2.7.4.3 Peak airway pressure 2.7.4.4 PEEP
2.7.4.5 Fraction of inspired oxygen (FiO2) 2.7.4.6 Respiratory rate
2.7.4.7 Flow rate
2.7.4.8 Complications of mechanical ventilation 2.7.4.9 Alarms
2.7.4.10 Summary of mechanical ventilator settings
2.7.4.1 Ventilator modes
The variable methods by which the patient and the ventilator interact to deliver effective ventilation are called modes (Lewis et al, 2004:1783). According to Urden et al, 2006:671), ventilator modes refer to how the machine will ventilate the patient. In other words, selection of a particular mode of ventilation determines how much the patient will participate in his/her ventilatory pattern. The ventilator mode selected is based on how much work of breathing the patient ought to or can perform and is determined by the patient‟s ventilator status, respiratory drive and arterial blood gases. Generally, ventilator modes are controlled or assisted. With controlled
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ventilator support, the ventilator does all the work and with assisted ventilator support, the patient and the ventilator share the work of breathing (Pierce, 2002:56).
Modes are further categorised as volume modes and pressure modes, non- conventional and newer modes. Many of these modes may be used in conjunction with each other. However, in the midst of all the wide variety and availability, the modes of ventilator operation constitute one of the most essential and challenging areas of requisite knowledge for the nurse caring for patients undergoing mechanical ventilation (Mims et al, 2004:50). This is because the mode essentially determines the level of ventilator support offered and the way the machine (ventilator) is functioning at a given time. Therefore, to confidently and competently manage a patient-ventilator system and to ensure patient safety, familiarity with ventilator modes is fundamental to the professional nurses working in the critical care unit. For the purpose of this study, only volume and pressure modes and their effect in ensuring the safety of the critically ill patient while connected to the mechanical ventilator will be discussed.
Volume modes
Volume modes refer to the ventilator modes that are dependent on a preset volume of air in the ventilator. Volume modes include continuous mandatory ventilation, assist-control and synchronised, intermittent mandatory ventilation mode (Morton and Fontaine, 2009:594). These modes can be referred to as common modes of ventilation as they are frequently used in the critical care units and have been utilised traditionally in the majority of critically ill patients (Hamed et al, 2006:78). A brief description of the volume modes and the complications related to their use is given below.
Continuous Mandatory Ventilation (CMV)
With CMV, the patient receives a pre-set number of breaths per minute with a pre- set tidal volume at regular intervals. Patient effort does not trigger a mechanical breath and the ventilator performs all the work of breathing. This has led to the recommendation that this mode should be best reserved for patients with no respiratory effort because of the dysfunction of the central nervous system, for
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instance, Guillain-Barrè syndrome or high-level spinal cord lesions, patients with apnea, respiratory arrest or those whose clinical condition requires sedation or paralysis or anaesthesia (Hamed et al, 2006:78; Pierce, 2007:214; Elliot et al, 2007:284).
CMV was used widely before the advent of assist-control ventilation and some manufacturers might even use the term CMV to refer to the assist-control mode. Many ventilator brands available might have a true CMV modes and assist-control would function as if they were CMV. However, despite of this confusion, the nurse should be aware of the differences and know the implications for caring for patients receiving this mode of ventilation. Because the patient cannot achieve a spontaneous breath, attempts to breathe results in patient effort with no flow delivered. This can lead to sensations of air hunger and significantly increase the work of breathing. With CMV, alveolar ventilation and the respiratory contribution to acid-base balance are completely controlled by the clinician. Acid-base balance should thus be closely monitored and ventilator settings must be adjusted to changing physiological scenarios such as fever, change in nutritional intake and stress. Respiratory muscle weakness and atrophy may result if CMV is used for an extended period, thus prolonging the weaning process (Hamed et al, 2006:78).
Adverse hemodynamic effects may occur with use of this mode because every breath is delivered under positive pressure. Patient-ventilator asynchrony caused by the flow rate or respiratory settings that are inadequate to meet the patient‟s ventilatory needs may occur (Elliot et al, 2007:284; Pierce, 2007:215). As the tidal volume and rate are controlled, the patient might be at risk from hypercapnia, hypoventilation and volutrauma (Urden et al, 2006:672). CMV may require the use of sedatives or neuro-muscular blocking agents, which may compromise patient safety in the event of a ventilator mishap and may prolong the weaning times and, ultimately, the safety and cost in critical care units, which have been proven to impact on the quality of patient care (Shelledy, Rau and Thomas-Goodfellow, 1995:68). The professional nurse should be aware of these implications for practice in using the CMV mode in the mechanically ventilated patient as these adverse effects might be harmful to the patient, thereby compromising his/her safety.
