Assessing Mechanical Ventilation

Posted on July 11, 2024 by Pat Neligan

One of the most intimidating things about entering the ICU for the first time is the “life support machine” – the mechanical ventilator. Although I have posted an extensive series of tutorials on Mechanical Ventilation, covering most of the modes, oxygen therapy and applied respiratory physiology, I have attempted, in this tutorial, to distill everything to the “least you have to know” in 40 minutes. Keep in mind that modern machines look more like iPhones, and are far easier to use than the devices I grew up with that looked to me, on day 1, like something in the cartoon below.

I start with a discussion about the difference between normal breathing, CPAP and Positive Pressure Ventilation (PPV). PEEP is, effectively, CPAP during PPV. I then go on to discuss pressure limited modes of ventilation; worldwide this are the most widely used modes in ICU. I limit my discussion to Pressure Assist Control, Volume Guaranteed Pressure Control (VG_PC) and Pressure Support Ventilation (PSV). VG-PC is a popular and flexible option as an ICU’s default mode. However, as it is a pressure controlled mode, there is significant variability in tidal volume and airway pressure from minute to minute.

Several important rules are emphasized: the tidal volume should, in general be lower than 6ml/kg of ideal body weight, the plateau pressure lower than 30cmH2O and the driving pressure lower than 15cmH2O. I introduce the Spontaneous Breathing Index (SBI = RR/TV in L). The magic number is 100. We use the SBI to determine the success of weaning on PSV.

Volume Controlled Ventilation is the predominant mode use in the Operating Rooms (Theatres), and Volume Assist Control is a popular mode in North America. In ICU you must set a peak inspiratory flow and be aware that this may be insufficient during assisted breaths and lead to dys-synchrony. Volume Control is often used in ARDS to “lock in” the Tidal Volume (TV) but the operator must be aware that the TV that matters is not what is dialed up on the ventilator, but what the patient exhales.

I go on to discuss how to assess the patient on invasive mechanical ventilation, by looking at whether they are breathing spontaneously, in which case we determine whether they are suitable for a Pressure Support wean or not, or whether or not there is a problem with oxygenation (increase FiO2, PEEP, Mean Airway Pressure and seriously consider Prone Positioning) or Ventilation (increase Respiratory Rate, Tidal Volume or both, reduce PEEP).

The final part of the tutorial looks at Non Invasive Ventilation (NIV), and I explain how, in general we only use 2 modes on standalone devices – CPAP and Spontaneous Timed (S/T). The latter is similar to PSV with a backup rate, but I point out that instead of PEEP+PS the breath is EPAP + IPAP and IPAP is not built upon IPAP, as is the case with PSV. If one is delivering NIV on an ICU ventilator, then “leak” adjustment or “leak sync” should be used.

@ccmtutorials

The kPa Rules – Part 1: Oxygen

Posted on March 7, 2024 by Pat Neligan

In the early 1970s much of the world adopted the System International (SI) approach to scientific measurement. Unfortunately, the remainder of the world ignored it. This means that, today, we have different units presented in the scientific literature depending on the location of the source of the publication.

The USA is the most notable non SI country and this presents a problem in that the majority of English language textbooks and journals in medicine as well as a lot of the international guidelines and clinical pathways are derived in the US. In critical care this is important – as blood gasses are reported in mmHg in the USA (and most of the literature) and in kPa elsewhere – notably in Europe.

In many of my tutorials I have reported clinical “rules” such as the PaO2/FiO2 ratio, the Alveolar Gas Equation and the majority of the calculations in acid base – in mmHg. This series of two tutorials serve to right the balance. However there is a twist.

