Tag Archives: inspiratory time

Pressure Assist Control in ARDS (Acute Hypoxic Respiratory Failure)

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This is the second tutorial on Pressure Assist Control (PAC).

The tutorial focuses on the use of PAC in acute hypoxic respiratory failure (AHRF or ARDS). In this tutorial I cover the use of Pressure Assist Control in Acute Hypoxic Respiratory Failure (AHRF or ARDS). My major objective is to ensure that you understand that cookbook approaches to ARDS using volume control cannot be ported over to pressure control – in particular the ever increasing use of PEEP.

During previous tutorials I explained that increasing mean airway pressure (Pmean) can be achieved more effectively with increased inspiratory time (Ti) than by increasing PEEP. Pmean can also be increased by increasing respiratory rate but you must be really careful with Auto-PEEP as that reduces tidal ventilation, Pmean and increases dead space. This tutorial starts with an explanation of the Pendulluft effect in hypoxemia and with increased airway resistance – basically prolonging inspiration results in better overall gas distribution. How one manages worsening hypoxemia and lung compliance is key to your skill as an operator of a mechanical ventilator.

Early in the course one tends to maintain driving pressure and inspiratory time in PAC while increasing PEEP. However, ultimately one runs into the 30cmH2O barrier. At that point one must adjust. An important adjustment is to stop the patient from breathing – or more likely – gasping. “Gasping” is a term that I will use in these tutorials to describe the patient generating massive transpulmonary pressures (and likely lung stretch) with minimal impact on ventilation. In fact, the increased work of breathing causes a deterioration in oxygenation due to lower mixed venous oxygen tensions consequent of increased oxygen consumption (not covered in this tutorial).

The second adjustment one must make is to increase the inspiratory time and reduce the PEEP – keeping in mind your tidal volume target. At this point respiratory rate must fall and Auto-PEEP controlled. The final part of the tutorial covers the major drawback of PAC ventilation – expiratory dys-synchrony: what happens when the patient wants to exhale during inspiration.

I guarantee you’ll learn something. @ccmturorials http://www.ccmtutorials.org

Pressure Assist Control (Part 1)

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Virtually all “modern” modes of mechanical ventilation are built on a pressure controlled platform – the original of the species is Pressure Assist Control (PAC). This tutorial introduces PAC as it would be used on a patient admitted, for example, to ICU, with relatively normal lungs.
The tutorial commences with a clinical scenario followed by a guide to the settings on both Puritan Bennett and Drager ventilators. At this point in the course I am going to start spending more time on Drager devices as these ventilators were built from the ground up to be used as pressure controlled machines. There are nuances to the Drager ventilator that may be slightly counter-intuitive to clinicians who are familiar to other brands: in particular the use of a pressure limit (Pinsp) rather than a driving pressure above PEEP. I explain this with examples. I then explain how pressure control works and remind you of flow and time triggering.
All pressure controlled modes are time cycled with decelerating flow patterns. Care must be taken to ensure that inspiratory time is sufficiently long so as to ensure that the airway is adequately pressurized but not to long as will cause Auto-PEEP.
If you want to understand mechanical ventilation you absolutely must be able to interpret and craft ventilator waveforms – and this tutorial focuses on identifying abnormal waveforms in pressure control and correcting them. Hence there is a section on “Crafting the Pressure Waveform” and a section on “Crafting the Flow Waveform.”
Finally I discuss inspiratory time and tidal volumes

Pressure Controlled Ventilation – Fundamentals Part 2: Mean Airway Pressure

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In the previous tutorial I introduced some of the fundamental elements of pressure control ventilation – time cycling, decelerating flow, pressure ramps etc. This time I discuss, in detail, the concept of mean airway pressure (Pmaw) and describe why increasing Pmaw is an effective way of treating patients with extensive lung disease. In volume controlled ventilation this can be achieved by titrating PEEP upwards and increasing respiratory rate. Care must be taken to keep the plateau pressure below 30cmH2O in the majority of patients. In pressure control Pmaw is generally increased by increasing inspiratory time – extreme care must be taken, though, to avoid escalating Auto-PEEP as this corrodes tidal volume and actually reduces Pmaw.

If auto-PEEP is unavoidable, as it is with inverse ratio ventilation, then extrinsic PEEP should be reduced to ensure that tidal ventilation is maintained. Pmaw can be achieved in volume control by adding an inspiratory pause, and in pressure control by increasing respiratory rate – but these are less effective approaches – in volume control because of necessary flow limitation and in pressure control because of fixed inspiratory times, and Auto-PEEP.
I guarantee you’ll learn something.

@ccmtutorials

Pressure Controlled Ventilation – The Fundamentals Part 1

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It is time to discuss Pressure Controlled Ventilation. In general if a patient has normal lungs or minimal disease, it really does not matter what mode of ventilation you use, pressure or volume controlled. However, there are some major advantages to using Pressure Control – principally in Acute Hypoxic Respiratory Failure. There are also many disadvantages. This is the first of two tutorials that cover the fundamentals of Pressure Control. I start with a discussion of the terminology that I will be using – the Pressure Limit (PL), the Inspiratory Pressure (IP), the Driving Pressure (DP)/Inspiratory Ramp, the Inspiratory Time (Ti) and the Expiratory Time (Texp). Pressure Controlled Ventilation (PCV) is pressure targeted/limited and volume variable. Breaths are time cycled – in inspiration, expiration or both. The flow pattern is always decelerating.

Following the introduction of a clinical scenario – a patient who is developing ARDS, I describe the process of PCV. I explain that tidal volumes are variable in all settings and all modes of PCV and later describe how changing patient position, chest wall elastance and airway resistance can all impact the tidal volume. I discuss why pressure control is the best option for mechanically ventilating children (particularly where there is no endotracheal tube cuff and a significant air leak) and why you need to pay attention to the rise time and respiratory rate. Finally I discuss the major disadvantages of using PCV. I guarantee you’ll learn something!
@ccmtutorials