智能脱机模式在机械通气脱机阶段的应用

脱机是从完全通气支持向自主呼吸转变的一个过程,不同的通气技术被用于患者的脱机,常用的脱机方法包括T型管自主呼吸实验、同步间歇指令通气（SIMV）以及压力支持通气（PSV）。它们各有优缺点。新的通气模式开始应用到机械通气的脱机阶段,它们包括适应性支持通气（ASV）模式、Automode模式、分钟指令通气模式（MMV）和Smartcare模式等。它们减轻了医务人员的工作负担,提高了脱机的效率。     

Differenzialdiagnose und Management von Weaningproblemen

Of 6?ventilated ICU patients, 1?patient will experience prolonged weaning from mechanical ventilation. This is associated with a substantially increased morbidity and mortality. After one unsuccessful spontaneous breathing trial, the organ system preventing successful weaning should be identified. The mechanisms limiting weaning should be assessed toward the end of a failing spontaneous breathing trial using physical examination, arterial and venous blood gases, echocardiography, and other tools. Disturbed mechanics of the respiratory system, weakness of the ventilatory muscles, and weaning-induced cardiac dysfunction including myocardial ischemia are common causes of difficult or prolonged weaning. The respective therapeutic strategies are reduction of the work of breathing, ventilatory muscle training, and reducing cardiac preload as well as afterload. Weaning problems are preventable by a structured weaning strategy, including early spontaneous awakening and breathing trials as well as early rehabilitation of patients with ICU-acquired weakness.     

External work output and force generation during synchronized intermittent mechanical ventilation. Effect of machine assistance on breathing effort

We measured the mechanical work performed by 12 acutely ill patients during synchronized intermittent mandatory ventilation to determine the influence of volume-cycled machine assistance on inspiratory timing, respiratory muscle force development, and external work output. The frequency and tidal volume of spontaneous breaths increased at lower levels of mechanical ventilation, but inspiratory time fraction did not vary across the spectrum of machine support. As machine support was withdrawn, inspiratory work and pressure-time product increased progressively for both spontaneous and assisted breathing cycles. On a per cycle basis, work output was greater for assisted than for spontaneous breaths at all levels of comparison. Although the mean pressure developed by the patient during assisted cycles averaged approximately equal to 20% less than during adjacent unassisted cycles, contraction time averaged approximately equal to 20% longer, so that the pressure-time products were nearly equivalent for both types of cycle. Two indices of force reserve indicated that our patients taxed their maximal ventilatory capability at all but the highest levels of support. We conclude that under the conditions of this study the ventilatory pump continued to be active at all levels of machine assistance. Although work per liter related linearly to the proportion of minute ventilation borne by the patient, force generation differed little for spontaneous and machine-aided breaths at any specified level of support. Whether judged on the basis of mean developed pressure (work per liter of ventilation) or pressure-time product, little effort adaptation to volume-cycled machine assistance appears to occur on a breath-by-breath basis.     

Respiratory function during pressure support ventilation

Pressure support ventilation (PSV) is a pressure assist form of mechanical ventilatory support that augments the patient's spontaneous inspiratory efforts with a clinician selected level of positive airway pressure. To understand the effects of PSV on respiratory function, experiments were performed on 15 stable patients requiring synchronized intermittent mandatory ventilation (SIMV), as well as on a mechanical model simulating these patients' ventilatory systems. In the clinical study, gas exchange, airway pressures, blood pressure and heart rate were measured while SIMV was replaced by enough PSV to approximate the baseline SIMV tidal volume (VT). Measurements were repeated while this PSV level was then reduced in three 5 cm H2O steps every 10 to 15 minutes. It was found that PSV was a reasonable form of mechanical ventilatory support in patients with spontaneous ventilatory drives. It improves patient comfort, reduces the patient's ventilatory work, and provides a more balanced pressure and volume change form of muscle work to the patient. The clinical significance of these properties during the weaning process remain to be determined.     

