Pressure-controlled ventilation versus controlled mechanical ventilation with decelerating inspiratory flow

Comparison of respiratory mechanics measurements in the volume cycled ventilation (VCV) and pressure controlled ventilation (PCV)

Abstract Introduction Monitoring respiratory mechanics may provide important information for the intensivist, assisting in the early detection of pulmonary function changes of patients hospitalized in ICU. Objective: To compare measurements of respiratory mechanics in VCV and PCV modes, and correlate them with age and oxygenation index. Materials and methods: Cross-sectional study conducted in the adult ICU of the Hospital Nossa Senhora da Conceição, in Tubarão - SC. A hundred and twenty individuals were selected between March and August 2013. The respiratory mechanics measurements were evaluated using compliance and resistance static measures of the respiratory system in PCV and VCV modes between the 1st and 5th day of hospitalization. Simultaneously, the oxygenation index PaO2/FiO2 was collected. Results: The obtained results were: compliance (VCV) = 40.9 ± 12.8 mL/cmH2O, compliance (PCV) = 35.0 ± 10.0 mL/cmH2O, resistance (VCV) = 13.2 ± 4.9 cmH2O/L/s, resistance (PCV) = 27.3 ± 16.2 cmH2O/L/s and PaO2/FiO2 = 236.0 ± 97.6 mmHg. There was statistical difference (p < 0.001) between the compliance and resistance measures in VCV and PCV modes. The correlations between the oxygenation index and compliance in VCV and PCV modes and resistance in VCV and PCV modes were, respectively, r = 0.381 (p < 0.001), r = 0.398 (p < 0.001), r = -0.188 (p = 0.040), r = -0.343 (p < 0.001). Conclusion: Despite the differences between the respiratory mechanics measurements the monitoring using VCV and PCV modes seems to show complementary aspects.

Pressure-Controlled vs Volume-Controlled Ventilation in Acute Respiratory Failure: A Physiology-Based Narrative and Systematic Review

BACKGROUND

Mechanical ventilation is a cornerstone in the management of acute respiratory failure. Both volume-targeted and pressure-targeted ventilations are used, the latter modes being increasingly used. We provide a narrative review of the physiologic principles of these two types of breath delivery, performed a literature search, and analyzed published comparisons between modes.

METHODS

We performed a systematic review and meta-analysis to determine whether pressure control-continuous mandatory ventilation (PC-CMV) or pressure control-inverse ratio ventilation (PC-IRV) has demonstrated advantages over volume control-continuous mandatory ventilation (VC-CMV). The Cochrane tool for risk of bias was used for methodologic quality. We also introduced physiologic criteria as quality indicators for selecting the studies. Outcomes included compliance, gas exchange, hemodynamics, work of breathing, and clinical outcomes. Analyses were completed with RevMan5 using random effects models.

RESULTS

Thirty-four studies met inclusion criteria, many being at high risk of bias. Comparisons of PC-CMV/PC-IRV and VC-CMV did not show any difference for compliance or gas exchange, even when looking at PC-IRV. Calculating the oxygenation index suggested a poorer effect for PC-IRV. There was no difference between modes in terms of hemodynamics, work of breathing, or clinical outcomes.

CONCLUSIONS

The two modes have different working principles but clinical available data do not suggest any difference in the outcomes. We included all identified trials, enhancing generalizability, and attempted to include only sufficient quality physiologic studies. However, included trials were small and varied considerably in quality. These data should help to open the choice of ventilation of patients with acute respiratory failure.

Pressure-controlled mechanical ventilation is more advantageous in the follow-up of patients with chronic obstructive pulmonary disease after open heart surgery

OBJECTIVE

Cardiopulmonary bypass deteriorates pulmonary functions to a certain extent. Patients with chronic obstructive pulmonary disease (COPD) are associated with increased mortality and morbidity risks in the postoperative period of open-heart surgery. In this study we compared 2 different mechanical ventilation modes, pressure-controlled ventilation (PCV) and volume-controlled ventilation (VCV), in this particular patient population.

PATIENTS AND METHODS

Forty patients with severe COPD were assigned to 1 of 2 groups and enrolled to receive PCV or VCV in the postoperative period. Arterial blood gases, respiratory parameters, and intensive care unit and hospital stays were compared between the 2 groups.

RESULTS

Maximum airway pressure was higher in the VCV group. Pulmonary compliance was lower in the VCV group and minute ventilation was significantly lower in the group ventilated with PCV mode. The respiratory index was increased in the PCV group compared with the VCV group and with preoperative findings. Duration of mechanical ventilation was significantly shorter with PCV; however, intensive care unit and hospital stays did not differ.