In this first tutorial I am not just rehashing the approach to oxygenation by swapping out mmHg for kPa. In fact, the use of kPa to measure and monitor oxygenation provides us with a significant helping hand. Effectively, as atmospheric gas is effectively 100kPa and Oxygen exerts 21% of that – Dalton’s law – then it is clear that the partial pressure of inspired oxygen (PiO2) is 21kPa. Oxygen is poorly soluble in blood and water – the solubility co-efficient is 0.225 – meaning that the quantity of oxygen dissolved in blood is the PaO2 x 0.225 kPa. Oxygen follows Henry’s law – meaning that solubility is related to temperature (37 degrees C) and pressure – the PiO2. In the best case scenario the PaO2 – the partial pressure of oxygen in arterial blood is 13kPa. That means that the gradient between PiO2 and PaO2 is, at a minimal, 8kPa. The greater the stretch between the two the larger the lung injury or ventilation perfusion mismatch.

The oxygen content of blood is 1.34 x Hb x SaO2/100 + (PaO2 x 0.225). I explore the impact of different FiO2s and ambient pressure on the blood oxygen content. Although dissolved oxygen is very low breathing air – the use of supplemental oxygen may dramatically increase it – particularly in hyperbaric conditions.

Finally I address the issue of PaO2/FiO2 as a way of quantifying oxygenation. The PF ratio, as we call it, is a significant component of the ARDS definition. A PF ratio of 200 in mmHg is equivalent to 25 in kPa and a ratio of 100 in mmHg is equivalent to 12.5 in kPa. An easier way to look at this, though, is to divide the PiO2 by the PaO2 – the numbers look similar but you now have a proportion in kPa. That PF ratio of 25 in kPa resolves to 0.25 meaning that only 25% of inspired oxygen is reaching the pulmonary veins (PaO2). Likewise a PF ratio of 12.5 in kPa (100 in mmHg) resolves to 0.125 – which means that only 1/8th of the inspired oxygen is delivered to arterial blood. I think that this is a really good way of assessing oxygenation – and a way of clarifying hypoxemia in your brain.

Proportional Assist Ventilation [PAV+]

Posted on February 14, 2024 by Pat Neligan

Proportional assist ventilation has been around in various shapes and forms since the late 1990s. The most advanced current iteration – PAV+ – is unique to Puritan Bennett ventilators. It is a closed loop mode of ventilation. That means that the ventilator dynamically changes the level of assistance that the patient receives in response to patient effort.
PAV+ is neither volume controlled nor pressure controlled but is patient (and operator) controlled. The operator adjusts the percentage support that the ventilator delivers to the patient. The patient breathes – triggering the ventilator – and the ventilator amplifies the patient’s breath. Consequently the more work that the patient does to generate muscular effort the more work the ventilator performs to match the patient’s workload.

It has been known for some time that the diaphragm becomes both atrophic and dysfunctional in acute critical illness, in particular due to disuse during control of mechanical ventilation. In most assisted modes, all the patient needs to do is trigger the ventilator. Patient workload may be inversely proportional to ventilator workload. Frequently the patient’s diaphragm and ventilator are out of synchrony.

PAV+ is patient triggered and flow cycled so it should be seen as a form of pressure support ventilation. PAV+ contrasts with standard pressure support in that the degree of support changes from breath to breath and indeed within breath depending on patient effort. Pressure support delivers a fixed airway pressure for every single breath irrespective of patient effort. Consequently if we map patient effort to ventilator workload there is only one point where the two will intersect. Conversely in proportional assist ventilation the workload of the ventilator and the workload of the patient increase and decrease linearly.

PAV+ works by utilizing very high quality flow and pressure sensors. The ventilator determines when the patient initiates the breath and when the breath is completed. Having instructed the ventilator what proportion of work of breathing that the ventilator should perform, one observes, using a work of breathing bar, if the patient is doing satisfactory work or whether they need to increase or decrease their workload. The work of breathing (WOB) is determined by the ventilator by measuring compliance, resistance and intrinsic peep dynamically every 9 to 12 breaths. As such a Green Zone between 0.3 and 0.7 joules per litre is indicative of ideal work of breathing for the patient; I call this the “sweet spot.” As long as the patient’s WOB resides within the sweet spot of the toolbar the bedside clinician can be satisfied that the patient is both comfortable and safe.

