2 Mechanical Ventilation, part 2 (Troubleshooting)

Question 1

Assist control (AC) is also known as

Answer 1

Continuous Mandatory Ventilation (CMV)

Question 2

One approach in adjusting settings in mech vent

Answer 2

BUR, VT , and IFR will affect CO2.
FIO2 and PEEP will affect O2

Question 3

The more efficient way of increasing ventilation.

Answer 3

Increasing tidal volume.
Because increasing RR will also result in increased dead space (in every breath, there’s about 150cc of dead space)

Question 4

Alveolar ventilation can be defined as

Answer 4

Alveolar ventilation = RR x (TV - dead space)

Question 5

Benefit of increased PEEP in heart failure

Answer 5

May decrease venous return, resulting in decreased burden on the right atrium

PEEP also opens up collapsed alveoli

Question 6

Remarks on ARDS

Answer 6

It is a restrictive lung pathology, where the compliance is low.
This may result in increased pressures.

Question 7

Remarks on COPD

Answer 7

The compliance in COPD is actually high.
Also, patients are at risk of gas trapping, so make sure the expiratory flow limb goes back to zero before the next breath. This can be facilitated by decreasing the RR up to RR 10.
- if patient is hypotensive, disconnect the patient from the ventilator to allow the patient to exhale the air out.

Question 8

Formula for airway pressure

Answer 8

Total airway pressure = Flow x Resistance + Alveolar pressure

Question 9

Problems causing high airway pressures can be most easily be distinguished to be coming from the ventilator and circuit or from the endotracheal tube or patient by

Answer 9

disconnecting the patient from the ventilator and manually ventilating the patient

Question 10

Formula for alveolar pressure

Answer 10

alveolar pressure = (volume/compliance) + PEEP

Question 11

Remarks on alveolar pressures

Answer 11

1. It is the alveolar pressures and NOT the airway pressure which is important in terms of lung damage and hemodynamic effects
2. Alveolar pressure is represented by the plateau pressure, which should be <30 cm H20

Question 12

Remarks on high alveolar pressures

Answer 12

1. alveolar pressure = (volume/compliance) + PEEP
   Therefore, a high alveolar pressure is due to an excessive tidal volume, gas trapping, PEEP, or low compliance.
2. Excessive alveolar pressures may cause acute lung injury and air leaks (e.g., pneumothorax, pneumomediastinum)
3. Most ventilators terminate inspiration if the airway pressure reaches the set upper pressure limit. As this usually occurs relatively early in inspiration, it results in the patient receiving a much lower tidal volume.

Question 13

Machine and tubing problems that can cause high airway pressure in volume preset modes (and low tidal volume in pressure preset modes)

Answer 13

VENTILATOR
Inappropriate settings
Ventilator malfunction
CIRCUIT
Kinking
Pooling of condensed water vapor
Wet filters causing ↑ resistance
ENDOTRACHEAL TUBE
Kinking
Obstructed with sputum, blood, etc
Endobronchial intubation

Question 14

Patient problems that can cause high airway pressure in volume preset modes (and low tidal volume in pressure preset modes)

Answer 14

PATIENT
Bronchospasm
↓ lung compliance (e.g., pulmonary edema, consolidation, collapse)
↓ pleural compliance (e.g., pneumothorax)
↓ chest wall compliance (e.g., abdominal distention)
Patient-ventilator dysynchrony
Coughing

Question 15

In constant pressure modes, the most useful information is obtained from the

Answer 15

flow waveform

Question 16

Hypotension occurring immediately after the initiation of positive pressure ventilation is usually due to:

Answer 16

1. Hypovolemia, exacerbated by reduced venous return due to positive intrathoracic pressure
2. Drugs used for induction. Almost all anesthetic induction agents cause vasodilation and myocardial depression
3. Gas trapping due to over-enthusiastic ventilation
4. Loss of sympathetic tone as a result of the use of hypnotic/sedative agents

Hypovolemia and drug-induced hypotension are by far the most common so the appropriate initial response is usually to give fluid

Question 17

What to do when there’s desaturation

Answer 17

[Assess patient first]
1. Set FiO2 = 1.0
2. Assess chest movement
3. If there is chest movement, examine patient for problems such as endobronchial intubation, pneumothorax, collapse, pulmonary edema, bronchospasm, pulmonary embolism
4. If chest is not moving, manually ventilate the patient.

Question 18

Whenever a problem occurs (such as dysynchrony), always _______

Answer 18

clinically assess the patient first

Question 19

Dysynchrony can be classified based on

Answer 19

Its occurrence in each phase of the breath:
1. Expiratory-inpsiratory cycling (triggering)
2. Inspiration
3. Inspiratory-expiratory cycling

Question 20

Interpret the waveform

Answer 20

Pressure waveform in volume control mode
High peak pressures with low plateau pressures.
Indicative of airway problem

Question 21

Interpret the waveform

Answer 21

Pressure waveform in volume control mode
High plateau pressure in relation to the peak pressure.
Indicative of reduced pulmonary compliance (e.g., ARDS)

Question 22

Interpret the waveform

Answer 22

Flow waveform in pressure-control mode
Slow decay in the dotted inspiratory flow which does not reach zero by end of inspiratory time
Decay in the dotted expiratory flow is linear and does not reach zero by end of expiratory time
Suggests increased resistance

Question 23

Interpret the waveform

Answer 23

Flow waveform in pressure-control mode
Rapid decay in both the dotted inspiratory and expiratory flow.
Both reach baseline before the end of inspiratory and expiratory times respectively
Suggestive of decreased compliance

Question 24

Interpret the waveform

Answer 24

Flow waveform in volume control mode
The dotted expiratory flow limb does not go back to zero.
Indicative of gas trapping.

Question 25

Interpret the waveform

Answer 25

Pressure waveform in volume control mode
Gradual decrease in the pressures, until a trigger causing inspiration.
Signifies autotriggering, where the most common cause is circuit leak.
Dysynchrony in the expiratory-inspiratory cycling

Question 26

Autotriggering can be confirmed by

Answer 26

Turning the PEEP to zero setting the trigger to “pressure”.
- the circuit must fall to a negative value for the ventilator to be triggered
- this can only happen if the patient makes an inspiratory effort
- so the abolition of triggering in this instance confirms the occurence of autotriggering

Question 27

Interpret the waveform

Answer 27

There’s concavity of the upstroke in inspiratory pressure.
Indicative of inadequate flow rate, where the patient is trying to breathe in faster than the ventilator. (the patient is “leading” the ventilatory)
Mgt: increase the inspiratory flow rate (IFR)
Dysynchrony in the inspiratory phase.

Question 28

Interpret the waveform

Answer 28

Flow waveform
Demonstrates a premature inspiratory-expiratory cycling
Associated with
- _Fixed (and short) inspiratory time_
- In pressure support mode:
— _low levels of pressure support_
— short respiratory time constant (e.g., ARDS)
— relatively high cycling off threshold
— _dynamic hyperinflation_
Dysynchrony in the inspiratory-expiratory cycling