CONCLUSION

There is not a single widely accepted and established mode of ventilation for patients with COPD undergoing open-heart surgery. Our modest experience indicated promising results with PCV mode; however, further studies are warranted.

Pressure modes of invasive mechanical ventilation

Pressure modes of invasive mechanical ventilation generate a tidal breath by delivering pressure over time. Pressure control ventilation (PC) is the prototypical pressure mode and is patient- or time-triggered, pressure-limited, and time-cycled. Other pressure modes include pressure support ventilation (PSV), pressure-regulated volume control (PRVC, also known as volume control plus [VC+]), airway pressure release ventilation (APRV), and biphasic ventilation (also known as BiLevel). Despite their complexity, modern ventilators respond to patient effort and respiratory system mechanics in a fairly predictable fashion. No single mode has consistently demonstrated superiority in clinical trials; however, empiric management with a pressure mode may achieve the goals of patient-ventilator synchrony, effective respiratory system support, adequate gas exchange, and limited ventilator-induced lung injury.

Severe hypoxemic respiratory failure: part 1--ventilatory strategies

Approximately 16% of deaths in patients with ARDS results from refractory hypoxemia, which is the inability to achieve adequate arterial oxygenation despite high levels of inspired oxygen or the development of barotrauma. A number of ventilator-focused rescue therapies that can be used when conventional mechanical ventilation does not achieve a specific target level of oxygenation are discussed. A literature search was conducted and narrative review written to summarize the use of high levels of positive end-expiratory pressure, recruitment maneuvers, airway pressure-release ventilation, and high-frequency ventilation. Each therapy reviewed has been reported to improve oxygenation in patients with ARDS. However, none of them have been shown to improve survival when studied in heterogeneous populations of patients with ARDS. Moreover, none of the therapies has been reported to be superior to another for the goal of improving oxygenation. The goal of improving oxygenation must always be balanced against the risk of further lung injury. The optimal time to initiate rescue therapies, if needed, is within 96 h of the onset of ARDS, a time when alveolar recruitment potential is the greatest. A variety of ventilatory approaches are available to improve oxygenation in the setting of refractory hypoxemia and ARDS. Which, if any, of these approaches should be used is often determined by the availability of equipment and clinician bias.

Comparison of volume-controlled and pressure-controlled ventilation during laparoscopic gastric banding in morbidly obese patients

BACKGROUND

There are no guidelines on ventilation modes in morbidly obese patients. We investigated the effects of volume-controlled (VCV) and pressure-controlled ventilation (PCV) on gas exchange, respiratory mechanics, and cardiovascular responses in laparoscopic gastric banding procedures.

METHODS

After Institutional Review Board approval, 24 adult consenting patients scheduled for laparoscopic gastric banding were studied. Anesthesia was standardized using remifentanil, propofol, rocuronium, and sevoflurane. All patients started with VCV with a tidal volume of 10 ml kg(-1) ideal body weight, respiratory rate adjusted to obtain an end-tidal carbon dioxide of 35-40 mmHg, positive end-expiratory pressure of 5 cmH2O, an inspiratory pause of 10% and an inspiratory/expiratory ratio of 1:2. Fifteen minutes after pneumoperitoneum, the patients were randomly allocated to two groups. In Group VCV (n = 12), ventilation was with the same parameters. In Group PCV (n = 12), the airway pressure was set to provide a tidal volume of 10 ml kg(-1) ideal body weight without exceeding 35 cm H2O. Respiratory rate was adjusted to keep an end-tidal carbon dioxide of 35-40 mmHg. Arterial blood samples were drawn after surgical positioning and 15 min after allocation. Analysis of variance (ANOVA) was used for statistical analysis.

RESULTS

With constant minute ventilation, VCV generates equal airway pressures and cardiovascular effects with a lower PaCO2 as compared to PCV (42.5 (5.2) mmHg versus 48.9 (4.3) mmHg, p < 0.01 ANOVA). Arterial oxygenation remained unchanged.

CONCLUSIONS

VCV and PCV appear to be an equally suited ventilatory technique for laparoscopic procedures in morbidly obese patients. Carbon dioxide elimination is more efficient when using VCV.

Pulmonary contusions and critical care management in thoracic trauma

Many victims of thoracic trauma require ICU care and mechanical ventilatory support. Pressure and volume-limited modes assist in the prevention of ventilator-associated lung injury. Ventilator-associated pneumonia is a significant cause of posttraumatic morbidity and mortality. Minimizing ventilator days, secretion control, early nutritional support, and patient positioning are methods to reduce the risk of pneumonia.