Discontinuation of mechanical ventilation

The vast majority of patients who undergo mechanical ventilation are able to discontinue ventilatory assistance within a few days. Typically, patients who require only short-term mechanical ventilation do not have severe underlying lung disease, and the problem for which they require ventilatory support is most commonly rapidly reversible. In these patients on short-term ventilatory support, parameters of spontaneous ventilatory requirements and respiratory muscle strength, including minute ventilation, maximal voluntary ventilation, vital capacity, and maximal inspiratory pressure, are useful in predicting the success of discontinuation of mechanical ventilation. Ventilatory support can generally be discontinued by a variety of techniques in these patients without the need for weaning from the ventilator per se. The smaller group of patients in whom it is not possible to discontinue mechanical ventilation within less than 7 days comprises individuals who frequently have severe acute or chronic lung disease, multisystem extrapulmonary disease, or neuromuscular disease. After a period of prolonged mechanical ventilatory support, these complicated patients require a process of progressive weaning in which they gradually become able to support spontaneous ventilation. Spontaneous ventilatory parameters do not correlate well with weaning ability in patients on long-term ventilatory support. A systematic and comprehensive approach in which attention is focused on optimizing pulmonary and nonpulmonary factors that affect the weaning process provides the best chance for successful withdrawal of ventilatory support after long-term mechanical ventilation. Inadequate ventilatory drive, respiratory muscle weakness and fatigue, increased work of breathing, excessive CO2 production, and cardiac failure are potential mechanisms that may play a role in inhibiting successful weaning. Adverse factors relevant to each of these mechanisms must be addressed and corrected to whatever extent possible. Studies have not demonstrated the superiority of either classic T-piece weaning or IMV weaning methods in difficult-to-wean patients on long-term ventilatory support. Both techniques may be used successfully as long as all patient variables that may adversely affect weaning ability are corrected or optimized and close care and attention to the details of the weaning process itself are provided.(ABSTRACT TRUNCATED AT 400 WORDS)     

Spontaneous breathing and total body oxygen consumption in children recovering from open-heart surgery.

The effects of withdrawal of ventilatory support on cardiopulmonary function, oxygen consumption and carbon dioxide production were assessed in 25 infants and children within seven days (2.9 +/- 2.5 days; mean +/- SD) of an open heart operation, during weaning from ventilatory support. The average age of the patients was 3.4 +/- 3.5 years and weight 12.4 +/- 8.3 kg. Heart rate, blood pressure, arterial and central venous blood gas values, and oxyhemoglobin saturations were measured during controlled mechanical ventilation and during spontaneous breathing with continuous positive airway pressure. Simultaneously, VO2 and VCO2 were measured using indirect calorimetry. Withdrawal of ventilatory support effected an expected, significant decrease in arterial pH (7.42 +/- 0.10 to 7.37 +/- 0.06; p less than 0.001) and an increase in PaCO2 (34 +/- 6 to 40 +/- 5 mm Hg; p less than 0.0001), while arterial blood oxyhemoglobin saturation, heart rate, and blood pressure remained unchanged. A significant increase in central venous oxyhemoglobin saturation (67.9 +/- 11.9 to 74.8 +/- 8.3 percent; p less than 0.001) indicated improvement in systemic blood flow during spontaneous breathing. Average VO2 and VCO2 did not change significantly. A decrease in VO2 by more than 5 percent was seen in seven patients, an increase by more than 5 percent in nine, and a change within +/- 5 percent in nine patients. The change in VO2 was inversely related to the difference between measured and expected VO2 during mechanical ventilation (r = -0.73) and to body temperature (r = -0.69). The results indicate that factors other than the oxygen uptake by the respiratory muscles may have significant effects on total body VO2 in infants and children after open-heart surgery. Therefore, monitoring of VO2 during withdrawal of ventilatory support may not be an accurate indicator of respiratory work and oxygen cost of breathing in these patients.     

Long-term effects of spontaneous breathing during ventilatory support in patients with acute lung injury.

