Interaction between intrinsic positive end-expiratory pressure and externally applied positive end-expiratory pressure during controlled mechanical ventilation

Central airway occlusion underestimates intrinsic positive end-expiratory pressure: a numerical and physical simulation

Intrinsic positive end-expiratory pressure (PEEP) occurs when airway outflow is higher than zero at end-expiration. Differences in the time constant among alveolar units may result in an uneven distribution of intrinsic PEEP. The authors conducted a computer simulation of a 2-compartment respiratory system and calculated intrinsic PEEP for each alveolar unit and confirmed it with a test-lung experiment. Ventilator settings, including respiratory rate, inspiratory time, pause time, and external PEEP, were tested at various values in combination with various airway resistance and alveolar compliance values. The simulation was performed by calculating the flow, pressure, and volume every millisecond. The data demonstrated that the larger the difference of time constant between 2 respiratory units, the greater the difference in intrinsic PEEP between the units. A higher respiratory frequency and a larger percentage of inspiratory time resulted in an increase in the intrinsic PEEP at the central airway, as well as a wide difference in the intrinsic PEEP between airway units. These phenomena were confirmed by a 2-compartment test-lung study. The authors demonstrated and verified an uneven distribution of intrinsic PEEP in 2 different experiments, which raised a warning that some respiratory units might have much higher intrinsic PEEP than the intrinsic PEEP measured clinically.

“The Use of Positive End-Expiratory Pressure in Mechanical Ventilation”

An improvement in oxygenation for patients who have acute respiratory failure using PEEP was described close to 40 years ago. Since then, a considerable amount of research has allowed clinicians to use this therapeutic modality in various ways. In patients receiving mechanical ventilation, the term positive end-expiratory pressure (PEEP) refers to pressure in the airway at the end of passive expiration that exceeds atmospheric pressure. The use of PEEP mainly has been reserved to recruit or stabilize lung units and improve oxygenation in patients who have hypoxemic respiratory failure. It has been shown that this helps the respiratory muscles to decrease the work of breathing and the amount of infiltrated-atelectatic tissues. The beneficial effects of the use of PEEP include: the improvement of oxygenation, recruitment of lung units, and improvement of compliance. Other effects can be adverse, like decreasing cardiac output, increased risk of barotrauma, and the interference with assessment of hemodynamic pressures.

Bench-to-bedside review: adjuncts to mechanical ventilation in patients with acute lung injury

Mechanical ventilation is indispensable for the survival of patients with acute lung injury and acute respiratory distress syndrome. However, excessive tidal volumes and inadequate lung recruitment may contribute to mortality by causing ventilator-induced lung injury. This bench-to-bedside review presents the scientific rationale for using adjuncts to mechanical ventilation aimed at optimizing lung recruitment and preventing the deleterious consequences of reduced tidal volume. To enhance CO₂ elimination when tidal volume is reduced, the following are possible: first, ventilator respiratory frequency can be increased without necessarily generating intrinsic positive end-expiratory pressure; second, instrumental dead space can be reduced by replacing the heat and moisture exchanger with a conventional humidifier; and third, expiratory washout can be used for replacing the CO₂-laden gas present at end expiration in the instrumental dead space by a fresh gas (this method is still experimental). For optimizing lung recruitment and preventing lung derecruitment there are the following possibilities: first, recruitment manoeuvres may be performed in the most hypoxaemic patients before implementing the preset positive end-expiratory pressure or after episodes of accidental lung derecruitment; second, the patient can be turned to the prone position; third, closed-circuit endotracheal suctioning is to be preferred to open endotracheal suctioning.

Ventilation of patients with asthma and obstructive lung disease

Mechanical ventilation in a patient with obstructive airway disease may be a lifesaving measure; however, it may also be associated with significant morbidity and mortality. It is important for a physician to be familiar with the potential complications of mechanical ventilation in this group of patients and to know how to avoid them by carefully applying safe ventilator strategies. The cornerstone of such strategies is to minimize minute ventilation, maximize time for expiration, and avoid hyperinflation of the lung. Several bedside parameters (iPEEP, VEI, Pplat) that reflect presence of gas trapping and potential hyperinflation may be measured. In addition to mechanical ventilation, management should include inhaled bronchodilators and systemic corticosteroid therapies. In the event controlled hypoventilation is necessary, sedation with or without the use of muscle relaxants may be required. Unconventional therapies such as the use of Heliox, magnesium sulfate, ketamine, and inhalational anesthetics may be attempted in severe cases that do not respond to conventional management. With appropriate use of ventilator strategies, a reduction in the mortality and morbidity of patients with obstructive airway disease requiring mechanical ventilation has recently been noted.

An intrinsic positive end-expiratory pressure lung model, with and without flow limitation

OBJECTIVE

To design an intrinsic positive end-expiratory pressure (PEEP) lung model that has the property of air flow limitation.

DESIGN

Mechanical lung model study of intrinsic PEEP.

SETTING

Lung models were set up in the research laboratory.

INTERVENTIONS

Intrinsic PEEP lung models were created with and without flow limitation. In the model with flow limitation, intrinsic PEEP was created by replacing a portion of the expiratory circuit with a collapsible Penrose tube and by placing this portion the circuit under water. The expiratory circuit became a part of the respiratory airway, with flow limitation occurring at the Penrose drain. In the model without flow limitation, intrinsic PEEP was generated with a fixed linear resistor, which was inserted in the expiratory circuit to produce a similar level of intrinsic PEEP. Multiple levels of external PEEP, both above and below the initial intrinsic PEEP, were applied.