As the tidal volume relates to the patient’s neural activity that results in diaphragmatic power one should not be unduly concerned about high or low tidal volumes in this mode.

If one wishes to put a patient on proportional assist ventilation it is imperative that one determines if the patient is breathing spontaneously and taking an adequate minute ventilation prior to using this mode. The reason for this is that there is no backup rate in PAV+. Usually one starts with 70% support: that means 70% of the work of breathing is performed by the ventilator on 30% by the patient. After a couple of minutes, once one has observed the work of breathing bar, one can make adjustments either to increase the workload of the ventilator or to reduce it by keeping the patient within that Green Zone sweet spot. Generally failure of the patient to settle on this mode is manifest by a respiratory rate of more than 35. Once the patient has been on 20% support for an hour or more and is awake, obeying commands, protecting their airway, and not being suctioned frequently then the patient can be extubated.

Studies that have looked at PAV+ versus pressure support have indicated that weaning is more rapid with PAV+.

Weaning From Mechanical Ventilation (the basics)

Posted on October 25, 2023 by Pat Neligan

This tutorial is about weaning from mechanical ventilation. This is not an easy topic because every professional in the ICU has their own weaning method and their own opinions regarding how best to wean and liberate patients. The literature is unhelpful. Some patients, for example those who have been intubated for a brief period of time, can be awoken and the tube removed after a couple of spontaneous breaths. Other patients require careful multidisciplinary activity over weeks to months to liberate. This tutorial focuses on the in-between group patient who have been intubated for a week or so, who require both clinical and mechanical assessment of their ability to wean and liberate from the ventilator.

Generally the first intervention in weaning is to change the patient over to a spontaneous breathing mode – pressure support or volume support and ensure that alveolar ventilation is adequate to maintain CO2 clearance.

Then a number of clinical and mechanical assessments can be made: is the patient awake, do they have a cough, are they triggering adequately, what is their rapid shallow breathing index (RSBI)? One can estimate muscle strength by performing an occlusion test – either a partial occlusion (P0.1) or a longer occlusion (NIF). Once the patient is assessed as being a candidate for weaning, then one can perform a spontaneous breathing trial (SBT) that is either supported (PS, VS, ATC) or unsupported (T-piece, C-circuit, Trach mask, Swedish Nose).

If the SBT is successful after 90 minutes – extubate the patient. SBTs may fail due to worsening hypoxemia, hypercarbia or hypocarbia, respiratory distress (increase RSBI or use of accessory muscles) or cardiovascular instability (hypotension, hypertension, tachycardia, bradycardia, arrhythmias) or falling levels of consciousness, agitation or acute delirium.

Why isn’t the patient breathing up? (Triggering the Ventilator)

Posted on October 17, 2023 by Pat Neligan

Is there anything more frustrating in the ICU when you decide to start weaning a patient – they look like they’re assisting the ventilator. You switch them over to a “spontaneous” mode and then……nothing…..no breaths….eventually the backup starts.

This tutorial is about triggering of mechanical ventilation. I will revisit how patients trigger the ventilator, the different systems used and introduce I-Sync – a new method of triggering.

Finally I will discuss the problem of Auto-PEEP and explain why, in the setting of Auto-PEEP, there is no point fiddling with the flow by or negative pressure.

I guarantee you will learn something. @ccmtutorials www.ccmtutorials.org

The Wibbly Wobbly Waveform – Expiratory Dysynchrony

Posted on October 11, 2023 by Pat Neligan

Expiratory dysynchrony is a major unrecognized problem in critical care. Usually it takes one of two forms: a terminal upstroke on the pressure waveform, indicating pressure cycling (breath too long) or a W shaped anomaly in the expiratory flow waveform – indicative of the breath being too short or too long. I call this the “Wibbly Wobbly Waveform”.

This tutorial looks at expiratory dysynchrony – why it happens and how to make adjustments to resolve the problem. I also introduce a relatively new technology: IE Sync.