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Assist-Control (A/C) mode
In using assist-control ventilation, the ventilator delivers a controlled ventilator breath in response to the patient-initiated breath. The patient cannot breathe independently of the ventilator. The patient can initiate inspiration and control the breathing frequency. If the patient fails to initiate inspiration, the ventilator automatically goes into a backup mode and delivers the pre-set rate and tidal volume until an inspiratory effort is sensed (Hamed et al, 2006:78; Pierce, 2007:217). The A/C mode thus allows the patient to control the rate of breathing and yet it guarantees the delivery of a minimal pre-set rate and volume. The ventilator performs the bulk of the ventilatory work. This mode of ventilation is often used as initial mode of ventilation to fully support a patient, such as when a patient is first intubated or when the patient is too weak to perform the work of breathing, for instance, when emerging from anaesthesia (Morton and Fontaine, 2009:594).
Disadvantages of using the A/C mode include respiratory alkalosis due to the patient‟s tendency to hyperventilate. Hyperventilation may also lead to the formation of auto-PEEP, because of the shortened expiratory time. The acid-base balance of the patient must be monitored closely. Respiratory muscle atrophy may result as the patient may not participate in breathing when becoming dependent on the ventilator. Variability in the patient‟s hemodynamic status may occur with this mode as every breath is delivered under positive pressure (Pierce, 2007:217). Professional nurses are often required to set the ventilator mode or to adjust it according to the patient‟s clinical condition. It is, therefore, important that they are aware of the disadvantages in using the A/C mode and should monitor the patient meticulously.
Synchronised Intermittent Mandatory Ventilation (SIMV) mode
In SIMV mode, the ventilator delivers gas at a pre-set tidal volume and rate while allowing the patient to breathe spontaneously. Ventilator breaths are synchronized with the patient‟s respiratory effort (Urden et al, 2006: 672). At times, SIMV might be confused with IMV (intermittent mandatory ventilation), but there is a definite difference between the two modes. With IMV, the mandatory breaths are delivered at a precise time, regardless of where the patient is in the ventilatory cycle, whereas with SIMV, the ventilator synchronises the delivery of the mandatory breath when it
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senses the patient‟s inspiratory efforts. The main difference between the SIMV and A/C modes is in the volume of the patient-initiated breaths. Patient-initiated breaths in the A/C mode results in the patient receiving a guaranteed tidal volume, whereas in SIMV, spontaneous breath tidal volume is variable because it depends on patient effort and lung characteristics (Pierce, 2007:218).
Traditionally, SIMV was created as a mode in which the patient could interact with the ventilator using the respiratory muscles, thus becoming a popular weaning mode. This was achieved by lowering the SIMV rate, allowing the patient to initiate more spontaneous breaths, and therefore, assuming a greater portion of the ventilatory work. As the patient demonstrates the ability to generate more work, the mandatory breath rate is decreased accordingly, thus making weaning off the ventilator possible (Morton and Fontaine, 2009:95). Although SIMV has been used traditionally as weaning mode, it was found in randomised trials that SIMV is, in fact, inferior to both spontaneous breathing trials and pressure-support ventilation as a mode of weaning (Brochard Rauss, Benito, Conti, Mancebo and Rekik, 1994:896-903; Estaban, Frutos, Tobin, Alia, Solsana and Valverdu, 1995: 345-350; Meade, Guyatt, Snuff, Griffith, Hand and Toprani, 2001:425S). According to Pierce (2007:218), compared to other weaning modalities, SIMV is associated with the longest weaning times and the lowest success rates.
SIMV is indicated for use in patients who have a normal respiratory drive but whose respiratory muscles are unable to perform all the work of breathing. Although using SIMV has been proven to have fewer disadvantages than the A/C mode, for instance, less respiratory alkalosis, less atrophy of the respiratory muscles and better distribution of gas within the lung, it does have disadvantages that the health care practitioner should be aware of. Patient-ventilator asynchrony, patient discomfort, inadequate ventilation and potentially barotrauma are possible complications linked to the use of SIMV. During this mode, the work of breathing might be increased, which will promote respiratory fatigue. The patients should be monitored for hypercapnia, further increase in the work of breathing and inadequate spontaneous tidal volume (Urden et al, 2006:672).
