Principles of IPPV Flashcards
What is inspiratory versus expiratory tidal volume
VTi - dialled into the machine by anaesthetist
VTe - measured coming out of the patient
What is normal VTi for an adult
7 - 9 ml/kg
What is ventilatory rate
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
What is the minute volume
The total volume of gas moved either in OR out of the lungs in one minute BUT NOT BOTH
How does the pressure - volume curve differ for spontaneous ventilation versus IPPV?
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
When does the highest airway pressure occur during IPPV
At the end of the inspiratory loop - positive pressure in the lungs must be limited to prevent barotrauma
- In a normal healthy patient, what peak pressure will generate appropriate tidal volumes?
- What conditions might increase the peak pressure required to generate adequate tidal volumes?
- At what peak airway pressure does barotrauma become likely?
- 20 mmHg (27 cmH2O)
- Obesity (thick and heavy chest wall), diseased lungs, abdominal pathology.
- 40 mmHg (54 cmH2O)
What is the I:E ration how is this related to respiratory rate? What is the normal I:E ratio
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
What is PEEP
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
How does PEEP affect the P-V loop
Moves it toward the right so that the end expiratory pressure is positive 5 -10 cmH2O
What is volume-limited ventilation
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
What is pressure-limited ventilation
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
Which two entities are limited on modern ventilators for safety?
Peak Airway Pressure
Inspiratory flow rate
What are the absolute indications for IPPV classified?
Patient factors
Surgical/anaesthetic factors
List the surgical/anaesthetic factors which constitute an absolute indication for IPPV
Muscle Relaxation Thoracic surgery (open chest) Neurosurgery (CO2 control) Unfavourable position (Prone) Alert extubation essential (e.g. after RSI)
List the patient factors which constitute an absolute indication for IPPV
Pulmonary disease
Obesity
Cardiac disease
Name four common ventilation modes and the reason why each was created
- Volume controlled (Commonly used in OT)
- Pressure controlled (Limit pressure to alveoli)
- Synchronized intermittent mandatory ventilation (SIMV) (developed to assist weaning phase of Ventilation)
- Pressure support ventilation (PSV) (developed to assist weaning phase of ventilation.
Classify and define the advantages of IPPV
Direct advantages (ventilation itself)
- Prevention of atalectasis
- High dose opioid techniques possible
Indirect advantages (NMB and Intubation)
NMB
- certainty of movement prevention
- lighter depth of anaesthetic required
Classify and define the disadvantages of IPPV
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
Describe the effects of the increase mean intrathoracic pressure during IPPV (versus SV) on the Respiratory System
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
Describe the effects of the increase mean intrathoracic pressure during IPPV (versus SV) on the Cardiovascular system
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.
How does IPPV cause reduced urine volume
Raised mean intrathoracic pressure leads to reduced CO and reduced renal perfusion. 2dry activation of RAAS –> Na and H2O retention
What are the indirect advantages of IPPV
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
What are the indirect disadvantages of IPPV
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
