Synchronized Intermittent Mandatory Ventilation

Anatomy and Physiology

Synchronized intermittent mandatory ventilation is a mode that comes installed on many ventilators. When SIMV was first invented, it required a "tee-piece" assembly modification to an existing ventilator. The "tee" is a corrugated tubing that connects to a heated nebulizer. One limb of the "tee" is open to the air and connected to a patient's port that has a one-way valve. The valve inserts into a hole drilled into the inspiratory limb, or "Y-piece," of the ventilator. This design allows the valve to close when the ventilator is on, delivering a preset tidal volume to the patient. When the patient takes a spontaneous breath, the valve opens and allows for inhalation of gas from the "tee-piece."[1] Newer generations of ventilators can support SIMV using a closed ventilator circuit with the assistance of the machine's microprocessor.[3]

Indications

Synchronized intermittent mandatory ventilation is typically used to help wean patients from the ventilator.[4] From a physiologic standpoint, SIMV has the advantage of avoiding acute respiratory alkalosis by allowing patients to achieve normal alveolar ventilation through an intact ventilatory drive.[5] One concern when using SIMV is that it can result in an increased work of breathing. One way to counteract this is by adding pressure support to the SIMV.[6] Mechanical ventilation, in general, is indicated in severe hypoxic and hypercapnic respiratory failure, often after a failed trial of non-invasive ventilation.[7]

Contraindications

Synchronized intermittent mandatory ventilation is a ventilator mode that enables partial mechanical assistance. This ventilator mode will provide a set number of breaths at a fixed tidal volume, but a patient can trigger a spontaneous breath with the volume determined by patient effort.[8] The maximal benefits of SIMV may only be realized by a patient who can take a spontaneous breath.

Equipment

Synchronized intermittent mandatory ventilation requires a ventilator that is programmable for the mode.

Personnel

Like any other mode of mechanical ventilation, SIMV requires trained respiratory therapists to monitor the ventilator and physician oversite.

Preparation

Preparation for ventilation with SIMV is similar to other modalities of mechanical ventilation. A patient must first have an advanced airway in place, and the patient must show improvement in their respiratory status with a plan to start the weaning process.

Technique or Treatment

Once the patient is ready to initiate the weaning process, it requires the appropriate settings on the ventilator. The parameters include tidal volume, respiratory rate, positive end-expiratory pressure (PEEP), the fraction of inspired oxygen (FiO2), and, if used, the pressure support setting. After the initiation of mechanical ventilation, it is best practice to obtain an arterial blood gas within 60 minutes and to titrate the ventilator settings accordingly.[9]

SIMV is rarely used for weaning. A survey of intensivists from various geographic regions showed SIMV was used 0 to 6% for weaning, depending on the region. More common methods of weaning are pressure support with PEEP (regional range of 56.5 to 72.3%) and T-piece (regional range of 8.9 to 59.5).[10]

Complications

Complications affecting patients undergoing mechanical ventilation include ventilator-associated pneumonia (VAP), barotrauma, acute respiratory distress syndrome (ARDS), pneumothorax, atelectasis, and post-extubation stridor. VAP is generally defined as a new persistent infiltrate on chest radiograph after a patient has been on mechanical ventilation for at least 48 hours, with at least three of the following associated symptoms: fever, leukopenia/leukocytosis, increased sputum production, rales, cough, or worsening gas exchange. ARDS is generally defined using the Berlin definition. The definition requires the measurement of the partial pressure of oxygen on a blood gas compared to the fraction of inspired oxygen the patient is currently receiving. There are three stages of ARDS: mild with a PaO2/FiO2 ratio less than or equal to 300 mm Hg, moderate with PaO2/FiO2 less than or equal to 200 mm Hg, and severe with PaO2/FiO2 less than or equal to 100 mm Hg.[11]

A study of pediatric patients in Egypt showed 39.9% of patients experienced complications, which equates to 29.5 complications per 1000 ventilation days. The complications were broken down into VAPs (27.3% or 20.19/1000 ventilator days), pneumothorax (10.6% or 7.82/1000 ventilator days), atelectasis (4.4% or 3.28/1000 ventilator days), and post-extubation stridor (2.4% or 1.76/1000 ventilator days).[12]

