Effect on respiratory function of pressure support ventilation versus synchronised intermittent mandatory ventilation in preterm infants

Our objective was to compare the effects of pressure support ventilation and synchronized intermittent mandatory ventilation on respiratory function in preterm babies. Twenty preterm infants (mean gestational age, 29 weeks; mean weight at study, 1,354 g) were evaluated. Patients received two repeated cycles of synchronized intermittent mandatory ventilation, alternated with pressure support ventilation, for a total of four alternated phases, each phase lasting 4 hr. Spontaneous respiratory rate, tidal volume, minute volume, and mean airway pressure were recorded hourly. The tidal volume released by the ventilator was limited to 6 ml/kg. During the two pressure support ventilation phases, a statistically significant reduction of respiratory rate and a significant increase of tidal and minute volume were noted, as compared to the two synchronized intermittent mandatory ventilation periods. Mean airway pressure significantly increased only after the first shift from synchronized intermittent mandatory ventilation to pressure support ventilation. The changes of minute volume and respiratory rate observed during pressure support ventilation did not persist after the return to synchronized intermittent mandatory ventilation. In conclusion, pressure support ventilation, as compared to synchronized intermittent mandatory ventilation, seemed to improve respiratory function in preterm infants.     

Does intermittent mandatory ventilation correct respiratory alkalosis in patients receiving assisted mechanical ventilation?

One of the claimed advantages of intermittent mandatory ventilation (IMV) over assisted mechanical ventilation (AMV) (assist-control) is the avoidance or correction of acute respiratory alkalosis, ostensibly by allowing patients to achieve normal alveolar ventilation (VA) and PaCO2 through the function of an intact ventilatory drive. However, although respiratory alkalosis in patients being hyperventilated with controlled mechanical ventilation (CMV) can be corrected by a change to IMV, CMV is seldom appropriate for patients with acute respiratory failure, and whether IMV affects respiratory alkalosis in patients triggering the ventilator in the AMV mode has not previously been tested. We studied 26 patients with acute respiratory alkalosis (pH greater than or equal to 7.48) while receiving AMV. Measurements of arterial blood gases and CO2 production (VCO2), and calculation of VA, were performed after 30 min of AMV, repeated after 30 min of IMV at a mandatory rate one half the previous AMV rate, and then repeated again 30 min after a return to the original AMV settings. Mean arterial pH decreased slightly from 7.51 during AMV to 7.48 during IMV, and returned to 7.51 on resumption of AMV (p less than 0.05 for both changes); corresponding mean values for PaCO2 were 28.6, 29.7, and 27.5 mmHg, respectively. These changes were related to an increase in VCO2 during IMV as compared with AMV (p less than 0.05), without a significant alteration in VA. When the mandatory rate was further reduced during IMV from one half to one fourth the prior, triggered AMV rate in 10 patients, no additional reduction in pH occurred.(ABSTRACT TRUNCATED AT 250 WORDS)     

Demand and continuous flow intermittent mandatory ventilation systems

A mechanical lung was used to evaluate the pressure and flow characteristics of four demand and two continuous flow intermittent mandatory ventilation (IMV) systems. The amount of negative pressure required to initiate inspiratory flow and peak expiratory resistance were measured. The inspiratory pressure required to initiate flow in the demand mode was also compared to pressures generated in the assist mode. In addition, the peak expiratory resistance was measured with four commercially available exhalation valves. Results showed that the ventilator manometer measuring internal machine pressures significantly underestimated the amount of negative pressure required to open the demand valve (p less than 0.01). There are major differences in the flow and pressure characteristics among demand and continuous flow IMV systems. Systems that impose high inspiratory elastic threshold loads and expiratory flow resistive loads may have a deleterious effect on the mechanics of breathing, and thereby limit weaning success and eventually impair the recovery of certain patients in respiratory failure. The basic methodology, especially the simple technique of inserting an aneroid manometer in line next to a patient's ET tube, for measuring proximal negative inspiratory force (NIF test) can be easily applied to any and all ventilators at any practitioner's individual institution.     

