基本通气模式（basic modes of mechanical ventilation）

 

前言

1.一种支持治疗，不能治疗基础疾病。

2.两个重要的生理目标：1.通过提供充分的通气和氧合使血气和酸碱平衡正常；2.降低病人的呼吸功。庐江县人民医院呼吸内科徐大林

3病理状态:

  1).当心衰或ARDS时-改善氧合-提高FiO₂、PEEP，平均气道压力。

  2).当患者高碳酸血症性呼吸衰竭（COPD、药物过量、神经肌肉疾病)，须关注更多的是提供充分的通气。

  3).如癫痫可能仅需要气道保护或简单的维持正常的呼吸功能。

4.四种基本常用的通气模式：辅助/控制（A/C）, 容量辅助控制（VAC），压力支持（PSV），同步间隙指令（SIMV）.

 

TERMINOLOGY

Independent variables set by clinicans and dependent variables measured by ventilators

Tidal volume(VT)

Respiratory rate/frequency(f)

Minute ventilation(VE): VT*f

Peak airway pressure(Paw)：the pressure that is required to deliver the VT to the patient..

Plateau pressure(Pplat): the pressure that is needed to distend the lung.

Peak inspiratory flow: the highest flow that is used to deliver VT to the patient during inspiratoy phase.

Mean airway pressure: the time-weighted average pressure during the respiratory cycle.

Inspiratory time: the amout of time it takes to deliver VT.

Positive end-expiratory pressure(PEEP): the amout of positive pressure that is maintained at end-expiration.

Fraction of inpiratory oxygen(FiO2)

 

MACHANISMS OF MECHANICAL VENTILATORY SUPPORT

1Initiated during the trigger phase: 1)machine timer; 2)pressure change(pressure trigger);3)flow change(flow trigger).

2.Two commonly used target or limit set by the clinican for the ventilator during inspiration:1)flow target, which is a flow rate and pattern set by the clinican; airway pressure thus varies; 2)pressure target, which is an inspiratory pressure limit set by clinican; flow and volume thus vary.

3. Four commonly used cycle-off criteria: 1)volume, in which a breath is terminated when a target volume is achieved; 2)time;3)flow;4)pressure. These four cycle-off mechanisms are also commonly used to classify mechanical ventilation into volume-cycled, time-cycled, flow-cycled, and pressure-cycled, respectively.

The cycle phase is then followed by expiration, which is mostly passive and depend on lung recoil pressure(compliance) and airway/circuit resistance. The product of compliance and airway resistance is called the time constant(Tc).Patients with a long Tc(COPD) will need a longer expiratory time to empty the lung completely, whereas patients with a short Tc(ARDS, pulmonary fibrosis) can empty the lung quickly.

BASIC MODES OF MECHANICAL VENTILATION

VAC, PAC, PSV, SIMV

1.       Volume Assist-Control mode: a breath initiated by the machine timer(control mode) or by the patient(assist mode). VT is then delivered by a flow-targeted mechanism(i.e., a fixed flow  set by the clinican) until a preset VT is reached. The ventilator terminates the breath(volume cycle-off) and allows expiration to proceed. The clinicans sets respiratory rate, VT, and peak inspiratory flow, in addition to FiO₂ and PEEP. The dependent variables are pressures(Paw and Pplat). The inspiratory time(Ti) is determined by the ratio of VT and inspiratory flow(Ti=VT/flow rate). The patients will receive at least a VE that is equal to the preset rate times the VT. One another advantages of this mode is that patients can be fully rested on the ventilator, except for triggering, assuming that the peak inspiratory flow is adequate. The problem of this mode is that patients tend to hyperventilate as they come out of deep sedation, thereby resulting in hypocapnia and respiratory alkalosis. In addition, patients who demand high inspiratory flow may  “fight” the ventilator if the flow rate is set too low.

2.       Pressure assist-control mode: also called pressure controlled mode. A breath can be initiated by the machine timer(control mode) or by the patient(assist mode). The clinican sets respiratory rate, inspiratory pressure, and Ti, in addition to FiO₂ and PEEP. The flow waveform is always decelerating in the PAC mode because the flow slows as it reaches the pressure limit. The dependent variables are VT and inspiratory flow. Patients who are placed on PAC mode breathe at a respiratory rate that is at least equal to the set rate. The magnitude of VT, however, depends on the resistance and compliance of the respiratory system and, sometimes, on Ti. Thus is a patient stops triggering the breath, he or she will continue to a breathe at a rate that is at least equal to the preset rate, but the VE will vary depending on the VT the patient receives. Like the VAC mode, the PAC mode can fully rest patients, except for triggering. On major advantage of the PAC mode compared with the VAC mode is that the Pplat can be more easily regulated, an important consideration in patients with ARDS during ventilation using the lung protective strategy. Decelerating flow patterns in PAC improve the distribution of ventilation in a lung with heterogeneous mechanical properties( as in acute lung injury). The PAC mode is also useful in patients who are ventilated using cuffless endotracheal tube(e.g., neonates and children an patients with bronchopleural fistula). Under these conditions, the PAC mode continues to attempt to pressurize the airway for the duration of the Ti despite the volume loss through the leak. In addition, because of the high variable inspiratory flow needed to deliver the volume, the PAC mode may be more comfortable for some patients who have strong respiratory drive and demand high flow that cannot be satisfied by the fixed flow in the VAC mode.