Pressure control ventilation

As mechanical ventilators become increasingly sophisticated, clinicians are faced with a variety of ventilatory modes that use volume, pressure, and time in combination to achieve the overall goal of assisted ventilation. Although much has been written about the advantages and disadvantages of these increasingly complex modalities, currently there is no convincing evidence of the superiority of one mode of ventilation over another. Pressure control ventilation may offer particular advantages in certain circumstances in which variable flow rates are preferred or when pressure and volume limitation is required. The goal of this article is to provide clinicians with a fundamental understanding of the dependent and independent variables active in pressure control ventilation and describe features of the mode that may contribute to improved gas exchange and patient-ventilator synchronization.

The open lung concept of mechanical ventilation: the role of recruitment and stabilization

This article describes the pathophysiologic basis and clinical role for lung recruitment maneuvers. It reviews the literature and presents the authors' clinical experience of over 15 years in the collaboration between Erasmus MC and the University of Rochester. The authors are hopeful that these lung-protective strategies are presented in a useful format that may be useful to the practicing intensivist, thus bringing laboratory and clinical research to bedside practice.

Ventilation-Perfusion Distribution Related to Different Inspiratory Flow Patterns in Experimental Lung Injury

In acute lung injury (ALI), controlled mechanical ventilation with decelerating inspiratory flow (.V(dec)) has been suggested to improve oxygenation when compared with constant flow (.V(con)) by improving the distribution of ventilation and perfusion (.V(A)/.Q). We performed the present study to test this hypothesis in an animal model of ALI. Furthermore, the effects of combined decelerating and constant flow (Vdot;(deco)) were evaluated. Thus, 18 pigs with experimental ALI were randomized to receive mechanical ventilation with either .V(con), .V(dec) or a fixed combination of both flow wave forms (.V(deco)) at the same tidal volume and positive end-expiratory pressure level for 6 h. Hemodynamics, gas exchange, and .V(A)/.Q distribution were determined. The results revealed an improvement of oxygenation resulting from a decrease of pulmonary shunt within each group (P < 0.05). However, blood flow to lung areas with a normal .V(A)/.Q distribution increased only during ventilation with .V(con) (P < 0.05). Accordingly, PaO(2) was higher with .V(con) than with .V(dec) and .V(deco) (P < 0.05). We conclude that contrary to the hypothesis, .V(con)provides a more favorable .V(A)/.Q distribution, and hence better oxygenation, when compared with .V(dec) and .V(deco) in this model of ALI.

IMPLICATIONS

In acute lung injury, mechanical ventilation with decelerating flow has been suggested to improve ventilation-perfusion distribution when compared with constant flow. We tested this hypothesis in an animal model. Contrary to the hypothesis, we found a more favorable ventilation-perfusion distribution during constant flow when compared with decelerating flow.

Ventilation in the patient with unilateral lung disease

Severe ULD presents a challenge in ventilator management because of the marked asymmetry in the mechanics of the two lungs. The asymmetry may result from significant decreases or increases in the compliance of the involved lung. Traditional ventilator support may fail to produce adequate gas exchange in these situations and has the potential to cause further deterioration. Fortunately, conventional techniques can be safely and effectively applied in the majority of cases without having to resort to less familiar and potentially hazardous forms of support. In those circumstances when conventional ventilation is unsuccessful in restoring adequate gas exchange, lateral positioning and ILV have proved effective at improving and maintaining gas exchange. Controlled trials to guide clinical decision making are lacking. In patients who have processes associated with decreased compliance in the involved lung, lateral positioning may be a simple method of improving gas exchange but is associated with many practical limitations. ILV in these patients is frequently successful when differential PEEP is applied with the higher pressure to the involved lung. In patients in whom the pathology results in distribution of ventilation favoring the involved lung, particularly BPF, ILV can be used to supply adequate support while minimizing flow through the fistula and allowing it to close. The application of these techniques should be undertaken with an understanding of the pathophysiology of the underlying process; the reported experience with these techniques, including indications and successfully applied methods; and the potential problems encountered with their use. Fortunately, these modalities are infrequently required, but they provide a critical means of support when conventional techniques fail.

The physiologic effects of inverse ratio ventilation

STUDY OBJECTIVES

The efficacy of inverse ratio ventilation in ARDS is not clear. Furthermore, the mechanism responsible has not been determined. We designed an animal study to determine if inverse ratio ventilation improves gas exchange and by what mechanism.

DESIGN

Prospective randomized, controlled design was used.