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In a study conducted by Shelledy et al (1995:67), it was found that SIMV with pressure support significantly increases minute volume and ventilatory equivalents when compared with assist-control or SIMV alone, and was thus the most efficient mode of full ventilatory support. They found no difference in ventilatory efficiency between assist-control and SIMV. However, very little objective evidence supports the choice of a particular mode of mechanical ventilation in the spontaneous breathing patient. One possible criterion for the selection of a particular mode is the effect on the work of breathing (Fenstermacher and Hong, 2004:260).
It was reported in a study done by Estaban et al (2000:1455) that assist-control ventilation is used in 40 to 72% of patients who are connected to mechanical ventilators in North America, South America, Spain and Portugal. In a survey published by Venus, Smith and Mathru (1987:530-533), 72 % of physicians in the United States listed SIMV as their preferred ventilator mode. Ten years later, SIMV on its own was used in only 6% of patients receiving mechanical ventilation in North American critical care units. However, the use of a combination of SIMV and PS or A/C appears to be the most commonly used mode of ventilation in the United States. In Uruguay, the popularity of the SIMV and PS combination is also quite common (Leung, Jubran and Tobin, 1997:1940). An interesting feature was the infrequent use of certain modalities, such as SIMV as a stand-alone mode, non-invasive ventilation, permissive hypercapnia and the other new modes of ventilation, as reported in the study conducted by Estaban et al (2000:1450-1458). No literature pertaining to South Africa could be found in studies conducted nationally on the utilisation and implementation of the different volume ventilator modes for the critically ill patient.
Pressure modes
During a pressure mode, a breath is delivered at a pressure that is constant for every breath. Pressure modes include pressure support ventilation, pressure-controlled ventilation, airway pressure release ventilation and volume-guaranteed pressure- option (Morton and Fontaine, 2009:597). A brief description of these pressure modes follows.
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Pressure-Support Ventilation (PSV)
In this mode of ventilation, a pre-set positive pressure is used to augment the patient‟s inspiratory efforts. It is a ventilator-generated flow augmentation of each individual breath that the patient triggers. PSV is a spontaneous breathing mode used as the primary mode of ventilation in patients with stable respiratory drives to overcome any imposed mechanical resistance (Urden et al, 2006:672). PSV allows the tidal volume and inspiratory flow to be more adaptable to the patient‟s own ventilatory demands. This manner of supporting the patient‟s own ventilatory effort may be responsible for an improved comfort and synchrony with the ventilator, and has been shown to reduce the work of breathing and prevent diaphragmatic fatigue in patients with respiratory failure (Chiumello, Pelosi, Calvi, Bigatello and Gattinoni, 2002:925).
PSV has been used to limit barotrauma and to decrease the work of breathing, which can result from endotracheal tube and breathing circuit resistance. In a comparison between PSV and A/C, patients on PSV showed significantly higher tidal volume, minute ventilation and inspiratory time in association with a significantly lower pressure in airways (Tejeda, Boix, Alvarez, Balanza and Morales, 1997:1323). Pressure support, when used in combination with SIMV, is thought to reduce the work of breathing spontaneous breaths and thus overcomes the resistance to ventilation imposed by demand valves, patient breathing circuits and artificial airways (Shelledy et al, 1995:68). Other advantages of this mode include improved patient- ventilator synchrony and patient comfort (Urden et al, 2006:672).
Pressure-support ventilation has been proposed in limiting acute lung injury. It is recommended as a ventilator mode that has been used to limit barotrauma and to reduce the patient‟s work of breathing (Petrucci and Lacoveli, 2004:195; Hamed et al, 2006:79).
In maintaining patient safety while caring for the mechanically ventilated patient, it is not only important that the professional nurses are aware of the most beneficial mode to use, but also the use of the ventilation mode that is the least harmful for the critically ill patient.
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Pressure-Controlled Ventilation (PCV)
PCV is a pressure-limited, time-cycled ventilator mode. The desired pressure level is set, as is the inspiratory time and the respiratory rate. With every breath, the ventilator delivers an inspiratory flow until the pre-set airway pressure limit is achieved. As with PSV, the flow decelerates throughout inspiration; however, the cycle is time and not flow. The decreased flow at the end of inspiration results in less turbulent, more laminar flow and a more even distribution of the breath. This decelerating waveform has long been shown to improve lung mechanics and gas exchange during mechanical ventilation (Hamed et al, 2006:79). The PCV mode is used to control plateau pressures in conditions such as ARDS in which the compliance is decreased and the risk for barotrauma is high. It is used when the patient has persistent oxygenation problems despite a high FiO2 and high levels of PEEP and in patients in whom it is desirable to control peak inspiratory pressures (Pierce, 2002:57; Pierce, 2007:224; Morton and Fontaine, 2009:597).