Improved gas exchange has been observed during spontaneous breathing with airway pressure release ventilation (APRV) as compared with controlled mechanical ventilation. This study was designed to determine whether use of APRV with spontaneous breathing as a primary ventilatory support modality better prevents deterioration of cardiopulmonary function than does initial controlled mechanical ventilation in patients at risk for acute respiratory distress syndrome (ARDS). Thirty patients with multiple trauma were randomly assigned to either breathe spontaneously with APRV (APRV Group) (n = 15) or to receive pressure-controlled, time-cycled mechanical ventilation (PCV) for 72 h followed by weaning with APRV (PCV Group) (n = 15). Patients maintained spontaneous breathing during APRV with continuous infusion of sufentanil and midazolam (Ramsay sedation score [RSS] of 3). Absence of spontaneous breathing (PCV Group) was induced with sufentanil and midazolam (RSS of 5) and neuromuscular blockade. Primary use of APRV was associated with increases (p < 0.05) in respiratory system compliance (CRS), arterial oxygen tension (PaO2), cardiac index (CI), and oxygen delivery (DO2), and with reductions (p < 0.05) in venous admixture (QVA/QT), and oxygen extraction. In contrast, patients who received 72 h of PCV had lower CRS, PaO2, CI, DO2, and Q VA/Q T values (p < 0.05) and required higher doses of sufentanil (p < 0.05), midazolam (p < 0.05), noradrenalin (p < 0.05), and dobutamine (p < 0.05). CRS, PaO2), CI and DO2 were lowest (p < 0.05) and Q VA/Q T was highest (p < 0.05) during PCV. Primary use of APRV was consistently associated with a shorter duration of ventilatory support (APRV Group: 15 +/- 2 d [mean +/- SEM]; PCV Group: 21 +/- 2 d) (p < 0.05) and length of intensive care unit (ICU) stay (APRV Group: 23 +/- 2 d; PCV Group: 30 +/- 2 d) (p < 0.05). These findings indicate that maintaining spontaneous breathing during APRV requires less sedation and improves cardiopulmonary function, presumably by recruiting nonventilated lung units, requiring a shorter duration of ventilatory support and ICU stay.     

Oxygen cost of breathing. Changes dependent upon mode of mechanical ventilation

We describe a patient with respiratory failure who demonstrated marked increases in O2 consumption (VO2) when breathing with synchronized intermittent mandatory mechanical ventilation (SIMV). When the mode of ventilation was changed to facilitate inspiratory gas flow (pressure-support) during spontaneous breathing, O2 consumption decreased 27 percent. Several important factors contributing to the increased O2 cost of breathing in patients requiring mechanical ventilation are reviewed, including the high internal resistance of demand-flow SIMV systems.     

Respiratory muscle dysfunction secondary to chronic tracheostomy tube placement

In patients requiring periodic mechanical ventilation, a deflated, fenestrated tracheostomy tube may impair respiratory muscle performance during spontaneous breathing. We describe a patient with severe chronic airflow obstruction (CAO) whose respiratory muscle performance and exercise duration improved after tracheostomy tube removal. Duty cycle, Pdi/Pdi max, and the tension time index were all lower during exercise after tracheostomy tube removal. We conclude that a deflated and fenestrated tracheostomy tube significantly increases airways resistance and can further limit ventilatory muscle performance in patients with airflow obstruction. Patients requiring intermittent ventilatory support may benefit from permanent tracheostomy fistulas that allow for intermittent self cannulation. This would avoid loading of the respiratory muscles when breathing spontaneously.     

Ventilatory muscle support in respiratory failure with nasal positive pressure ventilation

Long-term intermittent mechanical ventilation results in improvements in ventilatory performance and clinical status between ventilation sessions in patients with chronic respiratory failure. The application of intermittent positive pressure ventilation through a nasal mask (NPPV) is a simple, noninvasive method for the provision of chronic intermittent ventilatory support. We investigated the effects of NPPV on inspiratory muscle activity in three normal subjects and nine patients with acute or chronic ventilatory failure due to restrictive (four subjects) or obstructive (five subjects) respiratory disorders. NPPV resulted in reductions of phasic diaphragm electromyogram amplitude to 6.7 +/- 0.7 percent (mean +/- SEM) of values obtained during spontaneous breathing in the normal subjects, 6.4 +/- 3.2 percent in the restrictive group, and 8.3 +/- 5.1 percent in the obstructive group. Simultaneous decreases in activity of accessory respiratory muscles were observed. The reductions in inspiratory muscle activity were confirmed by the finding of positive intrathoracic pressure swings on inspiration in all subjects. With NPPV, oxygen saturation and PCO2 remained stable or improved as compared with values obtained during spontaneous breathing. These results indicate that NPPV can noninvasively provide ventilatory support while reducing inspiratory muscle energy expenditure in acute and chronic respiratory failure of diverse etiology. Long-term assisted ventilation with NPPV may be useful in improving ventilatory performance by resting the inspiratory muscles.     