SETTING

University of Missouri Pulmonary/Critical Care Animal Laboratory.

PARTICIPANTS

Nine dogs with oleic acid-induced lung injury as control animals to assess stability of the model, nine in the experimental model.

INTERVENTIONS

Conventional ventilation with full recruitment extrinsic positive end-expiratory pressure (PEEP) was compared with two other modes of ventilation. One was inverse ratio with extrinsic PEEP and the second was inverse ratio with intrinsic PEEP equal to full recruitment PEEP. Full recruitment levels of PEEP were defined by optimizing compliance, then increasing PEEP by 2.5 cm/H2O. Each type of ventilation was maintained for 45 min after the edema had stabilized. Comparison of lung injury over time requires stability of the model over time. Therefore, we also assessed the stability of the preparation over time by examining compliance, extravascular lung water, and venous admixture in nine control dogs with equivalent lung injury over the same time span.

MEASUREMENTS AND RESULTS

Mean airway pressure was increased by both types of inverse ratio ventilation, while compliance remained stable. Venous admixture was reduced (conv=0.32+/-0.12, inverse ratio with extrinsic PEEP=0.24+/-0.10, inverse ratio with intrinsic PEEP=0.28+/-0.11) with inverse ratio with extrinsic PEEP, but the improvement was less with inverse ratio with intrinsic PEEP, even though the mean airway pressure was higher.

CONCLUSIONS

We conclude that increasing mean airway pressure by prolongation of the inspiratory time improves gas exchange in our model of ARDS, but when mean airway pressure is increased further, allowing the development of intrinsic PEEP, the beneficial effects on gas exchange are less. Increasing mean airway pressure with intrinsic PEEP is not equivalent to other methods of increasing mean airway pressure.

Decelerating flow ventilation effects in acute respiratory failure

PURPOSE

The purpose of this article is to analyze the effect of a pressure-regulated volume-controlled ventilation mode on lung mechanics and gas exchange in patients with acute respiratory failure.

MATERIALS AND METHODS

We ventilated 10 patients with two pressure-limited modes: pressure-controlled ventilation (PC) and pressure-regulated volume-controlled ventilation (PRVC) in random order, for 1 hour each. Patients were stabilized on volume-controlled ventilation (VC) for 30 minutes before, between, and at the end of PC and PRVC to reach baseline conditions. At the end of every VC period and at 30 and 60 minutes of PC and PRVC, respiratory mechanics, gasometrics, and hemodynamic parameters were collected.

RESULTS

We found no significant differences between the three VC periods. Comparing VC with the two pressure-limited ventilation modes, peak pressure decreased from 29.4+/-9.1 cm H2O (VC) to 25.9+/-8.4 (PC 60 minutes) and 26.1+/-8.2 (PRVC 60 minutes), and PaCO2 decreased significantly from 38.6+/-3.1 mm Hg (VC) to 36.7+/-2.8 (PC 60 minutes) and 36.8+/-2.9 (PRVC 60 minutes).

CONCLUSIONS

Pressure-limited ventilation allows mechanical ventilation for the same tidal volume as VC but results in a lower peak inspiratory pressure and a slightly lower PaCO2. The mechanism responsible for this gas exchange effect is unknown but is probably related to a better air distribution of the decelerated flow. The clinical relevance of this phenomenon remains to be established.

Pressure-controlled ventilation in ARDS: a practical approach

Patients with ARDS typically have functionally small lungs. A growing body of clinical and experimental evidence has demonstrated that mechanical ventilation that results in high transpulmonary pressure gradients and overdistention of lung units will potentiate the acute lung injury in patients with ARDS. A relative form of "lung rest" using low tidal volume mechanical ventilation that prevents alveolar overdistention has therefore been advocated. This may be achieved with low-volume, volume-cycled ventilation with a decelerating inspiratory flow or pressure-controlled ventilation (PCV). The goal of this article is to provide a simple and practical approach to the management of PCV in patients with ARDS. Implicit in our approach is the use of a ventilator with PCV software and waveform capabilities.

[Current aspects of acute respiratory distress syndrome in children]

Acute respiratory distress syndrome (ARDS) is a frequent condition in pediatric intensive care units. The mortality remains high despite advances in conventional mechanical ventilation and aetiological treatment. Several animal studies have documented lung injury during mechanical ventilation with high tidal volume, and clinical investigations have shown that in human ARDS, most ventilation is distributed to the small areas of remaining aerated lung resulting in overdistension of these areas and lung injury ("baby lung" theory). Nevertheless the usefulness of extrapulmonary gas exchange remains much debated. New ventilatory strategies have been developed in order to reduce ventilator-induced lung injury and to improve systemic oxygenation but multicentric randomized clinical trials are needed before these strategies can be validated.

