Ventilator Weaning Parameters: Can You be Sure They’re Ready?

Intensive care unit. Young lady in white lab coat checking digital device for measuring blood pressure. Middle aged man with endotracheal tube in mouth sleeping after surgery

It is obvious that positive pressure ventilation (PPV) is not a normal way to breathe, and for many years we have known that the longer someone remains on PPV, the more likely damage to the lung and other organs will occur. To begin, simply placing someone on artificial ventilation and forcing air into the lung under high flow conditions in order to create positive pressure in the airway, forcing the expansion of delicate lung tissue during inspiration is actually creating a direct opposite pressure gradient to normal spontaneous breathing. So, the question should always be pondered by all good clinicians of their mechanically ventilated patient: when can you wean a patient from a machine that, although a temporary life-saving bridge, is likely causing harm with every passing minute?

As a reminder, not every patient on PPV is intubated. Patients who are on non-invasive PPV and those who are connected to PPV via trachestomy tube should also be weaned prior to attempting removal from PPV. Worth noting, certain patients that have been on prolonged mechanical ventilation (greater than 14-21 days) or those who occasionally require non-invasive PPV shortly after being removed from invasive PPV are categories of patients in which weaning may be more complex. Certain other patient population such as in a long-term care facility or those that are to be terminally weaned also create a completely different view of weaning.

Generally, PPV is a time-proven intervention of modern medicine that when indicated buys time to allow the body to recover and PPV used judiciously overall is arguably more lifesaving than lung damaging. It must be noted that we are not discussing the many varieties of modes nor the means in which a patient can be ventilated, only when and how weaning should be accomplished. That said, let’s establish the most important consideration before attempting to wean. Unequivocally, whatever caused the patient to need PPV in the first place must be resolved, otherwise weaning would be futile right? Thereafter, there are many considerations.

For starters, before considering weaning from mechanical ventilation, some semblance of stability and relative consistency in both the metabolic rate and respiratory status of your patient must be maintained or at the least temporarily attained. Otherwise, the patient’s drive to breath may supersede any weaning attempts. Many studies have been accomplished and articles written that highlight various techniques and/or steps to the process of weaning, but consensus usually points to a spontaneous breathing trial (SBT) as the truest test of PPV removal readiness. The path to an SBT and the means in which a true SBT is accomplished remains an ongoing subject of debate but will typically require some sort of weaning process.

The term weaning implies a gradual decrease in intervention; therefore, it should stand to reason that patients should be progressively given more autonomy and control over their breath rate, ventilating volume, while generally being able to take on a significant portion of the work of breathing. Physiologic signs of distress (e.g., tachypnea, tachycardia, abdominal or ‘see saw’ respirations) that indicate ongoing respiratory compromise should be monitored as progressive weaning is accomplished. Even though institutions across the globe have various weaning criteria and protocol, some degree of patient stability should be present before weaning is accomplished in most circumstances. 

Let’s get into the more detailed process of weaning. Patients that are difficult or unable to be weaned and eventually removed from PPV usually fall into one of the following general categories:

  • Persistent need for airway protection
  • Increased metabolic demand (high O2 consumption, high CO2 production)
  • Persistent pulmonary compromise (e.g., pneumothorax, pleural effusion, diffuse infiltrates/pneumonia, atelectasis, pulmonary edema)
  • Ventilator dependency (psychologically or physiologically after prolonged PPV)

The above categories do not necessarily represent a comprehensive list, nor do patients that fall into one of the above categories always fail weaning. Some patients just need more time, requiring a slower step-by-step process.

So, what exactly is the best process?

Textbooks are riddled with so-called weaning parameters that attempt to create a list of all the minimum criteria for acceptability. Although there are a handful of certain metrics/numbers that should be a consistent target, almost no parameter represents a stand-alone indicator of a “green light” to wean and remove from PPV. Logically, the lower the requirements for FiO2, PEEP, overall pressure support, and sedation, the more probability there would be for weaning success. The shorter amount of time a patient has been on PPV, the more probability for weaning success. Additionally, the lesser degree of cardiovascular and pulmonary disease present overall, with the absence of other organ failure (i.e., liver, kidney), the more probability for weaning success. But wait… there’s more. 

