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Anesthesia > Test 1- Mechanical Ventilation > Flashcards

Test 1- Mechanical Ventilation Flashcards

1
Q

Importance of ventilation during anesthesia

A

Anesthesia can affect ventilation

Alter sensitivity to CO2
Relaxes respiratory muscles (FRC decreases) Atelectasis develops
Make V/Q matching worse

Ventilation can affect anesthesia
Uptake of inhalational anesthetics depends on ventilation

Controlled ventilation facilitates reliable uptake and smooth plane of anesthesia

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2
Q

Ventilation

A

Ventilation is ALL ABOUT CO2!!!

The process involved in the movement of air (gas) in and out of the alveoli

Defined by PaCO2

Normal PaCO2 ≈ 34 - 45 mmHg

Patient should also have normal resp. rate, rhythm and effort

Monitored with arterial blood gas (PaCO2) or capnography

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3
Q

Oxygenation

A

The process of oxygenation of arterial blood

Defined by PaO2

Hypoxemia:
PaO2 < 60 mmHg SaO2 < 90%

Monitored with arterial blood gas (PaO2) or pulse oximetry

Strongly depends on inspired O2%

When breathing air

Also depends on ventilation
When breathing 100% O2

Oxygenation cannot be improved by more ventilation
Could be improved by special respiratory manoeuvers (see recruitment later)
Apneic oxygenation is possible (ventilation may not be needed for oxygenation)!

Using 100% oxygen can insure good oxygenation in most circumstances!

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4
Q

Resistance limits

A

Resistance limits flow
Resistance = ∆Pressure / Flow

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5
Q

Compliance limits

A

Compliance limits volume

Compliance = Vol. / Flow

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6
Q

Indications for mechanical ventilation (MV)

A

There is a need to decrease PaCO2 #1 indication under anesthesia

There is a need to increase PaO2
It’s easier to provide high FiO2 if the patient is intubated and breathing 100% O2

If the patient is already intubated and breathing 100% O2 increasing oxygenation will only be possible with special respiratory manoeuvers and not with conventional ventilation

There is a need to decrease respiratory effort
Mostly happens in the ICU as a treatment for respiratory failure

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7
Q

Indications for MV during anesthesia

A

Convenient control of respiratory function

Prolonged anesthesia
Maintain a more stable anesthesia plane Neuromouscluar blockade

Thoracic surgery
Chest wall or diaphragmatic trauma
Obesity, increased abdominal pressure Head down positioning (Trendellenburg) Laparoscopy
Control of intracranial pressure

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8
Q

Indications for MV in the ICU

A

Depression of the respiratory center in the brain Inadequate thoracic expansion
Inadequate lung expansion
Airway obstruction

Respiratory arrest (or Cardio Pulmonary Arrest) Pulmonary edema, ARDS

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9
Q

Side effects of MV

A

Impairs venous return and cardiac output

May cause hypotension especially in hypovolemic patients

Treatment

Volume loading
 Decreasing airway pressures (change ventilator settings) Switch off the ventilator
 Inotropic drugs (e.g. dobutamine)

Others: pneumothorax and lung injury

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10
Q

Side effects of hypercapnia

A

Direct effects of CO2 Peripheral vasodilation

Decreased myocardial contractility
Bradycardia, possible cardiac arrest (very extreme case!) Increased intracranial pressure

Indirect effects of CO2 via catecholamine release Tachycardia, arrhythmias

Increased myocardial contractility Increased blood pressure

CO2 narcosis
>95 mmHg progressive narcosis >245 mmHg complete narcosis

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11
Q

Risk of not ventilating properly#

A

If you don’t control ventilation during thoracic surgery and let the lung be collapsed for prolonged time, not only CO2 will accumulate but the patient may quickly turn hypoxemic and you may encounter sudden death of the patient!

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12
Q

Should I ventilate during anesthesia?

A

Debated issue especially in horses

The point is how to balance between compromising either

cardiovascular or respiratory function (and oxygenation)

Permissive hypercapnia may be acceptable up to 60-70 mmHg

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13
Q

2 types of ventilation?

A

Manual or Mechanical

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14
Q

Ventilation modes

A

Volume controlled ventilation
Device sets the volume, pressure is a dependent variable

If compliance decreased (pneumothorax) pressure would increase Difficult to control the tidal volume in very small patients

Pressure controlled ventilation
Device sets the pressure, volume is a dependent variable

If resistance increased (airway obstruction) volume would decrease Works well regardless of body size

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15
Q

Clinical recommendation

A

If lung volume changes during procedure (e.g. thoracotomy)

Use pressure controlled ventilation

If trans-pulmonary pressure changes (e.g. laparoscopy) Use volume controlled ventilation

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16
Q

Source of driving power

A

Electrically driven: e.g. using a linear motor

Pneumatically driven: using pressurized gas source. More common

17
Q

What are the different control variables for different ventilators?

