Mechanical Ventilation

Question 1

What is happening physiologically during inspiration?

Answer 1

• Contraction of diaphragm and intercostal mm +/- abd mm
• Enlargement of chest cavity > drop in pleural pressure = drop in alveolar pressure
• Air moves from atmosphere to alveoli

Question 2

What is happening physiologically during expiration?

Answer 2

• Contraction of resp mm ceases
• Elastic recoil of chest wall and lungs incr alveolar pressure over the atm pressure
• Air moves from alveoli to atm

Question 3

During spontaneous ventilation, as transpulmonary pressure and alveolar pressure drop, the alveolar volume ______

Answer 3

Increases

Question 4

Describe the work of breathing

Answer 4

• Energy required by resp mm to produce an inspiration - under normal conditions expiration is passive (No WOB)

Question 5

Work of breathing is needed to do what 3 things?

Answer 5

1. To expand lungs against elastic forces
2. To overcome the viscosity of the lung and chest wall structures
3. To overcome airway resistance

Question 6

Airway resistance has to do with what?

Answer 6

Atm P - Alveolar P

Volume of airflow

Question 7

Define tidal volume

Answer 7

Volume of air inspired or expired with each normal breath; 10-20 ml/kg

Question 8

Define minute ventilation (Vm)

Answer 8

Total amount of new air moved into the resp passages each minute

Vm = TV x RR

Question 9

Define alveolar minute ventilation (Va)

Answer 9

Total volume of new air entering the alveoli each minute

Va = RR x (TV - anatomical dead space volume)

Question 10

What are the normal FiO2 and FiN2 of atmospheric air?

Answer 10

FiO2 = 21% (159 mmHg)

FiN2 = 78% (590 mmHg)

Question 11

What is the PaCO2 and PaO2 (arterial) during normal alveolar ventilation? PvCO2 and PvO2?

Answer 11

PaCO2 = 40 mmHg; PaO2 =100 mmHg

PvCO2 = 50 mmHg; PvO2 = 40 mmHg

Question 12

What 4 things cause CNS depression and affect spontaneous ventilation? What is the effect of these on spontaneous ventilation?

Answer 12

general anesthesia, sedatives, opioids, CNS dz; decr alveolar ventilation and reduce central drive

Question 13

What types of thoracic abnormalities affect spontaneous ventilation?

Answer 13

Open chest, pneumothorax, pleural effusion, external pressure on the chest, obesity

Question 14

What things cause increased intra-abdominal pressure that affect spontaneous ventilation? What is the effect of these on spontaneous ventilation?

Answer 14

Pregnancy, GDV, abdominal fluids, large abd masses, pneumoperitoneum, obesity; decr alveolar ventilation, reduce compliance

Question 15

Define compliance

Answer 15

Measure of lung’s ability to stretch and expand; is the change in the volume for any given applied pressure

Question 16

What are the two main effects on spontaneous ventilation we are concerned about with hypoventilation?

Answer 16

Hypoxemia (unless high FiO2 is provided) and hypercapnea

Question 17

What are four causes of hypoxemia?

Answer 17

1. Hypoventilation - inability of resp system to maintain a normal alveolar ventilation = CO2 not eliminated adequately
2. Diffusion limitation
3. Shunt
4. Venilation-perfusion inequality

*plus low FiO2

Question 18

What are situations where ventilatory support is mandatory?

Answer 18

• Open chest sx
• Use of neuromuscular blocker agents
• Resp arrest
• Lung dz where normoxemia is not maintained by supplementing O2
• Hypercapnia
• Patients that cannot tolerate incr in CO2 (ie. brain tumors)

Question 19

What are situations where ventilatory support is highly recommended?

Answer 19

• Low lung/chest compliance
• Obese
• GDV
• Pregnant
• Horses
• Laparoscopic sx

*these animals are prone to severe hypoventilation therefore ventilatory support can become mandatory!

Question 20

What are situations where ventilatory support is beneficial?

Answer 20

Dorsal recumbency

Any patient under general anesthesia

Question 21

During positive pressure ventilation, positive pressure is generated in the breathing system producing movement of air into the alveoli. Therefore, during inspiration the alveolar pressure is _______ compared to the atmospheric pressure (as opposed to spontaneous ventilation).

Answer 21

Positive

Question 22

What is a demand valve?

Answer 22

Provides high flow of oxygen (up to 160 L/min); used in large animals before connecting to anesthetic machine or in the process of weaning from the ventilator

Question 23

What is the name of the bag used for manual positive pressure ventilation in small animals?

