Principles of IPPV

Question 1

What is inspiratory versus expiratory tidal volume

Answer 1

VTi - dialled into the machine by anaesthetist

VTe - measured coming out of the patient

Question 2

What is normal VTi for an adult

Answer 2

7 - 9 ml/kg

Question 3

What is ventilatory rate

Answer 3

Same as RR in spontaneously breathing patient –> number of breathing cycles per minute –> often shortened to the symbol ƒ to denote frequency of respiratory cycles

Question 4

What is the minute volume

Answer 4

The total volume of gas moved either in OR out of the lungs in one minute BUT NOT BOTH

Question 5

How does the pressure - volume curve differ for spontaneous ventilation versus IPPV?

Answer 5

Spontaneous ventilation:

The inspiratory loop is on the negative side of the y axis (as the pressures are negative during inspiration). The expiratory loop is on the positive side as pressure becomes positive during expiration

IPPV:
The entire P-V loop leans to the right. I.e. The inspiratory loop is the inferior loop with increasing pressure and volume. The superior loop is the expiratory loop with decreasing pressure and volume. However, the pressure is always positive so the entire loop is on the right (positive side) of the y axis

Question 6

When does the highest airway pressure occur during IPPV

Answer 6

At the end of the inspiratory loop - positive pressure in the lungs must be limited to prevent barotrauma

Question 7

1. In a normal healthy patient, what peak pressure will generate appropriate tidal volumes?
2. What conditions might increase the peak pressure required to generate adequate tidal volumes?
3. At what peak airway pressure does barotrauma become likely?

Answer 7

1. 20 mmHg (27 cmH2O)
2. Obesity (thick and heavy chest wall), diseased lungs, abdominal pathology.
3. 40 mmHg (54 cmH2O)

Question 8

What is the I:E ration how is this related to respiratory rate? What is the normal I:E ratio

Answer 8

Ratio of the inspiratory time to expiratory times. The actual length of the inspiratory and expiratory times is determined by the set ventilatory rate.

E.g. RR = 10 breaths per minute. with I:E of 1:2

1 respiratory cycle every 6 seconds

so at an I:E 1:2 thats 2 seconds for inspiration and 4 seconds for expiration.

1:1.5 to 1:3

Question 9

What is PEEP

Answer 9

Positive End Expiratory Pressure

PEEP involves artificially increasing the pressure at which the patient’s lungs come to rest at the end of expiration –> to a level just above the atmospheric pressure

Question 10

How does PEEP affect the P-V loop

Answer 10

Moves it toward the right so that the end expiratory pressure is positive 5 -10 cmH2O

Question 11

What is volume-limited ventilation

Answer 11

In volume–limited ventilation, the inspiratory tidal volume is set, and the peak airway pressure that this volume generates is variable and dependent on individual patient lung compliance

Question 12

What is pressure-limited ventilation

Answer 12

In pressure-limited ventilation, the peak airway pressure to be generated is set, and the tidal volume delivered is variable and dependent on individual patient lung compliance

Useful in small children, some forms of lung pathology and some surgeries

Question 13

Which two entities are limited on modern ventilators for safety?

Answer 13

Peak Airway Pressure

Inspiratory flow rate

Question 14

What are the absolute indications for IPPV classified?

Answer 14

Patient factors

Surgical/anaesthetic factors

Question 15

List the surgical/anaesthetic factors which constitute an absolute indication for IPPV

Answer 15

Muscle Relaxation
Thoracic surgery (open chest)
Neurosurgery (CO2 control)
Unfavourable position (Prone)
Alert extubation essential (e.g. after RSI)

Question 16

List the patient factors which constitute an absolute indication for IPPV

Answer 16

Pulmonary disease
Obesity
Cardiac disease

Question 17

Name four common ventilation modes and the reason why each was created

Answer 17

1. Volume controlled (Commonly used in OT)
2. Pressure controlled (Limit pressure to alveoli)
3. Synchronized intermittent mandatory ventilation (SIMV) (developed to assist weaning phase of Ventilation)
4. Pressure support ventilation (PSV) (developed to assist weaning phase of ventilation.