Weaning from mechanical ventilation by long-term nasal positive pressure ventilation in two patients with acute respiratory distress syndrome associated with pneumococcal sepsis

Only few data concerning weaning by nasal positive pressure ventilation (NPPV) are available, and successful weaning by using NPPV in patients with acute respiratory distress syndrome (ARDS) and severe complications has not yet been described. Two cases with ARDS and both preexisting thoracopulmonary disease (infundibulum abnormality and suspected COPD) and associated complications (recurrent sepsis, acute renal failure, need for lobectomy, severe malnutrition) could not be weaned by invasive ventilatory techniques. Both patients presented with rapid shallow breathing and PaCO(2) values >60 mm Hg during intermittent trials of spontaneous breathing, although the primary pathology and associated complications had been resolved. Patients were successfully adapted on NPPV in a stepwise approach after 93 days and 67 days of invasive ventilation. In one patient withdrawal from NPPV was possible after 2 months. In the other patient the duration of daily ventilation could be significantly reduced from 18 to 6 h/day after 9 months on NPPV. Therefore, patients with ARDS who cannot be weaned by invasive ventilatory strategies might be removed successfully from invasive mechanical ventilation by using NPPV even when there are preexisting thoracopulmonary disease and major complications during invasive ventilation.     

Effects of assisted mechanical ventilation on control of breathing

During spontaneous breathing, the respiratory muscle pressure (Pmus) waveform is determined by a complex system consisting of a motor arm, a control center, and various feedback mechanisms that convey information to the control center. During assisted mechanical ventilation, the pressure delivered by the ventilator (Paw) is incorporated into the system and may alter the Pmus waveform, which in turn modifies the function of the ventilator. Thus, the response of Pmus to Paw and the response of Paw to Pmus constitute the two components of patient-ventilator interaction as well as of control of breathing during assisted mechanical ventilation. The response of Paw to Pmus depends on: 1) the mode of ventilatory support; 2) the mechanics of the respiratory system, and 3) the characteristics of the Pmus waveform. On the other hand the response of Pmus to Paw is mediated through four feedback systems: 1) mechanical; 2) chemical; 3) reflex, and 4) behavioral. It follows that the system that controls the act of breathing may be considerably modified by mechanical ventilation. The physician dealing with a mechanically ventilated patient should take into account the interaction between the respiratory effort and the function of the ventilator and be aware that the ventilatory output may or may not reflect the various aspects of control of breathing.     

Inspiratory pressure support prevents diaphragmatic fatigue during weaning from mechanical ventilation

Persistent inability to tolerate discontinuation from mechanical ventilation is frequently encountered in patients recovering from acute respiratory failure. We studied the ability of inspiratory pressure support, a new mode of ventilatory assistance, to promote a nonfatiguing respiratory muscle activity in eight patients unsuccessful at weaning from mechanical ventilation. During spontaneous breathing, seven of the eight patients demonstrated electromyographic signs of incipient diaphragmatic fatigue. During ventilation with pressure support at increasing levels, the work of breathing gradually decreased (p less than 0.02) as well as the oxygen consumption of the respiratory muscles (p less than 0.01), and electrical signs suggestive of diaphragmatic fatigue were no longer present. In addition, intrinsic positive end-expiratory pressure was progressively reduced. For each patient an optimal level of pressure support was found (as much as 20 cm H2O), identified as the lowest level maintaining diaphragmatic activity without fatigue. Above this level, diaphragmatic activity was further reduced and untoward effects such as hyperinflation and apnea occurred. When electrical diaphragmatic fatigue occurred, the activity of the sternocleidomastoid muscle was markedly increased, whereas it was minimal when the optimal level was reached. We conclude that in patients demonstrating difficulties in weaning from the ventilator: (1) pressure support ventilation can assist spontaneous breathing and avoid diaphragmatic fatigue (pressure support allows adjustment of the work of each breath to provide an optimal muscle load); (2) clinical monitoring of sternocleidomastoid muscle activity allows the required level of pressure support to be determined to prevent fatigue.     

