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PRVC
- 1. Pressure regulated volume control Charles Gomersall Dept of Anaesthesia & Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital Version 1.0 April 2003
- 2. Disclaimer • The information in this tutorial is thought to be accurate but the author, the Prince of Wales Hospital and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this tutorial
- 3. PRVC • Assist-control ventilation • Pressure control titrated to a set tidal volume • Time cycled
- 4. Assist control • Breaths: – Ventilator initiated (control breaths) – Patient initiated (assist breaths) – Set minimum frequency • Characteristics of each inspiration are the same – Not affected by whether breath is control breath or an assist breath
- 8. Assist control • Set – Minimum respiratory rate • Patient’s spontaneous respiratory rate < set rate ⇒ ventilator gives additional control breaths to make up difference • Patient’s spontaneous rate > set rate ⇒ no control breaths
- 9. PRVC • Set – Minimum respiratory rate – Target tidal volume
- 10. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 11. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 12. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 13. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 14. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 15. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 16. PRVC Measure VT Compare to set VT Same inspiratory pressure ↓ inspiratory pressure ↑ inspiratory pressure Test breath Less Equal More
- 17. PRVC • Set – Minimum respiratory rate – Target tidal volume – Upper pressure limit • Maximum delivered pressure = 5 cm H2O below pressure alarm limit
- 18. PRVC • Set – Minimum respiratory rate – Target tidal volume – Upper pressure limit – FIO2=1.0
- 19. PRVC • Set – Minimum respiratory rate – Target tidal volume – Upper pressure limit – FIO2 – Inspiratory time or I:E ratio
- 20. PRVC • Longer inspiratory time – Improved oxygenation • Higher mean airway pressure • Re-distribution – Lower peak airway pressure • More time available to deliver set tidal volume • Shorter inspiratory time – Less risk of gas trapping and PEEPi – Less effect on cardiovascular system
- 21. PRVC • Expiratory time is the remaining time before the next breath
- 22. PRVC • Set – Minimum respiratory rate – Target tidal volume – Upper pressure limit – FIO2 – Inspiratory time or I:E ratio – Rise time – PEEP
- 23. Advantages • Decelerating inspiratory flow pattern • Pressure automatically adjusted for changes in compliance and resistance within a set range – Tidal volume guaranteed – Limits volutrauma – Prevents hypoventilation
- 24. Disadvantages • Pressure delivered is dependent on tidal volume achieved on last breath – Intermittent patient effort ⇒ variable tidal volumes
- 27. Disadvantages • Pressure delivered is dependent on tidal volume achieved on last breath – Intermittent patient effort ⇒ variable tidal volumes • Less suitable for patients with asthma or COPD
- 28. PRVC • Assist-control ventilation • Pressure control titrated to a set tidal volume • Time cycled
Editor's Notes
- Pressure regulated volume control is a form of assist control ventilation in which, like pressure control, a constant pressure is applied throughout inspiration. However, unlike pressure control, this pressure may vary from breath to breath with changes in the patient’s airway resistance and respiratory system compliance, as the ventilator tries to ensure the set tidal volume. The ventilator changes (or cycles) from inspiration to expiration at the end of a set period of time.
- As with all assist control modes breaths can either be initiated by the ventilator (control breaths) or by the patient (assist breaths). Regardless of whether the breath is ventilator initiated or patient initiated the ventilator delivers the same type of breath, aiming for the same tidal volume with the same inspiratory time.
- As this is essentially a form of pressure control ventilation the ventilator provides a square wave of pressure. Because of this the flow pattern is a decelerating pattern. This pattern of flow is associated with better oxygenation.
- Note that a square wave of pressure is applied for a constant period of time regardless of whether the breath is a control breath
- Or a assist breath
- The operator sets a minimum respiratory rate. If the patient’s spontaneous rate is below this the ventilator delivers sufficient control breaths to make up the difference. If, on the other hand the patient’s spontaneous rate is greater than the set rate the ventilator delivers no control breaths.
- A target tidal volume also needs to be set
- The ventilator titrates the pressure to the set tidal volume in the following way. It starts by giving a test breath
- And then it measures the tidal volume
- And compares it to the set tidal volume
- If it is less than the set volume the inspiratory pressure is increased
- If it is equal to the set volume the inspiratory pressure is left unchanged and
- If it is more than the set volume the inspiratory pressure is decreased and then the next breath is delivered
- The ventilator measures the tidal volume again and makes the necessary adjustment and continues to do so on a breath by breath basis
- Clearly there is a danger with this mechanism that the ventilator may increase the inspiratory pressure to dangerously high levels so it is vital to set the upper pressure limit to an appropriate level. The maximum delivered pressure is limited to 5 cmH2O below the upper pressure alarm limit, so to ensure that potentially dangerous pressures are not delivered the upper pressure alarm limit should be set at 35-40 cm H2O. Do not set the limit above this level without consulting an ICU specialist.
- Always set the fractional inspired oxygen concentration to 1 initially. It is better to expose the patient to unnecessarily high concentrations of oxygen for a short period of time than to run the risk of the patient being hypoxic
- inspiratory time is usually set to 30% giving an I:E ratio of approximately 1:2. This is similar to our normal breathing pattern
- Longer inspiratory times result in improved oxygenation due to a higher mean airway pressure and the longer time available for re-distribution of gas from more to less compliant alveoli. At the same time peak airway pressures are reduced because more time is available to deliver the set tidal volume. On the other hand shorter inspiratory times reduce the risk of gas trapping and intrinsic PEEP by leaving more time for expiration. The cardiovascular effects of mechanical ventilation may also be lessened by the lower mean intrathoracic pressure.
- Note that expiratory time is not set, it is merely the remaining time before the next breath.
- The other controls that need to be set are rise time and PEEP.
- The advantage of PRVC is the decelerating inspiratory flow pattern with automatic adjustment of the inspiratory pressure for changes in compliance and resistance resulting in a guaranteed tidal volume. This should limit both the risk of volutrauma and hypoventilation. If the upper pressure limit is set appropriately the risk of barotrauma should also be limited.
- The disadvantage of the mode is that the pressure delivered is dependent on the tidal volume achieved on the previous breath. If the patient intermittently makes a significant inspiratory effort this will result in very variable tidal volumes with both high and low volumes being delivered
- This is illustrated on this slide. The first breath is a control breath with the patient making no inspiratory effort. As the set tidal volume is achieved the same pressure is delivered on the next breath. This second breath, however, is triggered by the patient, who makes a significant inspiratory effort. As a result, although the airway pressure is the same as the previous breath, the transpulmonary pressure is much greater resulting in a much bigger tidal volume.
- The ventilator therefore reduces the inspiratory pressure for the next breath. This breath is not triggered by the patient who makes no inspiratory effort and therefore the lower pressure results in a tidal volume below the set tidal volume.
- PRVC is probably less suitable for patients with severe asthma or COPD. Patients with severe asthma or COPD are extremely difficult to ventilate and the help of an ICU specialist should be urgently sought.
- To recap PRVC is a time cycled form of assist control ventilation in which the inspiratory pressure is titrated to achieve a set tidal volume.