Asynchrony is another complication, defined as a mismatch between the patient's demand and the ventilator supply of measures such as ventilation rate, flow, volume, or pressure.[13] Studies of neonatal patients show that a neurally adjusted ventilatory assist has significant fewer asynchrony events that SIMV.[14] In adult patients with acute respiratory distress syndrome, there was no significant difference in ventilator asynchrony amongst patients in assist/control mode and SIMV. Additionally, there was no difference in the duration of mechanical ventilation or hospital length of stay.[15]

Clinical Significance

If the patient does not trigger a breath, only the scheduled mandatory breaths will be delivered. Perceived benefits of SIMV include improved patient comfort on the ventilator, reduced work of breathing, reduction in ventilator dyssynchrony, and ease of ventilator weaning. Clinical trials evaluating some of these benefits have not overwhelmingly supported these benefits. SIMV, and specifically SIMV-PS, continues to be a commonly used ventilator mode in the many US intensive care units and especially in surgical ICUs. One of the newer modes of mechanical ventilation, airway pressure release ventilation (APRV), is a variant of SIMV-PS. In APRV, the inspiratory time is longer than the expiratory time, providing an inverse I to E ratio to improve oxygenation.[3]

SIMV was a popular method of mechanical ventilation when it was initially invented. Newer studies demonstrate that it may not be the most effective mode of ventilation. A study of preterm infants shows that SIMV has significantly worse mean airway pressure, duration of time from the onset of weaning to extubation, duration of nasal continuous positive airway pressure support after extubation, and an extubation failure rate when compared to pressure support ventilation with volume guarantee.[16] In adult patients receiving coronary artery bypass grafting, adaptive support ventilation showed statistically lower amounts of atelectasis, number of changes in mechanical ventilator settings, number of ventilator alarms, and hospital length of stay when compared to SIMV.[17] After placing a patient on mechanical ventilation, many intensivists begin to plan their strategy to wean and ultimately liberate the patient from mechanical ventilation. Studies have shown that SIMV is the least efficient technique of weaning when compared to pressure support ventilation and intermittent T-piece trials.[18] Patients with acute respiratory distress syndrome also have shown increased ventilator weaning duration time with SIMV.[19] Similarly, in patients who underwent orthotopic liver transplantation, pressure support SIMV had a significantly higher number of modifications to ventilator settings and duration of mechanical ventilation than patients in adaptive support ventilation.[20]

Enhancing Healthcare Team Outcomes

The management of patients receiving SIMV requires interprofessional teamwork amongst clinicians, pulmonologists, nurses, respiratory therapists, and other allied health professions. Respiratory therapists generally are trained to closely monitor the ventilator and be able to troubleshoot when problems occur. The nursing staff has direct bedside monitoring of the ventilated patient, usually in an intensive care unit or similar facility. In neonatal patients, SIMV has been associated with a higher risk of bronchopulmonary dysplasia and increased with duration of ventilation when compared to high-frequency oscillatory ventilation. With all types of ventilation, only one individual should be assigned to change the parameters. Anytime a parameter is changed, they should chart the change and notify the nurse. Daily morning rounds should be undertaken with the team members to ensure that everyone is aware of the treatment plan. Nurses who play a crucial role in managing patients on ventilators should always monitor the patient for adverse events and complications associated with ventilation. [Level I] Communication between members of the interprofessional healthcare team should be immediate to ensure that outcomes are not compromised.[21] [Level V]

Nursing, Allied Health, and Interprofessional Team Interventions

Nursing interventions for SIMV are similar to interventions required for all patients who are receiving mechanical ventilation. Evidence-based interventions for ventilator patients include the ABCDEF bundle. This bundle stands for assess, prevent, and manage pain; to have a daily spontaneous awakening and breathing trials; choice of analgesia and sedation; to assess, prevent, and manage delirium; early mobility and exercise; and family engagement.[22] Implementing this bundle has led to proven substantial reductions in the number of days on mechanical ventilation, hospital length of stay, and total overall intensive care unit and hospital cost.[23]

Nursing, Allied Health, and Interprofessional Team Monitoring

Nursing and respiratory therapists must closely monitor any patient on mechanical ventilation, including those on SIMV. Ventilator asynchrony and dyssynchrony result when the patient and ventilator are out of sync and should be minimized. There are seven types of events: ineffective effort, double trigger, premature cycling, delayed cycling, reverse triggering, flow starvation, and auto-cycling. Graphical displays on ventilators can help nurses, and respiratory therapists identify ventilator asynchrony. These graphics include pressure and flow waveforms. Further education may be required to identify these events on waveforms monitoring, but prompt identification will increase the amount of time a patient spends in synchrony with their ventilator.[24]