Mandatory Ventilation

• operator sets the number of full mandatory ventilator breaths
• patient can breathe spontaneously inbetween
• machine will try to synchronize mandatory and respiratory efforts
• generally used for patients with less severe disease than A/C

Question 14

What are the two most common types of ventilator support for continuous spontaneous ventilation? Describe them

Answer 14

continuous positive airway pressure (CPAP)

• ventilator does not deliver breaths
• all breaths are spontaneous breaths/completely patient generated (rate, inspiratory time and TV all determined by patient)

pressure support ventilation (PSV)

• ventilator does not deliver breaths but tidal volume is augmented by the ventilator
• good for patients with adequate respiratory drive but inadequate ventilatory strength
• can be used alone or in conjunction with CPAP or SIMV

Question 15

What is the reported normal tidal volume in dogs and cats?

Answer 15

10-15 ml/kg

Question 16

When placing an animal on a ventilator, what is the maximum tidal volume that should be used initially

Answer 16

do not exceed 10 mL/kg to prevent lung injury from overdistension

Question 17

What are common tidal volumes targeted in animals with severe lung disease?

Answer 17

6-8 mL/kg

Question 18

When placing an animal on a ventilator what is a desirable initial peak airway pressure?

Answer 18

10-15 mm Hg

Question 19

When placing an animal on a ventilator, what are reasonable trigger values to start with?

Answer 19

• an airway pressure drop of - 2 mm Hg
• a gas flow change of 2 L/min

Question 20

What are the benefits of PEEP?

Answer 20

• improves oxygenation
• increased recruitment of collapsed alveoli
• may reduce ventilator-induced lung injury

Question 21

What is the minimum PEEP set in most ventilator settings?

Answer 21

2 mm Hg to prevent atalectasis

Question 22

Explain “breath stacking”. What is recommended to avoid this?

Answer 22

As respiratory rate increases, expiratory time is sacrificed to fit in the amount of breaths per minute. The animal is then not able to fully exhale before the start of the next inspiration, which creates an intrinsive positive end-expiratory pressure.

To avoid this an I:E ratio of at least 1:1 or higher is recommended. Common starting point is 1:2. If the respiratory rate is increase, the inspiratory time may need to be decreased to accomplish sufficient I:E ratio.

Question 23

Fill in the blanks for initial ventilator settings, assuming the patient has normal lungs

Answer 23

Question 24

Fill in the blanks for initial ventilator settings assuming the patient has lung disease

Answer 24

Question 25

Explain how and why inspiratory pressure and tidal volumes are adjusted in an animal with lung disease?

Answer 25

* the inspiratory pressure needs to be higher due to decreased lung compliance, necessitating higher pressures for achieving same tidal volume for adeqaute ventilation * the tidal volume needs to be lower since excessive distension is a major reason for ventilator-induced lung injury.

Question 26

After connecting a patient to the ventilator, what are the first steps to assess safe ventilation?

Answer 26

* observe patient's chest rise and whether it is appropriate * auscultate both sides of the chest to make sure both sides of the lung are ventilated * evaluate all monitoring available * perform arterial blood gas once stable, or alternatively venous blood gas plus SpO2

Question 27

What are the PaO2, SPO2 and PCO2 goals during ventilations?

Answer 27

* PaO2 80-1200 mm Hg * SPO2 \> 95% * PCO2 35-55 mm Hg (35-40 in patients with brain disease)

Question 28

How do you adjust the settings if a patient has a too high PaO2 (\>120)?

Answer 28

1. decrease FiO2 until \< 60% 2. once FiO2 \< 60% start decreasing PEEP and peak airway pressure

Question 29

How do you adjust the ventilator settings if the patient's PaO2 is too low (\< 80 mm Hg)?

Answer 29

1. Increase the FiO2, increase to 100% if the PaO2/SPO2 drop is acute and/or severe. 2. Increase PEEP and peak airway pressures to improve pulmonary oxygenation efficiency.

Question 30

How do you adjust the ventilator settings in a patient with too high or too low PCO2?

