Author Archives: Pat Neligan

Unknown's avatar

About Pat Neligan

Pat Neligan lives and works in Galway, Ireland

Volume Guaranteed Pressure Control (Pressure Regulated Volume Control)

Posted on by
Reply

This tutorial is about Volume Guaranteed Pressure Controlled (VG-PC) Ventilation – otherwise known as PC-VG, PRVC, VC+ etc. It is a modern mode of ventilation that aims to deliver a desired tidal volume (volume control) using the pressure controlled paradigm (unlimited flow). As such it is a mode that is often labelled “dual controlled” although, in some ways, it is neither volume controlled, pressure controlled nor both.
Confused? Most are. I have labelled the mode VG-PC – because that is the best approximation – but volume is not necessarily guaranteed and it is certainly not limited. So why bother using this mode. Simply – it works! As a general use “unit default” mode of ventilation VG-PC has few peers: it is nimble enough to be used as the mode of ventilation of choice for patients admitted to ICU following intubation: postoperatively or with respiratory failure.
If the lungs deteriorate – then the mode is versatile enough to deal with it. Being time cycled – mean airway pressure can be easily altered. If compliance or resistance of position changes – then the tidal volume “guarantee” changes the inspiratory pressure from breath to breath to ensure that things remain stable. If the patient breaths spontaneously, using the assist control or SIMV paradigm, flow is increased to meet patient demands. As such it is a very forgiving mode of ventilation, ideal for novices, reassuring to the ICU clinicians. This tutorial explains VG-PC, demonstrates how it is set up in three different ventilators – Puritan Bennetts, Dragers Evitas, Servo I and GE (Aisys) anesthetic machines. I explain the operation of this mode and its strengths and weaknesses. I guarantee you’ll learn something. @ccmtutorials http://www.ccmtutorials.com

Breathe Easy – Automatic Tube Compensation

Posted on by
Reply

This tutorial is about Automatic Tube Compensation (ATC). ATC is a setting that has been included in most modern ventilators. Its aim is to reduce the work of breathing associated with the drop in pressure across the endotracheal tube. The ventilator senses pressure, flow and resistance and changes the pressure during the breath to ensure that the patient has the sensation that they are breathing through their own airway. There are two configurations of ATC – one is as an alternative to pressure support in patients who are essentially weaned from mechanical ventilation: essentially a spontaneous breathing trial. The second configuration is as an accessory to all pressure limited modes – such that the pressure waveform is crafted in inspiration and expiration to reduce the workload of breathing during both phases of respiration. @ccmtutorials http://www.ccmtutorials.org

SIMV-Pressure Control

Posted on by
Reply

This weeks tutorial is on SIMV-Pressure Control. Although this is one of the lesser used modes of ventilation, I sometimes see my colleagues using it in the operating room. And for good reason. Anesthesia ventilators are not set up in the same way as ICU vents. In particular – if you choose “PC” Pressure Control – that is what you get – pressure control; NOT pressure assist control. Hence there is no real provision for patient ventilator interaction. If you choose “SIMV” as pressure control, volume control or volume guaranteed pressure control, then the patient can breath and interact with the ventilator and receive pressure supported breaths. Consequently, conventional SIMV modes, these days, are far more likely to be used in the operating room than in the ICU.

The second reason that I wanted to cover SIMV Pressure Control is to set the groundwork for a different mode “BiLevel Pressure Control” that is built on a similar platform, looks a bit like SIMV, and has significant benefits for those of you who might choose SIMV-PC in ICU.

Most modes of ventilation offer two ways of supporting the spontaneous breath – assist control and SIMV. In SIMV-PC the spontaneous breath can be unsupported, pressure supported or partially supported using Automatic Tube Compensation (ATC). This tutorial covers the type of patient to whom you might deliver SIMV-PC; how to set up the mode; what it looks like on a ventilator screen and the strengths and weaknesses of the mode. @ccmtutorials http://www.ccmtutorials.org

Pressure Assist Control in ARDS (Acute Hypoxic Respiratory Failure)

Posted on by
Reply

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)

Posted on by
Reply

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

Posted on by
Reply

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

Posted on by
Reply

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

Positive End Expiratory Pressure – Phasic Shunting, Auto-PEEP & ARDS

Posted on by
Reply

In the previous tutorials I explained how hypoxemia results from low lung volumes, resulting in low functional residual capacity, airway closure and atelectasis. We looked at the mechanisms by which CPAP reduces the work of breathing in obstructed airways and how, following lung recruitment, PEEP maintains FRC.