3.       Pressure support ventilation mode: In the PSV mode, all breaths are initiated by the patient. The VT is delivered by a pressure-targeted mechanism(i.e., an inspiratory pressure set by the clinican). The ventilator continues to deliver the breath until the inspiratory flow has decreased to a specific level(e.g., at 25% of the peak inspiratory flow(flow cycle-off). Because each breath on the PSV mode is initiated by the patient, it is very important to ensure that the patient has reliable breathing effort before being placed on the PSV mode. Otherwise, hypoventilation or apnea may occur. The magnitude of VT deliverd may differ from breath to breath, depending on lung mechanics and the patient’s respiratory drive. One of the major advantage of the PSV mode is that it allows the patient to determine the rate, the duration of inspiration, and the size of VT. This may enhance patient-ventilator synchrony and provide patient comfort. As the patient recovers, the clinican can determine how much work the ventilator can take from the patient by altering the pressure support level, thus making this a very attractive mode for weaning. PSV can also be added to another mode(e.g., SIMV) to support spontaneous breath.

4.       Synchronized Intermittent Mandatory Ventilation: In the IMV mode, machine-triggered(mandatory breathes are delivered a preset frequency by a flow-targeted volume-cycled(volume IMV) or pressure-targeted time-cycled(pressure IMV) mechanism. Between mandatory breaths, the patient can breathe spontaneously. There are two problems with the original design of the IMV mode:(1)it is possible for the patient and the ventilator to insipire in series, thus “stacking” one breath on top of another an leading to high airway pressures; and (2)the workload of spontaneous breaths remains quite high because the patient still has to open a demand valve and inspire without assistance through an endotracheal tube. The first problem has been addressed by fitting a sensor in the ventilator to detect and synchronize the patient’s spontaneous breaths(up to the mandatory rate) in a manner similar to the assist-control mode. The “S” in SIMV mode thus denotes the ability of this mode to synchronize the mandatory breath with the patient’s own inspiratory effort. The synchronization decreases the conflicts between the patient’s breathing efforts and mandatory machine breaths. The second problem of increased work of breathing during spontaneous breaths is solved by introducing pressure support for the spontaneous breaths. Thus, in the SIMV mode, the patient receives three types of breaths: the control(mandatory) breaths that are flow targeted volume-cycled or pressure targeted time-cycled (as in pressure IMV mode)， assisted( synchronized) breaths that are also flow targeted volume cycled or pressure targeted time cycled, and spontaneous that can be pressure supported. In clinical practice, SIMV and PSV are frequently combined and are prescibed as one setting(SIMV/PSV). The IMV mode was initially developed as a method of partial ventilatory support to facilitate liberation from mechanical ventilation. The demand valve placed in the breathing system allows the patient to breathe spontaneously while also receiving mandatory breaths. As the patient’s respiratory function improves, the number of mandotary breaths is decreased. The patient can be extubated when he or she is breathing with minimal mandatory breaths. Large clinical trials, however, show that SIMV is associated with a higher number of weaning failures compared with PSV, and SIMV tends to liberate patients more slowly from mechanical ventilation than PSV or T-piece methods.

 

PRESSURE VERSUS VOLUME MODES

There ha been an ongoing debate about whether pressure modes or volume modes are superior. In reality, if set up appropriately, both modes are likely equivalent in supporting gas exchange, hemodynamics, and pulmonary mechanics. One advantage of pressure modes is that clinician  can easily regulate inspiratory pressure in patients who need a protective lung strategy(e.g., patients with ARDS). It is also easier to adjust Ti and thus the inspiratory-expiratory(I:E) ratio in PAC mode in patients who need to maintain high mean airway pressure for oxygenation. In addition, pressure modes(PAV and PSV) provide higher initial flow to meet the strong demands in some critically ill patients compared with volume modes(VAC and SIMV). This approach improves patient-ventilator synchrony and decreases inspiratory work of breathing.