Answer 30

* too high PCO2 → increase tidal volume and/or RR * too low PCO2 → decrease tidal volume and/or RR

Question 31

How does severe lung disease indirectly "protect" animals from ventilator-associated cardiovascular compromise?

Answer 31

animals with severe lung disease have such "stiff" lungs (decreased compliance) that usually only little of the positive pressure applied transmits to the cardiovascular system

Question 32

What is the reported prevalance of VAP (ventilator associated pneumonia) in people?

Answer 32

10 - 48 %

Question 33

What consitutes VAP (ventilator associated pneumonia)?

Answer 33

VAP is recognized when new pulmonary infiltrates develop ≥48 hours after beginning ventilation.

Question 34

What are strategies that can be applied to reduce occurence of VAP?

Answer 34

* oral hygiene and frequent chlorhexidine mouth rinse * using sterile technique when handling circuit, endotracheal, or tracheostomy tube * reduce number of times circuit is being disconnected * replace ventilator circuits only every 5 days * personell to wear gloves and freuquently wash their hands

Question 35

How frequently should the ventilator circuit be replaced?

Answer 35

Only every 5 days earlier though if gross contanimation is observed

Question 36

How can ventilator-induced lung injury (VILI) contribute to MODS?

Answer 36

pulmonary inflammation from VILI ⇒ inflammatory mediators ⇒ enter systemic circulation ⇒ contribute to systemic inflammatory response ⇒ organ damage

Question 37

What are the 2 major mechanisms by which VILI occurs?

Answer 37

* Overextension of the lungs by excessive tidal volumes * repetitive alveolar collapse from inadequate PEEP

Question 38

When the blood gas goals cannot be achieved even with use of higher airway pressures or PEEP, what is a reasonable next step?

Answer 38

Change the patient's blood gas goals, e.g., PaO2 \> 60 mm Hg, PCO2 60-70 mm Hg

Question 39

What are the general recommendations to avoid oxygen toxicity?

Answer 39

* FiO2 of 100% should not be adminsitered for longer than 12-24 hours * For long term ventilation FiO2 should be \< 60%

Question 40

What is thought to be the major cause for ventilator induced penumothorax, and what can be applied to avoid this?

Answer 40

overdistention of the lungs, avoid high tidal volumes.

Question 41

How would airway pressure and tidal volume each change respectively in a patient developing a pneumothorax under volume- or pressure-controlled ventilation

Answer 41

volume controlled ventilation ⇒ pressure would increase pressure controlled ventilation ⇒ tidal volume would decrease

Question 42

What are most common causes for an acute/rapid decline in oxygenation?

Answer 42

* pneumothorax * machine malfunction * disconnection of circuit * loss of O2 supply

Question 43

What are the most common causes for a gradual decline in oxygenation?

Answer 43

* progressing pneumonia * ARDS * VILI

Question 44

List possible causes for hypercapnia in the ventilated patient:

Answer 44

* increased apparatus dead space (too much tubing) * ET tube kink or obstruction * incorrect circuit assembly (leaks, exhalation obstruction, etc.) * pneumothorax * increased pulmonary dead space (e.g., overdistension of alveoli or pulmonary embolism) * inadequate tidal volumes or respiratory rate * insufficient respiratory time

Question 45

List possible causes for patient-ventilator-dsysynchrony

Answer 45

* Hypoxemia * Hypercapnia * Pneumothorax * Hyperthermia * inappropriate ventilator settings * inadequate anesthesia depth

Question 46

What are the physiologic goals a patient must achieve before disconnecting the ventilator?

Answer 46

* the original disease process is stable or improving * the animal has a normal respiratory drive * the animal does no longer require ventilator support for adequate gas exchange * patient is oxygenating adequately (PaO2 to FiO2 ratio of at least 150 to 200)

Question 47

What are common reasons to not wean an animal from the ventilator?

Answer 47

* requires \> 60% FiO2 to oxygenate appropriately * cardiovascular instability * high peak inspired airway pressure (\>25 cm H2O) needed * high PEEP levels (\>5 cm H2O) needed

Question 48

During a spontaneous breathing trial, what are indications the trial has failed?

Answer 48

* hypoxemia * hypercapnia * hyperthermia * tachycardia * hypotension * tachypnea