Help – The Patient is Fighting the Ventilator

Posted on October 4, 2023 by Pat Neligan

The patient is turning purple in the bed, alarms are going off, he is desaturating: he is “fighting the ventilator.” Although a widely used description I believe that it is misused to redefine the problem away from an issue of ventilator operator competency and reframe it as a patient problem. It is not. Most of the time that patient have negative interactions with the ventilator it is a problem of triggering, flow or expiratory cycling. The treatment is not deep sedation and controlled ventilation. The treatment requires skill and nuance, and does not always work. This tutorial looks at inspiration and reasons why it may go wrong.

The most frequently seen patient ventilator dysynchrony is scooping of the pressure waveform, usually associated with flow limited volume controlled ventilation. This can be resolved by increasing the peak flow or changing to pressure control.

In general the ambition to establish a patient on spontaneous assisted ventilation is laudable, but oftentimes we have no idea about what is going on underneath the pressure, flow and volume waveforms. In this tutorial I try and correct the narrative about patient-ventilator interaction when using pressure support. I suggest that volume support in some situations may be a superior approach. I point out that the tidal volume in pressure support has little to do with patient effort and more to do with lung compliance.

I finish the tutorial with a discussion about the inspiratory rise time and explain why you must be careful when using older ventilators.

@ccmtutorials http://www.ccmtutorials.org

Help- The Patient’s Airway Pressures are STILL HIGH!

Posted on September 27, 2023 by Pat Neligan

In the previous tutorial we looked at the problem of high airway pressures and addressed inspiratory airway resistance in two ways: peak to plateau pressure gradient and dynamic and static inspiratory resistance.

In this tutorial we will look at three more ways of assessing airflow resistance: the identification and measurement of Auto-PEEP, Flow-Volume Loops and capnography.

Subsequently I discuss high airway pressure due to low total respiratory system compliance. I explain that when “compliance” is low – this may be a problem with the lungs as well as the chest wall – including the abdomen. I finish with the introduction into this course of Abdominal Compartment Syndrome.

50 Tutorials Uploaded! Now – Help the Patient’s Airway Pressures Are High!

Posted on September 19, 2023 by Pat Neligan

The alarm goes off like an air raid siren – everybody starts to panic – somebody starts to do the saturation countdown. There is nothing quite as distressing for the anesthesiologist or intensivist than for the ventilator to pressure cycle and fail to deliver tidal volumes due to high airway pressure.

Generally high pressures are caused by one of three things – a problem with the equipment (kinked tubing, patient biting the tubing etc.), an airway resistance problem (e.g. bronchospasm) or a pulmonary compliance problem (e.g. consolidation or pulmonary edema) or a combination of these. The first thing that the clinician should do when there pressure alarm goes off – is to silence the alarm and increase the Pmax.

Then go looking for the problem: start at the mouth and work your way back to the machine. If you can’t find a fault, put the patient on a manual breathing circuit and commence ventilation. If the patient is easy to bag, there is a machine problem, if difficult – then there is a problem with pulmonary resistance or compliance. In this first tutorial I look at assessing airway resistance. I do this in two ways. First I discuss peak to plateau pressure gradients and then look at airway resistance: dynamic versus static and how to calculate it. I will finish the discussion in the next tutorial.

Volume Pressure Loops – they are on every ventilator and anesthetic machine – look at them

Posted on September 12, 2023 by Pat Neligan

This tutorial looks at the pressure waveform in patients undergoing anesthesia or mechanically ventilated in ICU. All modern ventilators will provide a pressure time waveform and display volume pressure (often called “pressure volume” loops).

This tutorial commences with a discussion about pressure-flow loops – to demonstrate the relationship between flow and airway pressure. I then discuss and describe normal airway pressure versus time waveforms.

Subsequently I explore normal and abnormal dynamic volume pressure loops. I briefly discuss static VP-curves and why they are important in ARDS. Finally I demonstrate how you can measure real plateau pressure and static compliance by pushing one button and performing an inspiratory hold.