The inspiratory workload of patient-initiated mechanical ventilation

We quantified inspiratory effort during patient-triggered ventilator cycles in 20 critically ill patients receiving assisted mechanical ventilation (AMV). An index of the patient's work per liter of ventilation (WP) was defined as the difference in the mechanical work done by the ventilator during controlled and assisted breathing cycles at similar settings of tidal volume and flow. WP was estimated graphically from plots of airway pressure against inflation volume for peak flow settings of 60 L/min and 100 L/min. During patient-initiated cycles, effort did not cease with the onset of gas delivery. Values for WP varied widely but at both flow settings frequently equalled or exceeded the total workload expected for a spontaneously breathing normal subject. Furthermore, the patient's component of the mechanical workload during AMV was often a large percentage of the work performed during spontaneous breathing 30 s after discontinuing ventilator support (at 60 L/min: mean 62.6%; range 30.3 to 116.3%). The addition of deadspace to the external circuit increased VE and WP significantly. Both the maximally negative pressure generated against an occluded airway and the deflection of esophageal pressure in the first 100 ms after the onset of inspiratory effort were highly correlated with WP, suggesting the importance of strength and ventilatory drive as determinants of patient effort. WP correlated poorly with measures of chest mechanics, and there was no separation of WP values for the two flow rates we studied, perhaps because both settings exceeded the patient's spontaneous demand for airflow (FD).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of assisted mechanical ventilation on control of breathing

Summary During spontaneous breathing, the respiratory muscle pressure (Pmus) waveform is determined by a complex system consisting of a motor arm, a control center, and various feedback mechanisms that convey information to the control center. During assisted mechanical ventilation, the pressure delivered by the ventilator (Paw) is incorporated into the system and may alter the Pmus waveform, which in turn modifies the function of the ventilator. Thus, the response of Pmus to Paw and the response of Paw to Pmus constitute the two components of patient-ventilator interaction as well as of control of breathing during assisted mechanical ventilation. The response of Paw to Pmus depends on: 1) the mode of ventilatory support; 2) the mechanics of the respiratory system, and 3) the characteristics of the Pmus waveform. On the other hand the response of Pmus to Paw is mediated through four feedback systems: 1) mechanical; 2) chemical; 3) reflex, and 4) behavioral. It follows that the system that controls the act of breathing may be considerably modified by mechanical ventilation. The physician dealing with a mechanically ventilated patient should take into account the interaction between the respiratory effort and the function of the ventilator and be aware that the ventilatory output may or may not reflect the various aspects of control of breathing. Received: 13 July 1998 Accepted: 26 October 1998     

气道压力释放通气治疗急性肺损伤和急性呼吸窘迫综合征的研究进展

气道压力释放通气是以持续气道正压为工作基础的压力控制机械通气模式,其从高气道压向低气道压释放压力产生通气量进行气体交换.它能在肺保护通气策略基础上维持肺复张、开放及减少呼吸机相关肺损伤.它对急性肺损伤/急性呼吸窘迫综合征患者在降低气道峰压及平均压、提高氧合、改善血流动力学和胃肠灌注、减少镇静和麻醉药物使用等方面产生广泛有利影响;而且,自主呼吸对重力依赖区塌陷肺组织的重新开放在其中发挥重要作用,其应用受到研究者和临床医师越来越多的关注.     