Comparison of volume control and pressure control ventilation: is flow waveform the difference?

OBJECTIVE

To examine the hypothesis that a decelerating inspiratory flow waveform is responsible for improvements in gas exchange during pressure control ventilation for acute lung injury.

DESIGN

Prospective, controlled, crossover study.

MEASUREMENTS AND MAIN RESULTS

Twenty-five patients with acute lung injury requiring mechanical ventilation with a positive-end expiratory pressure > or = 10 cm H2O, ventilator frequency of > or = 8 bpm, inspired oxygen concentration of > or = 0.50, peak inspiratory pressure > or = 40 cm H2O, and requiring sedation and paralysis were studied. Patients were ventilated at a tidal volume of 10 mliters/kg, respiratory frequency was set to maintain a pH > 7.30 and PaCO2 < 50 mm Hg, and positive end-expiratory pressure (PEEP) set to maintain Pao2 > 70 mm Hg or Sao2 > 93% with an Fio2 < or = 0.50. In random sequence, ventilator mode was changed from volume control with a square flow waveform, pressure control ventilation with a decelerating flow waveform, or volume control ventilation with a decelerating flow waveform. Tidal volume, minute ventilation, and airway pressures were continuously measured at the proximal airway. After 2 hours of ventilation in each mode, arterial and mixed venous blood gases were drawn and cardiac output determined by thermodilution. Dead space to tidal volume ratio was determined from mixed expired gas concentrations and Paco2. During volume control ventilation with a square flow waveform, Pao2 was decreased (75 +/- 11 mm Hg vs. 85 +/- 9 mm Hg and 89 +/- 12 mm Hg), p < 0.05, and peak inspiratory pressure was increased (50 +/- 9 cm H2O vs. 42 +/- 7 cm H2O and 39 +/- 9 cm H2O) p < 0.05 compared to volume control with a decelerating flow waveform and pressure control ventilation. Mean airway pressure was also lower with volume control with a square flow waveform (17 +/- 4 cm H2O vs. 20 +/- 4 cm H2O and 21 +/- 3 cm H2O) compared to volume control with a decelerating flow waveform and pressure control ventilation. There were no differences in hemodynamic parameters.

CONCLUSIONS

Both pressure control ventilation and volume control ventilation with a decelerating flow waveform provided better oxygenation at a lower peak inspiratory pressure and higher mean airway pressure compared to volume control ventilation with a square flow waveform. The results of our study suggest that the reported advantages of pressure control ventilation over volume control ventilation with a square flow waveform can be accomplished with volume control ventilation with a decelerating flow waveform.

Pressure-controlled and volume-cycled mechanical ventilation

Pressure and volume modes of mechanical ventilation are available as options in the current generation of ventilators, giving clinicians many choices when managing a mechanically ventilated patient. In volume cycled ventilation, tidal volume is set and airway pressures are measured, whereas in pressure-controlled ventilation, pressure is set and volume is measured. This article reviews the characteristics of these two ventilatory modes and discusses in detail conversion from one mode to the other. Pertinent clinical studies and recent direct comparisons of volume-cycled and pressure-controlled ventilation are reviewed.

Ventilatory management of pulmonary contusion patients

BACKGROUND

The goal of this study was to evaluate two modes of mechanical ventilation in patients with pulmonary contusion: pressure-controlled ventilation (PCV) and volume-controlled ventilation (VCV).

METHODS

One hundred and thirty-five patients with pulmonary contusion, defined as an infiltrate on admission chest x-ray and hypoxemia, were treated over 45 months; 59 patients who required more than 48 hours of mechanical ventilation were initially managed with VCV.

RESULTS

Twenty patients were converted from VCV to PCV when pulmonary function deteriorated. With PCV, peak inspiratory pressure decreased from 49 +/- 1 to 31 +/- 1 cm H2O, the alveolar-arterial oxygen difference decreased from 491 +/- 36 mm Hg to 300 +/- 36 mm Hg. These findings were significantly different (P < 0.05, by Student's paired t-test). Twenty patients managed with PCV had equivalent duration of mechanical ventilation and days in intensive care units to 39 patients with less pulmonary dysfunction managed with VCV. None of the 10 patients who died expired from pulmonary failure.

CONCLUSIONS

PCV is an alternative mode to VCV in patients with poorly compliant lungs after pulmonary contusion.

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.