Intensivists and respiratory therapists encounter cases of increased peak pressure in patients who are mechanically ventilated on the volume control mode of ventilation. This could be due to a simple problem such as secretions that require suctioning, or it could be related to a life-threatening problem such as pneumothorax that requires immediate intervention.
The peak airway pressure (Ppeak) is the highest value attained by the ventilator waveform during inspiration and is required to overcome airway resistance and lung and chest wall elastance (dynamic state). The plateau airway pressure (Pplat) is the pressure required to overcome the lung and chest wall elastance after the flow of the air has ceased and the patient's breath is held in inspiration (static state). Additionally, the Ppeak is related to the flow rate when ventilation is in volume control mode, with higher pressure resulting from a higher flow rate.
The equation of motion mandates a balance between the change of pressure on the left side of the equation (Ppeak-PEEP) and the resistance, flow, volume, and compliance on the right side of the formula in a square waveform/volume control mode of ventilation:
Peak inspiratory pressure -PEEP = (Resistance x Flow) + (Elastance of respiratory system x Tidal volume) (formula 1)
If you move the PEEP to the right side of the equation, it will look like this:
Peak inspiratory pressure = (Resistance x Flow) + (Elastance of respiratory system x Tidal volume) + PEEP (formula 2)
Therefore, an increase in Ppeak can arise from increased resistance, increased flow, increased tidal volume, increased elastance (or decreased compliance) of the lung or chest wall, and increased positive end-expiratory pressure (PEEP), including intrinsic PEEP (PEEPi). The intensivist should ensure that the patient is hemodynamically stable and able to oxygenate adequately first then take a methodical and sequential approach to assess and manage increased Ppeak which includes the following:
Review the ventilator settings and be sure that they are appropriate for the patient.
Perform an inspiratory hold maneuver to detect which component of the Ppeak is elevated.
Perform an expiratory hold maneuver to detect the presence of intrinsic PEEP responsible for the increased Ppeak.
Finally, check the esophageal pressure to differentiate between increased resistance due to lung problems versus chest wall or abdominal problems.
Review the adequacy of tidal volume ( VT), flow rate, and PEEP settings
As it is clear in formula 2, if the tidal volume, flow rate, or extrinsic PEEP are set higher than what is appropriate, then peak inspiratory pressure will be high. The intensivist should insure adequate tidal volume and PEEP as per ideal body weight and within the limitation of the protective lung strategy. The flow should be set adequately to prevent air hunger and allow enough time for full expiration.
Perform an inspiratory hold maneuver
The inspiratory hold maneuver is a method for detecting the Pplat, since it eliminates the impact of airway resistance and the flow on the tracheal pressure. This is available on virtually every ventilator and involves manually overriding the expiratory valve, forcing it to close, and putting an entire static volume strain on the respiratory system (including the ventilator circuit). The peak and plateau airway pressures enable us to understand resistance and compliance changes in volume control ventilation.
In patients with increased resistance, the Ppeak pressure is increased but the Pplat remains the same. This could be caused by issues in the ventilator settings, ventilatory circuit, tubing, or natural airways. Issues in the ventilatory circuit include a kink in the circuit or fluid accumulation and require examining the circuit, releasing the kink or twist, or clearing the water condensate. Issues in the tube could be secondary to bitting on the tube, a biofilm-forming in a small ETT, mucus plugging, or herniation of the balloon requiring increased sedation, suctioning or changing the ETT. Bronchospasm, increased secretions, mucus plugging, or foreign body are the causes of increased resistance in the natural airway and require bronchodilators, suctioning, or removal of the foreign body. Finally, setting a high flow rate on the ventilator or the occurrence of patient-ventilatory asynchrony can cause a high peak inspiratory pressure and require adjustment of the ventilator settings or increased sedation.
If both the Ppeak pressure and the Pplat pressures are increased with no change in the difference between Ppeak and Pplat, then the issue is the compliance of the lung or the chest wall and abdomen. There is no way to differentiate if the elastance increase (the reverse of compliance) without having an esophageal pressure monitor, but first, you need to rule out an increase in the intrinsic PEEP as a cause of increased Pplat, therefore, an expiratory hold is needed.
Perform an expiratory hold maneuver
If the inspiratory hold maneuver revealed an elevated Ppeak and Pplat, then the intensivist should first rule out the possibility of intrinsic PEEP contributing to the baseline pressure. This is done with an expiratory hold maneuver and observing the rise of the baseline pressure in expiration. The presence of PEEPi will elevate the baseline pressure expiratory pressure and peak inspiratory pressure with the same value. Maneuvers to eliminate PEEPi include prolongation of the expiratory time by changing the flow delivery rate and inspiratory time, and decreasing tidal volume and respiratory rate.
Check the esophageal pressure (Peso) (if available)
If the expiratory hold maneuver did not reveal an elevated intrinsic PEEP and the Pplat is elevated then the increased Ppeak is caused by low compliance of the respiratory system. Esophageal pressure monitoring may not be available in your ICU, but if it is available, then the intensivist will be able to determine whether the decreased compliance is due to problems in the lung parenchyma as opposed to problems in the pleura, chest wall, or abdomen.
Do you have the ability to measure esophageal pressure in your ICU?
Yes
No
Are you kidding!
If both Pplat and Peso are elevated then the problem is in the pleura, chest wall, or abdomen:
Pleural issues: pneumothorax, pleural effusion, or hemothorax.
Chest wall issues: burns, fentanyl-induced chest wall rigidity (wooden chest syndrome), or the application of thoracic lumbar sacral orthosis (TLSO).
Abdominal issues: massive ascites or abdominal compartment syndrome.
On the other hand, if Pplat is elevated with a normal Peso, then the problem is in the lung parenchyma and may include:
Parenchymal issues: pneumonia, pulmonary edema, ARDS, pulmonary hemorrhage, atelectasis or pulmonary fibrosis.
Right main stem intubation.
The management of the above conditions varies from one condition to another and is not within the scope of this review.
Summary
Patients on the volume control mode of ventilation may have an increased peak pressure that could be due to increased airway resistance, increased lung elastance, or increased pleura, chest wall, or abdominal elastance. Systematically approaching this problem requires collaboration between intensivists and respiratory therapists who can work together to provide the best care for these patients.
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