A
  1. Flow: the ventilator delivers a constant flow to the patient
  2. Pressure: the ventilator delivers a constant pressure to the

patient

Analogy with electricity: flow is current, pressure is voltage. A battery may supply either constant current or constant voltage

18
Q

What are the cycle variables?

A

Triggers expiration when set value is reached Volume: volume controlled ventilation
Pressure: pressure controlled ventilation
Time: both

Flow: diminishing flow triggers expiration
Useful for pressure support ventilation (PSV) because it helps accommodating to the patient’s breathing pattern

19
Q

What are the limit variables?

A

When value is reached inspiration will be terminated
Volume limit: e.g. metal rod limits the expansion of the bellows.

Used e.g. in the North American Draeger

Pressure limit is used to prevent barotrauma as a consequence of inappropriate ventilator setting

20
Q

Pressure limiting valve at RUSVM

A

This is a safety pressure limit for the drive gas pressure. The patient will receive less pressure than this.

21
Q

Volume controlled ventilation can be achieved by using:

A

Volume controlled ventilation can be achieved by using: Flow controlled, time cycled ventilator (flow x time = volume)

Currently available ventilators at RUSVM belongs to this category

Flow controlled, volume limited, time cycled ventilator

22
Q

Pressure controlled ventilation can be achieved by using:

A

Pressure controlled ventilation can be achieved by using: Pressure controlled, time cycled ventilator (e.g. many modern ventilators)

Pressure controlled, pressure cycled ventilator
Flow controlled, pressure cycled ventilator (e.g. Bird ventilator)

23
Q
A

Volume controlled ventilation

Flow controlled, time cycled ventilator

24
Q
A

Pressure controlled ventilation

Pressure controlled, time cycled ventilator

25
Q

Defining tidal volume (Vt) using the I:E ratio

A

Remember: inspiratory time and flow together define Vt with flow controlled ventilators

I:E ratio = the ratio of inspiratory / expiratory times

I:E ratio and the resp. rate (RR) together define inspiratory time

Summary: use flow, RR and I:E ratio together to set the Vt

26
Q
A
27
Q

Defining Vt using the inspiratory time

A

Currently available ventilators at RUSVM has a control knob for inspiratory time (you are lucky!)

This along with the flow will define Vt

I:E ratio and expiratory time are a dependent variables

RR setting will not affect inspiratory time (and Vt)

But the desired RR will only be delivered if it’s possible with the inspiratory time you set

28
Q

PIP

A

Peak Inspiratory Pressure inflates the alveoli

29
Q

PEEP

A

Positive End Expiratory Pressure keeps the alveoli open

Indications for PEEP Open thorax

Lung parenchymal disease
Following alveolar recruitment maneuver

The benefit of PEEP is questionable during routine anesthesia case management

30
Q

IMV

A

Intermittent Mandatory Ventilation
Patient is allowed to breath freely between mechanical breaths

31
Q

SIMV

A

SIMV (Synchronised IMV)
Each spontaneous breath of the patient is assisted

32
Q

PSV

A

Pressure Support Ventilation
The patient is breathing freely but each breath is supported with pressure

Mechanical inspiration is terminated when flow stops (flow cycled) Better patient-ventilator synchrony than SIMV

33
Q

CPAP

A

Continuous Positive Airway Pressure

Assisted ventilation mode when both the inspiratory and the expiratory pressures are positive

34
Q

Ventilating healthy lungs

A

Tidal volume: 10 - 15 ml/kg

Respiratory rate: 10 - 15 breath/min

Inspiratory time: 1 - 2 sec

PIP: 10 - 20 cmH2O

PEEP: 0 - 2 cmH2O

(For ruminants maybe 6-10 ml/kg)

35
Q

Ventilating sick lungs

A

Tidal volume is smaller (baby lung): 4 - 8 ml/kg

Respiratory rate can be increased up to 60 breath/min

Inspiratory time can be increased but watch out for completeness of expiration

PIP can be increased to 35 (max 60) cmH2O

PEEP as needed: 5 - 20 cmH2O

36
Q

Atelectasis

A

Atelectasis (lung collapse)

General anesthesia causes collapse of the most dependent parts of the lungs in almost all patients

The lung collapses very rapidly after induction of anesthesia and persists for hours or days after surgery

Lung collapse impairs gas exchange and may contribute to development of pneumonia or lung injury

Cyclic recruitment: alveoli opens and collapses with each breath. May lead to lung injury.

37
Q

Alveolar recruitment maneuver (ARM)

A

Therapeutic maneuver aiming to open lung atelectasis and improve oxygenation

Types: CPAP and Cycling
Both should be followed by PEEP

38
Q

Clinical application of ARM

A

CAUTION: ARM is not yet a standard clinical procedure

Safe highest airway pressures may depend on species,

body size, clinical condition etc.

Such safe pressure limits are not yet established!

Other safety issues are concerning the cardiovascular

system!

Perform ARM only if you have Appropriate clinical indication

Reliable mechanical ventilator able to supply PEEP Sufficient monitoring
Sufficient clinical experience!

39
Q
A

Anesthesia (14 decks)

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