  Conversely, volume modes have the ability to deliver a constant VT and guarantee VE regardless of changes in lung mechanics. However, because the airway pressure is a dependent variable, it may not be as easy to monitor the alveolar pressure in patients with ARDS because an inspiratory pause maneuver needs to be performed to obtain the Pplat. The fixed flow used to deliver the VT also may be inadequate in some patients who have high flow demand.

 

SETTING UP THE VENTILATOR MODE FOR THE PATIENT

A clinican should ask the following questions when selecting and setting up a ventilator mode:

1.What are the objective for mechanical ventilation? Generally, the first objectives are to improve gas exchange and to correct acid-base imbalance, but sometimes the goal is merely to support patients for airway protection or inconsistent central respiratory drive(e.g., seizure, stroke, or drug ovedose). An additional objective in patients with acute respiratory failure is to unload respiratory muscles and relieve respiratory distress. These objective should be achieved with special attention to minimizing injury associated with mechanical ventilation, including O₂ toxicity and lung overstretch (ventilator-induced lung injury).

2.Does the patient have a reliable central respiratory drive? In general, partial support modes(e.g., PSV) can be safety applied to patients who have adequate central respiratory drive to support ventilation. In patients who have a tendency for apnea(e.g., patients with drug ovedose, brainstem stroke), a mode that provides guaranteed VE is preferable( assist-control or IMV). Patients who are deeply sedated and possibly paralyzed also fall into the latter category. The patient’s ability to initiate a breath(“trigger the ventilator”) should be reevaluated before one switches from assist-control modes to PSV.

3.What are the patient’s acute disease processes and comorbid conditions? In general, patients who have restrictive diseases and are in hypoxemic respiratory(e.g., ARDS, pulmonary edema, diffuse pneumonia) may require higher FiO₂ and PEEP to support oxygenation. Their VE requirement is usually higher and may be supported by using higher respiratory rates(20-25/min) and low VT. On the contrary, patients with obstructive lung disease or hypercapnic respiratory failure(e.g., status asthmaticus, severe COPD) usually do not require high FiO₂. High PEEP should be avoided because it may cause lung overinflaiton and barotraumas. The only exception is when the patient has developed a trigger problem resulting from intrinsic PEEP caused by airway obstruction. In this case, extrinsic PEEP can be carefully titrated to improve trigger sensitivity. The main treatments in this case, however, are effective bronchodilation and adequate expiratory time. It is important to reassess the presence of intrinsic PEEP, the trigger sensitivity, and the level of extrinsic PEEP frequently as the disease evolves. One should also ensure a maximum expiratory time so that the lung can empty the VT adequately, thus minimizing intrinsic PEEP. Therefore, if a control rate is needed for a deeply sedated patient with airway obstruction, a lower rate is preferable(8 to 15/minute).

4.How does the patient’s underling disease respond to treatment? Patients who receive mechanical ventilation should be assessed frequently to determine whether the current ventilator settings remain appropriate because the diseases or conditions that prompt the initiation of mechanical ventilation may change rapidly. If a patient’s respiratory status is worsening, more machine support may be needed. In contrase, if the patient is improving rapidly, machine support should be decreased to keep pace with the changing demands of the patient. Many patients become uncomfortable with excessive ventilatory support when their respiratory status is improving. If the ventilator settings are not accommodating, the patient may “fight” the ventilator. This, in turn, may lead to excessive sedation and may prolong ventilator weaning and intensive care unit stay. Thus, always “fit your ventilator to the patient, not the patient to the ventilator.”

 

DIAGNOSTICS DURING MECHANICAL VENTILTION

The following are simple bedside diagnostic guidelines for patients receiving basic modes of mechanical ventilation.

A.     VT

a.       In volume modes(VAC and IMV), the VT usually is set at 6 to 8 ml/kg of ideal body weight. For the patients with ARDS or similar bilateral alveolar processes, the lower end of the VT(6ml/kg) should always be used as long as the patient can tolerate it. Most ventilaors measure exhaled VT. If there is a significant decrease in the measured VT compared with the set VT, one needs to check for leaks in the ventilator circuit, including the cuff and tubing. Patients with a chest tube may also have lower measured VT if there is lung leak and a portion of the VT is lost through the chest tube(e.g., in bronchoopleural fistula).

b.       In pressure modes(PAC, PSV), a decrease in VT may indicate increases in airway resistance or decreases in lung compliance or both. This can happen in patients who develop bronchospasm, mucus plugging, barotraumas, or pulmonary edema. Migration of the endotracheal tube into the right mainstem bronchus may also decrease VT because it decreases lung compliance. An increase in VT may be associated with improvement in underlying respiratory conditions. Occasionally, patients may have very strong respiratory drive and may generate high VT even at low pressure support.