Question 18

Classify and define the advantages of IPPV

Answer 18

Direct advantages (ventilation itself)

1. Prevention of atalectasis
2. High dose opioid techniques possible

Indirect advantages (NMB and Intubation)
NMB
- certainty of movement prevention
- lighter depth of anaesthetic required

Question 19

Classify and define the disadvantages of IPPV

Answer 19

Direct factors (ventilation)
- Effects of raised intrathoracic pressure

Indirect factors (NMB and intubation)

• Risk of awareness with no self regulation of anaesthetic depth
• No respiratory monitoring of anaesthetic depth
• slower recognition of airway disconnection/obstruction
• Problems at intubation/extubation

Question 20

Describe the effects of the increase mean intrathoracic pressure during IPPV (versus SV) on the Respiratory System

Answer 20

RSP

• V/Q mismatch - over expanding alveoli compress surrounding capillaries
• Barotrauma - over-expansion/rupture of alveoli (pneumothorax/pneumomediastinum/surgical emphysema)
• Volutrauma - over-distension of “NORMAL” areas of the lung by high pressures leading to subsequent damaging inflammatory processes

Question 21

Describe the effects of the increase mean intrathoracic pressure during IPPV (versus SV) on the Cardiovascular system

Answer 21

Reduced VR and hence reduced CO

High airway pressures can cause direct compression of alveolar capillaries –> increased pulmonary vascular resistance –> Increased RV afterload –> Increased RVEDV. If severe, this increased RVEDV can bulge into the LV and reduce LV compliance leading to reduced LV SV.

IPPV can improve left ventricular function in heart failure by reducing venous return to the LV and ‘off-loading’ the heart.

Question 22

How does IPPV cause reduced urine volume

Answer 22

Raised mean intrathoracic pressure leads to reduced CO and reduced renal perfusion. 2dry activation of RAAS –> Na and H2O retention

Question 23

What are the indirect advantages of IPPV

Answer 23

As NMB is used there can be certainty that there will be no unexpected movement during delicate surgery.

Lighter anaesthesia required as motor responses are abolished by the paralysis

Question 24

What are the indirect disadvantages of IPPV

Answer 24

Increased risk of awareness as NMB are used and there is no self-regulation of depth of anaesthetic.

Disconnections of the breathing system/airway obstruction of ETT are identified later in mechanical ventilation compared to SV

Question 25

Provide recommended ventilator settings for an ADULT

Inspired oxygen 
Tidal volume 
Respiratory rate 
Inspiratory/expiratory (I:E) ratio 
Maximum airway pressure
PEEP

Answer 25

Volume controlled ventilation

Inspired oxygen 0.4 (40 %)
Tidal volume 8-10 ml/kg
Respiratory rate 10-12 breaths per min
Inspiratory/expiratory (I:E) ratio 1:2
Maximum airway pressure 30 cm H2O
PEEP up to + 5 cm H2O

Question 26

Provide recommended ventilator settings for an CHILD

Answer 26

Pressure controlled ventilation

Inspired oxygen 0.5 (50 %)
Inspiratory pressure 20 cm H20 irrespective of weight
Respiratory rate 20-25 for infants, 15-20 for older children
Inspiratory/expiratory (I:E) ratio 1:2
PEEP + 5 cm H2O (can be omitted if >4 years)

Question 27

Why should pressure controlled ventilation, higher FiO2 and higher respiratory rates be used for children

Answer 27

Lungs more susceptible to barotrauma/volutrauma/V/Q mismatch

Higher O2 consumption per kg

Higher CO2 production per kg

Question 28

What type of ventilation would you choose for a 22 year old asthmatic for an elective knee arthroscopy? He has a history of postoperative nausea and vomiting (PONV).

Select one option from the answers below.

Possible answers:
A. IPPV
B. SV

Answer 28

A. False.

B. True.

Not only will paralysis and neuromuscular reversal increase the chance of PONV, but intubation or ventilation via an LMA is more likely to precipitate bronchospasm or barotrauma than using a spontaneous ventilatory technique via an LMA.

Question 29

What is the CaO2 equation

Answer 29

CaO2 = 1.34 x [Hb] x SaO2/100 + (PaO2 x 0.23)

Units: mLO2 / L Blood