As the expiratory time is decreased when using PCV, the nurse must monitor for the development of hyperinflation or auto-PEEP. Regional alveolar over-distention and barotrauma may result from excessive total PEEP. When the PCV mode is used, the mean airway and intra-thoracic pressures rise, potentially resulting in a decrease in cardiac output and oxygen delivery (Morton and Fontaine, 2009:597). Therefore, it is necessary to monitor the patient‟s hemodynamic status closely. In caring for a patient who is on PCV mode in a critical care unit, it is the responsibility of the professional nurse to monitor for the complications linked to this mode in order to ensure the safety of the mechanically ventilated patient.
Airway Pressure Release Ventilation (APRV)
APRV is a spontaneous breathing mode used in patients to maintain alveolar recruitment without possible additional peak inspiratory pressures that could lead to barotrauma. During this mode, two different levels of CPAP (inspiratory and expiratory) are applied for set periods of time, allowing spontaneous breathing to occur at both levels (Urden et al, 2006:673). APRV has been used in trauma and ARDS patients to reduce airway pressure and lower minute volumes while allowing spontaneous breathing throughout the ventilator cycle, all with decreased sedation
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and neuromuscular blocking agent use. APRV mode allows lung protective strategies to be followed with limitation of plateau and peak pressures. Because the patient is breathing spontaneously throughout both high-and low-pressure phases, the need for sedation or neuro-muscular blocking agents may be limited. This may result in a shorter duration of critical care unit stay and a reduced incidence of adverse effects associated with critical illness neuropathy. The lower peak and mean airway pressure with APRV lead to a decreased transmitted intra-thoracic pressure and a reduction the central venous pressure of the mechanically ventilated patient. The reduced intra-thoracic pressure enhances venous return and, therefore, cardiac performances with a decreased pressure requirement to support arterial pressure and oxygen delivery (Kaplan, Bailey and Formosa, 2001:221-226).
APRV has been proven as a ventilatory mode which reduces barotrauma and circulatory compromise, patient-ventilatory asynchrony and leads to improved distribution of gas in the lungs (Pierce, 2007:244). This mode may improve oxygenation and prevent ventilator-induced lung injury in the critically ill patient. Weaning in this mode is possible and has been done successfully in the mechanically ventilated patient (Morton and Fontaine, 2009:597).
APRV offers several essential preconditions, which seem potentially advantageous for ventilation of even severe ARDS lungs. It provides a nearly continuous airway pressure level favourable in keeping the alveoli open, and a short expiratory time, which favours ventilation. It aids in the preservation of spontaneous breathing, which avoids the need for muscle relaxation and deep sedation. Furthermore, it reduces the risk for barotrauma or volutrauma and makes it possible to maintain relatively low airway pressures and thereby improves the conditions for pulmonary circulation and oxygen delivery (Burchardi, 1996:1065).
Disadvantages of APRV include that tidal volume is variable and depends on compliance and resistance factors in the patient-ventilator system (Pierce, 2007:244). However, considering all positive benefits of APRV, it appears to be the safest mode to use on the mechanically ventilated patient in minimising ventilator- induced lung injury and thereby ensuring the safety of the critically ill patient.
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Volume-Guaranteed Pressure Option mode (VPGO)
This mode is also known as pressure augmentation (PA) or as volume-assured pressure-support ventilation (VAPSV). VPGO mode ensures delivery of a prescribed tidal volume while using a decelerating flow pattern by means of a pressure breath. VAPSV is a spontaneous breathing mode used to treat acute respiratory illness and to facilitate weaning. In the acutely ill unstable patient, this option may provide pressure ventilation while guaranteeing tidal volume and minute ventilation at a set rate. In the spontaneously breathing patient, the option is used as a “safety” measure when pressure ventilation is desired (Rose, 2006:145). A variation of PSV is given with a set tidal volume to ensure that the patient receives minimum tidal volume with each pressure-support breath.
Advantages of this mode include increased patient comfort, decreased work of breathing, decreased respiratory muscle fatigue and promotion of respiratory muscle conditioning (Urden et al, 2006:673). The use of VAPSV in the acutely ill patient may provide pressure ventilation while guaranteeing tidal volume and minute volume at a set rate and may be useful in patients for whom the ability to cough and expose secretions are a problem (Morton and Fontaine, 2009:597).