Effect of intermittent mandatory ventilation on respiratory drive and timing

Seven patients receiving chronic ventilatory support were studied to better define the effects of intermittent mandatory ventilation (IMV) on the control and timing of spontaneous breathing between mandatory breaths. Each of these patients could sustain adequate spontaneous ventilation, as reflected by stable end-tidal carbon dioxide concentration (FETCO2), and arterial oxygen saturation (SO2) during periods of unassisted ventilation of sufficient duration to allow study. Inspiratory time (TI), respiratory cycle duration (Ttot), tidal volume (VT), and tracheal occlusion pressure (P0.1) were measured as IMV rate was progressively reduced. Respiratory timing was unaltered by decreasing IMV frequency; however, VT increased progressively. The P0.1 and mean inspiratory flow rate (VT/TI) also increased with each decrease in IMV rate, whereas FETCO2 and arterial SO2 remained constant. Thus, in these stable but ventilator-dependent patients, IMV did not alter respiratory timing or chemical stimuli, but it did alter respiratory drive as measured by VT/TI and P0.1.     

慢性阻塞性肺疾病急性发作期患者对比例辅助通气的生理反应

目的　探讨比例辅助通气 (PAV)不同辅助水平对慢性阻塞性肺疾病 (COPD)急性发作期患者生理反应的影响。方法　 9例COPD急性发作期患者接受三个不同比例辅助水平的PAV通气 ,观察患者吸气肌肉用力情况和呼吸方式的变化。结果　 (1)与自主呼吸 (SB)相比 ,PAV各辅助水平时的潮气量 (VT)、分钟通气量 (V·E)和呼吸频率 (RR)均稍增高 (P >0 0 5 )。各比例辅助水平之间的VT、V·E 和RR比较差异无显著性 (P >0 0 5 )。 (2 )与SB相比 ,各比例辅助水平时的跨膈压 (Pdi)、压力时间乘积 (PTP)和患者呼吸做功均明显减少 (P >0 0 1) ,Pdi、PTP和患者呼吸做功分别平均减少 8 36cmH2 O、11 4 9cmH2 O·s-1·L-1和 0 5 3J/L。随比例辅助水平的升高 ,Pdi、PTP和患者呼吸功无明显变化(P >0 0 5 )。 (3)PAV可减轻患者呼吸困难 (P <0 0 5 )。结论　本试验证实了无创PAV在COPD急性发作期患者中应用的可行性。患者感觉最舒适的PAV辅助比例水平是 (5 7± 11) %。根据患者感觉舒适情况而设定比例辅助水平的无创PAV可减轻患者的呼吸肌肉负担 ,最舒适水平时呼吸功减少5 7% ,Pdi减少 72 % ,PTP减少 6 5 % ;并改善患者的呼吸方式和呼吸困难     

Recent advances in mechanical ventilation

Important advances have been made over the past decade towards understanding the optimal approach to ventilating patients with acute respiratory failure. Evidence now supports the use of noninvasive positive pressure ventilation in selected patients with hypercapnic respiratory failure and chronic obstructive pulmonary disease, cardiogenic pulmonary edema, and for facilitating the discontinuation of ventilatory support in patients with chronic pulmonary disease. The concept of a lung protective ventilatory strategy has revolutionized the management of the acute respiratory distress syndrome. The process of liberation from mechanical ventilation is becoming more standardized, with evidence supporting daily trials of spontaneous breathing in all suitable mechanically ventilated patients. This article critically reviews the most important recent advances in mechanical ventilation and suggests future directions for further research in the field.     

Expiratory-Synchronized Sleep in a Quadriplegic Patient Using Inspiratory Neck Muscles To Breathe

In a patient with C3 quadriplegia causing complete diaphragm paralysis who developed inspiratory neck muscles (INM) hypertrophy to sustain ventilation, spontaneous breathing deeply altered sleep architecture, relegating sleep to the expiratory phase of the ventilatory cycle. A polysomnographic recording performed during mechanical ventilation (without INM activity), showed that sleep was abnormal but unaffected by the respiratory cycle. During spontaneous breathing, the polygraphic recordings showed expiratory microsleep episodes, with inspiratory arousals synchronous to bursts of INM activity. This case report illustrates the powerful adaptability of the respiratory and sleep control systems to maintain each vital function.