MEASUREMENTS AND MAIN RESULTS

At each level of external PEEP, peak airway pressure, plateau airway pressure, isovolume air flow, internal lung pressure, and intrinsic PEEP were measured. Peak airway pressure, plateau pressure, and internal lung pressure were minimally affected if the external PEEP was less than the intrinsic PEEP in the lung model with flow limitation. Intrinsic PEEP was reduced with external PEEP. However, if intrinsic PEEP was induced without dynamic airway closure or flow limitation, any level of external PEEP caused an immediate increase in peak airway pressure, plateau airway pressure, and internal lung pressure and a decrease in isovolume flow. External PEEP has little effect on the levels of intrinsic PEEP.

CONCLUSIONS

We demonstrated two different models of an intrinsic PEEP lung model. The interactions between intrinsic PEEP and externally applied PEEP were different. The lung model with collapsible tube closely simulated the human respiratory system with flow limitation. This lung model may be useful for the future study of intrinsic PEEP and pulmonary mechanics.

Volume-controlled versus biphasic positive airway pressure ventilation in leukopenic patients with severe respiratory failure

OBJECTIVE

To study comparatively the effects of volume-controlled vs. biphasic positive airway pressure mechanical ventilation on respiratory mechanics and oxygenation in leukopenic patients with severe respiratory failure.

DESIGN

Prospective, comparative study.

SETTING

Medical intensive care unit of a university hospital.

PATIENTS

Leukopenic (<1000 leukocytes/microliter) patients (n=20) after cytoreductive chemotherapy requiring mechanical ventilation for severe respiratory failure (Murray score of > 2.5).

INTERVENTION

Patients were assigned in a consecutive, alternating manner to receive either volume-controlled or biphasic positive airway pressure mechanical ventilation, starting within 12 to 24 hrs after endotracheal intubation.

MEASUREMENTS AND MAIN RESULTS

Tidal volume, inspiratory flow, peak inspiratory and positive end-expiratory pressures, FIO2, and arterial blood gas analyses were recorded hourly for a study period of 48 hrs. Biphasic positive airway pressure ventilation was associated with a significant reduction in peak inspiratory pressure (mean differences at 24, 36, and 48 hrs: 4.4, 3.4, and 4.2 cm H2O; p = .024, .019, and .013, respectively) and positive end-expiratory pressures (mean differences at 24, 36, and 48 hrs: 1.6, 1.4, and 1.5 cm H20; p = .023, .024, and .023, respectively) at significantly lower FIO2 (mean differences at 12, 24, 36, and 48 hrs; p = .007, .015, .016, and .011, respectively). PaO2/FIO2 ratios and CO2 removal were similar under ventilatory conditions.

CONCLUSIONS

Biphasic positive airway pressure ventilation offers the advantage of significantly reduced peak inspiratory and positive end-expiratory pressures at a lower FIO2 and with at least similar oxygenation and CO2 removal as achieved by volume-controlled mechanical ventilation. Our results are in line with previous reports on nonleukopenic patients and suggest that the positive effects of pressure-limited mechanical ventilation are independent of circulating white blood cells. Further studies are mandatory to demonstrate clinical benefit in this critically ill patient population.

Variability of intrinsic positive end-expiratory pressure in patients receiving mechanical ventilation

OBJECTIVE

Since variations in breathing pattern may affect the level of intrinsic positive end-expiratory pressure (PEEP), breath-to-breath variation of intrinsic PEEP was assessed.

DESIGN

Descriptive and prospective study.

SETTING

Medical intensive care unit of a university teaching hospital.

PATIENTS

Thirty-four patients requiring mechanical ventilation for a period of time due to respiratory failure.

MEASUREMENTS AND MAIN RESULTS

Intrinsic PEEP was determined using simultaneous recordings of the esophageal pressure and airflow. The breath-to-breath intrinsic PEEP, respiratory rate, tidal volume, inspiratory time, and fractional inspiratory time were measured. Intrinsic PEEP was noted in 33 of 34 patients. For all patients, the mean intrinsic PEEP was 3.59 cm H2O. The group mean standard deviation (SD) of the intrinsic PEEP over 35 breaths was 2.68 cm H2O. In 17 chronic obstructive pulmonary disease patients, the mean intrinsic PEEP was 4.69 cm H2O and the group mean SD of the intrinsic PEEP was 3.19 cm H2O. In the subgroup of patients with significant intrinsic PEEP, the mean intrinsic PEEP was 6.69 cm H2O and the group mean SD was 4.29 cm H2O. The group mean coefficient of variation of intrinsic PEEP for all 34 patients was 123%. Among the 15 patients with clinically significant intrinsic PEEP, the coefficient of variation was smaller (74%). We did not find significant correlation between the coefficients of variation of breathing pattern parameters and the coefficients of variation of intrinsic PEEP.

CONCLUSIONS

We conclude that the occurrence rate of intrinsic PEEP in mechanically ventilated patients is high. The degree of variability in intrinsic PEEP on a breath-to-breath basis is also high. It may be difficult to find a specific level of intrinsic PEEP. Addition of external positive end-expiratory pressure without considering the breath-to-breath variability may lead to overdistention of the lung.