B.      Respiratory rate

a.       In general, one should use a higher rate for patients with hypoxemic respiratory failure(20 to 25 breaths/minute) and a lower rate for patients with hypercapnic respiratory failure(8 to 15 breaths/m). Patients with severe metabolic acidosis may also require a higher rate and VE to compensate.

b.       The development of a high respiratory rate(tachypnea) in a patient who is previously stable may be associated with anxiety, worsening of respiratory conditions, development of complications(e.g., nosocomial pneumonia, sepsis, acute lung injury, fever), or inappropriate ventilator settings. A quick assessment for these latter conditions should be performed before tachypnea is attributed to anxiety. During the assessment, one also needs to pay attention to the development of intrinsic PEEP because the expiratory time tends to be shortened when respiratory rate increases.

c.       A low respiratory rate usually is associated with loss of respiratory drive, as seen in deep sedation or the development of acute neurologic events. In these situations, a backup rate may be added to ensure minimum VE in case the patient develops apnea. This is especially important if the patient is receiving only PSV.

C.      oxygenation

a.       When the patient is intubated for hypoxemic respiratory failure, an FiO₂ of 1.0 is usually prescribed initially. The FiO₂ can be adjusted to maintain an arterial partial pressure of O₂ of 60 mm Hg or higher and an O₂ saturation of at least 90%.

b.       When the PaO2 decrases acutely, one should look for causes of ventilation-perfusion mismatch or shunt. Common causes of acute hypoxemia in mechanically ventilated patients include nosocomial pneumonia, sepsis, pulmonary edema, acute lung injury, pulmonary embolism, and atelectasis/lung collapse. PaO2 can be increased by increasing FiO2 unless a large shunt is present. In patients with ARDS, pulmonary edema, or diffuse lung parenchymal diseases, increasing PEEP also improves oxygenation. The effect of increasing VT and respiratory rate(and hence VE) on oxygenation is quite small.

D.     Inspiratory flow rate

a.       In volume modes(VAC and IMV), the inspiratory flow rate is usually set at 40-90L/m. If the patient has high inspiratory flow demands, the inspiratory flow can be increased up to 90 to 120L/m. In these patients, higher flow rates may improve comfort, decrease inspiratory work, and, if they slow the respiratory rate, decrease intrinsic PEEP. Increases in inspiratory flow rate invariably result in increased Paw, which may trigger high-pressure alarms. If this happens or if the patient is still not comfortable, one can try pressure modes(PAC or PSV). The pressure modes usually provide much higher initial inspiratory flow than the volume modes while limiting the airway pressure.

b.       Volume modes(VAC, IMV) typically show a square flow pattern because of the fixed inspiratory flow rate, whereas pressure modes(PAC, PSV)usually employ a decelerating flow pattern. Modern ventilators allow clinicians to set the flow patterns even in the volume modes. A decelerating flow pattern is associated with lower inspiratory pressure.

E.      Paw and Pplat

a.       In volume modes, increases in Paw are associated with increased airway resistance or decreased respiratory system compliance. One can measure Pplat using an inspiratory pause function at end-inspiration to differentiate the two conditions. If increased airway resistance is the cause of increased Paw(e.g., bronchospasm, mucus plugging), the Pplat will be unchanged. If decrease respiratory system compliance is the cause(e.g., pneumothorax, pulmonary edema), the Pplat will increase.

b.       In pressure modes, because pressures are limiting, changes in airway resistance or respiratory system compliance are not associated with airway pressure changes. Rather, the VT will decrease.

F.      Trigger sensitivity

a. If the ventilator uses pressure trigger, the trigger sensitivity is usually set at -0.5 to -1.5cmH2O. If flow is trigger mechanism, the trigger sensitivity is usually set at 1 to 3 L/m. If the trigger is too sensitive, excessive numbers of breaths may be delivered inadvertently. At worst, the machince may even self-cycle. If the trigger is too insensitive, it may result in increased work of breathing. Both situations can cause patient’s discomfort and anxiety and may lead to patient-ventilator dysynchrony.

    b. The recognition of patient-ventilator dyssynchrony resulting from inappropriate triggering is important and can be facilitated by observing the patient’s chest wall movement and analyzing the pressure and flow waveforms. The most common cause of trigger insensitivity is the intrinsic PEEP. One should always be vigilant about the presence of intrinsic PEEP and, if present, minimize it.

 

CONCLUSIONS

Mechanical ventilators have become more sophisticated, but the basic physical principles involved in gas delivery by the ventilators are unchanged. Understanding how to use these basic modes not only allows the clinicans to manage critically ill patients more effectively but also provides a foundation for using the more advanced modes of mechanical ventilation.