In this tutorial I elaborate on these themes. I look at the problem of phasic V/Q mismatch (shunt) during expiration and how it may cause dis-correlation between pulse oximeters and blood gasses. PEEP prevents this at the expense of increasing dead space and negatively impacting ventilation. Optimal PEEP should restore lung compliance – compliance is low with low and high lung volumes. Compliance may also appear poor in pressure control when there is clinically significant auto-PEEP: the ventilator cannot distinguish auto-PEEP from driving pressure and lower than expected tidal volumes may result.

I explain the concept of the “Waterfall” effect to overcome Auto-PEEP. Finally, in our first visit to ARDS, I introduce the problem of deciding on optimal PEEP in that setting. I guarantee that you will learn something. @ccmtutorials http://www.ccmtutorials.org

@ccmtutorials

Why We Use CPAP and PEEP (part 1)

Posted on by
Reply

For most of us, the terms CPAP (continuous positive airway pressure) and PEEP (positive end expiratory pressure) have existed for all of our careers. But this was not always the case. Although mechanical ventilation, including the positive pressure variant, was a child of the 1950s – PEEP was not described until the late 1960s and even then was seen as a therapy for postoperative atelectasis in cardiac surgery patients. PEEP subsequently became the mainstay of therapy for hypoxic respiratory failure, but was always used in associated with positive pressure breaths. CPAP was developed in the early 1980s as a therapy for sleep disordered breathing. Over two decades the non invasive CPAP therapy and the invasive ventilation (pressure targeted breaths with PEEP) coalesced such that CPAP became a therapy for hypoxic respiratory failure and congestive heart failure, and pressure support (BiPAP or NIV) became a therapy for sleep apnea.

Strictly speaking PEEP and CPAP are different. It is possible to apply PEEP at end expiration and then commence the next breath from atmospheric pressure (try slapping your hand over your mouth mid expiration – then remove it and take a breath) – spontaneous PEEP. However this is almost never used in clinical practice. In CPAP the patients sinusoidal respiratory pattern persists – but starts and ends at an elevated baseline pressure. In PEEP the positive pressure breath starts and ends at that pressure (i.e. pre inspiration and end expiration). So, these days, in most scenarios PEEP and CPAP are indistinguishable. How they are delivered is, of course, different. Nevertheless they serve the same functions 1. To overcome airway resistance that causes disrupted or obstructed gas flow in expiration; 2. To reduce the work of breathing by reducing the magnitude of negative pleural pressure required to generate a tidal volume; 3. Most importantly – to restore functional residual capacity (FRC); 3. To prevent derecruitment of vulnerable lung units in the posterior dorsal segments of the lungs.

PEEP does not easily re-expand collapsed lung tissue – this is usually achieved by applying a recruitment maneuver (30cmH2O or more for 10 seconds during anesthesia, for 30 seconds in lung injury). The application of PEEP then prevents derecruitment. As such the majority of lung tissue may be re-expanded during anesthesia. This may not be the case in diseased lungs – the principle is to restore a functional residual capacity even if that effectively utilizes the inspiratory reserve volume.  

@ccmtutorials

Why Low Lung Volumes Are Bad

Posted on by
Reply

In previous tutorials I discussed the problem of ventilation perfusion mismatch, intrapulmonary shunt and physiologic dead space. I explained how different injuries to the lung (the 6 s approach – slimy, soggy, sticky etc.) resulted in poorly aerated airways and atelectasis. Before moving on to a discussion about CPAP/PEEP we need to explore the problem of low lung volumes. Although the lungs can hold up to 6L of air – in reality most of the time there is 2-2.5L in the alveoli. This is the resting lung volume that is found at end expiration and results when the tendency for the chest wall to spring outwards is balanced by the tendency for the lungs to collapse inwards. That resting lung volume is established by negative pleural pressure and it represents the expiratory reserve volume and residual volume – together the functional residual capacity (FRC).

FRC is the lung capacity in which most oxygenation takes place, in which lung compliance is highest, airway resistance lowest and pulmonary vascular resistance optimal. Loss of FRC (“low lung volumes”) – results in hypoxemia, increased work of breathing, autopeep and pulmonary hypertension.

During the tutorial I elaborate on lung volumes – how they are affected by position and age, how airway closure becomes a major issue as we get older – particularly in the supine position, and I introduce the volume pressure curve which is essential for understanding dynamic respiratory system compliance.