Answer 23

Ambu bag

Question 24

How does mechanical positive pressure ventilation differ from manual?

Answer 24

The bag is replaced by a bellow in a jar attached to a ventilator controller with high pressure oxygen flowing through

*often use O2 over room air in case there’s a hole in the bellow/jar

Question 25

Describe the concept of assisted ventilation?

Answer 25

* To reduce the work of breathing * Patient is able to initiate breath * Tidal volume is enhanced by manual or mechanical support

Question 26

What are 2 examples of assisted ventilation modalities?

Answer 26

Synchronized intermittent-mandatory ventilation (SIMV) Pressure Support Ventilation

Question 27

Describe controlled ventilation

Answer 27

* Complete control of ventilation * Operator sets tidal volume and respiratory rate * Also known as **IPPV (intermittent positive pressure ventilation)**

Question 28

What are 2 controlled ventilation modalities?

Answer 28

Volume controlled ventilation Pressure controlled ventilation

Question 29

Describe the process of volume controlled ventilation

Answer 29

* The **fixed delivered TV** will generate a certain airway pressure (normal = 10-20 cmH2O) = peak inspiration pressure (PIP) * Airway pressure will **depend on compliance of resp system** * pressure is limited - safety feature of ventilators to avoid barotrauma (\<30 cmH2O)

Question 30

Describe the concept of pressure controlled ventilation

Answer 30

* The **fixed PIP is set** at 10-20 cmH2O and the ventilator will deliver the TV reqiuired to generate the pre-set PIP * TV will **depend on the compliance** of the resp system * If pulmonary compliance is reduced, the TV will be reduced, resulting in a reduced alveolar ventilation

Question 31

Define positive end expiratory pressure (PEEP)

Answer 31

PEEP maintains a positive pressure during expiration to avoid collapse of the poorly ventilated alveoli Usually kept at 5-10 cmH2O Reduces venous return and can have a profound CV impact on hypovolemic BP

Question 32

What are you monitoring for during controlled ventilation?

Answer 32

1. Airway pressure: * avoid barotrauma, ideally PIP \< 20 cmH2O * avoid volutrauma, ideally TV \< 20 cmH2O, check PIP and compliance 2. End tidal CO2: * TV, PIP, RR and I:E adjusted to maintain normocapniahigh; CO2 = hypoventilation - low CO2 = hyperventilation 3. Compliance

Question 33

Describe the dual chamber ventilator

Answer 33

* Driving gas compresses bellow * Driving gas: compressed air or O2 * **Most common in vet med** * Ascending or descending bellow

Question 34

Describe a piston ventilator

Answer 34

* More sophisticated * Advanced methods of ventilation * Very precise * Does not require driving gas

Question 35

What are the physiological consequences of positive pressure ventilation on the CV system?

Answer 35

High and sustained intrathoracic pressure \> decr venous return \> hypotension - esp hypovolemic patients, close monitoring CV function after initiating IPPV

Question 36

What are the physiological consequences of positive pressure ventilation on the resp system?

Answer 36

High intrathoracic pressure and volume \> risk of barotrauma \> always monitor airway pressure, if thoracic cavity is open, compliance is sig incr \> TV and PIP should be adjusted to avoid overdistension Risk of pneumothorax - worsening previous pneumothorax

Question 37

Patient-Ventilatory Asynchrony occurs when?

Answer 37

The timing of the ventilator cycle is not simultaneous with the timing of the patient’s resp cycle

Question 38

What are some causes of patient-ventilator asynchrony?

Answer 38

Light anesthetic plane Nociception Hypercapnia Hypoxemia Hyperthermia

Question 39

Describe how positive pressure ventilation causes diaphragmatic hernia?

Answer 39

* In chronic cases \> re-expanding the lung can lead to: * Repercussion injury * Acute resp distress syndrome * Use low TV and high RR to try to maintain a normal minute ventilation * Permissive hypercapnia (ETCO2 50-55mmHg)

Question 40

Define atelectasis and what are some causes and consequences?

Answer 40

Collapse or closure of a lung resulting in reduced or absent gas exchange Causes: compression, absorption, decr surfactant Consequences: V/Q mismatch, hypoxemia, risk of post-operative pulm infection

Question 41

How do you deal atelectasis?

Answer 41

* Recruiting maneuvers (artificial sigh) * check CV fxn b/c CV impact may be profound esp in hypovolemic patients * Add PEEP after recruiting - prevents re-collapse of alveoli maintaining a positive pressure during expiration