Below lists some of the more universally acceptable minimums for criteria that indicate further weaning and/or PPV removal readiness:

  • FiO2 less than 0.5
  • PEEP less than 8 cmH2O
  • Blood pH near normal range (7.35 – 7.45)
  • PaO2 consistently at least 60 mmHg*
  • PaCO2 consistently less than 50 mmHg*
  • SpO2 consistently at least 90%
  • PaO2/FiO2 preferably greater than 200
  • PAO2-PaO2 gradient < 200**
  • RR less than 30 bpm
  • Vt at least 4 mls/kg of predicted body weight
  • Rapid shallow breathing index (RSBI): RR/Vt lower than 120
  • Vital capacity at least 10ml/kg or predicted body weight (no less than 1L for most adults)
  • Negative inspiratory force at least – 20 cmH2O
  • Preferably, a endotracheal cuff leak should also be confirmed for orally/nasally intubated patients

*Patients with established baseline of chronic hypoxemia and/or chronic hypercarbia may wean successfully at lower than normal PaO2 and higher than normal PaCO2

**At higher FiO2 (> 0.5), A-a gradient may be acceptable at a value greater than 200

Certainly, the above list represents target values only in order to lend assurance that a patient will be more likely successful when being removed from PPV. Again, no one metric or value absolutely indicates guaranteed success. Furthermore, certain patients, albeit rare, that may meet all the above criteria, may still fail the weaning process or require reinstatement of PPV after removal due to confounding variables. 

The step-by-step method recommended for weaning is not universal and highly dependent upon the patient and scenario at hand. Typically, the sickest patient that has been on PPV the longest amount of time would require the most conservative, progressive approach to weaning. In other words, there is an inverse relationship between acuity level (how sick), duration of PPV (how long the patient has been mechanically ventilated) and the amount of time it would take to successfully wean a patient to the point of PPV removal. 

If oxygenation has been the primary issue, gradually adjusting/weaning settings such as PEEP, FiO2, inspiratory time, not necessarily in that order, should be the primary target. If ventilation has been the primary issue, gradually adjusting/weaning settings such as pressure control (if on pressure-targeting mode of PPV), tidal volume (if on volume-targeting mode of PPV), respiratory rate, or expiratory time (rarely an actual setting), not necessarily in that order, should be the primary target. Also, do not discredit a recent chest x-ray as it may provide strong evidence whether a patient is truly ready to be further weaned and/or removed from PPV. 

In summary, it cannot be over emphasized that the human body is dynamic and unpredictable. Certain patients seem to meet criteria for weaning and all looks well just prior to removal from PPV only to be the one that quickly fails and require immediate replacement on PPV. Other patients despite all the negative indicators, are weaned and finally given a chance to be taken off PPV and end up doing quite well against all odds.

A number is a metric, an estimate, or an indicator of some value that is only relevant within a certain context. Yes, use the arterial blood gas for reference, …yes, monitoring of vital signs is a must, but in the end, you will likely know your patient quite well. Remember, you’ve likely been at bedside and have been caring for that specific individual as well as manipulating the PPV for many hours, so be sensitive to the patient’s needs and do not hesitate to be their advocate. Don’t be afraid to speak up if the patient needs more time. Likewise, when appropriate, be proactive to suggest a patient be given a chance off of PPV. If the physician is not convinced, offer to conduct a short SBT. So, go with your gut, but in the end, the overall clinical picture should be emphasized more so than a single number. 

About Tim Gilmore, PhD, RRT, RRT-NPS, RRT-ACCS, CPFT, AE-C 4 Articles
Dr. Gilmore is an Associate Professor of Cardiopulmonary Science (CPS) and the CPS Program Director at Louisiana State University Health Shreveport. He holds a PhD in Health Science from Nova Southeastern University, a Masters in Health Sciences and BS in Cardiopulmonary Science from LSU Health Shreveport and an AS in Respiratory Therapy from Bossier Parish Community College (BPCC). Dr. Gilmore’s research interests include ICU care, mechanical ventilation, asthma, and patient education. He is a member of the Louisiana Society for Respiratory Care, American Association for Respiratory Care, and National Board for Respiratory Care. He is a Registered Respiratory Therapist with specialty credentials as a Neonatal Pediatric Specialist and Adult Critical Care Specialist, with national certifications as a Pulmonary Functions Technologist and Asthma Educator. He also serves as a peer reviewer for Respiratory Care Journal and an item-writer for the NBRC.