Effect of intermittent ventilation on pulmonary hypertension in chronic respiratory failure

Intermittent non-invasive (or nocturnal mechanical ventilation) eliminates symptoms of hypoventilation and improves gas exchange in patients with chronic respiratory failure. Performing right heart catheterisation we studied the influence of nocturnal mechanical ventilation on pulmonary hemodynamics. We investigated 20 patients with restrictive thoracic diseases (Post-TBC: n = 9, scoliosis: n = 11, PaCO2: 59.8 +/- 7.6 mmHg) and 13 patients with COPD (n = 13, PaCO2: 58.5 +/- 7.8 mmHg). All patients were mechanically ventilated in controlled mode. During the study the medication was not changed; COPD patients with long-term oxygen maintained this therapy. Right-heart catheterisation was performed immediately before and after 1 year nocturnal mechanical ventilation. In patients with thoracic restriction NMV induced a marked reduction of pulmonary artery pressure (PAP) from 33.2 +/- 10.0 mmHg before to 24.8 +/- 6.2 mmHg after 1 year nocturnal mechanical ventilation. In the COPD group PAP increased from 25.3 +/- 6.0 mmHg before to 27.5 +/- 6.0 mmHg after 1 year nocturnal mechanical ventilation. In contrast to the COPD group in patients with chronic respiratory failure due to thoracic restriction nocturnal mechanical ventilation causes substantial reduction in pulmonary artery pressure after a one year application.     

Closed-loop control of respiratory drive using pressure-support ventilation: target drive ventilation

By using diaphragm electrical activity (multiple-array esophageal electrode) as an index of respiratory drive, and allowing such activity above or below a preset target range to indicate an increased or reduced demand for ventilatory assistance (target drive ventilation), we evaluated whether the level of pressure-support ventilation can be automatically adjusted in response to exercise-induced changes in ventilatory demand. Eleven healthy individuals breathed through a circuit (18 cm H2O/L/second inspiratory resistance at 1 L/second flow; 0.5-1.0 L/second expiratory flow limitation) connected to a modified ventilator. Subjects breathed for 6-minute periods at rest and during 20 and 40 W of bicycle exercise, with and without target drive ventilation (the target was set to 60% of the increase in diaphragm electrical activity observed between rest and 20 W of unassisted exercise). With target drive ventilation during exercise, the level of pressure-support ventilation was automatically increased, reaching 13.3 +/- 4.0 and 20.3 +/- 2.8 cm H2O during 20- and 40-W exercise, respectively, whereas diaphragm electrical activity was reduced to a level within the target range. Both diaphragmatic pressure-time product and end-tidal CO2 were significantly reduced with target drive ventilation at the end of the 20- (p < 0.01) and 40-W (p < 0.001) exercise periods. Minute ventilation was not altered. These results demonstrate that target drive ventilation can automatically adjust pressure-support ventilation, maintaining a constant neural drive and compensating for changes in respiratory demand.     

Respiratory drive and timing during assisted ventilation in dogs

In 8 anesthetized dogs, during isocapnic hyperoxia we studied the effect of assisted ventilation (AV) on ventilatory drive, inspiratory off-switch volume (Voff) and duration of inspiratory diaphragmatic activity (Tdi). Tidal volumes (Vt) during AV were double spontaneous Vt. Two electrodes were inserted in the diaphragm to obtain the electromyogram (EMG). The index of ventilatory drive was the EMG 0.3--i.e., the amplitude of the moving average EMG 300 ms after the onset of inspiratory activity. AV decreased EMG 0.3 but had no effect on Voff and Tdi. Vagotomy not only abolished the reduction in drive, but may have increased drive during AV.     

Randomized controlled trial of volume-targeted synchronized ventilation and conventional intermittent mandatory ventilation following initial exogenous surfactant therapy

We set out to evaluate the impact of volume-targeted synchronized ventilation and conventional intermittent mandatory ventilation (IMV) on the early physiologic response to surfactant replacement therapy in neonates with respiratory distress syndrome (RDS). We hypothesized that volume-targeted, patient-triggered synchronized ventilation would stabilize minute ventilation at a lower respiratory rate than that seen during volume-targeted IMV, and that synchronization would improve oxygenation and decrease variation in measured tidal volume (V(t)). This was a prospective, randomized study of 30 hospitalized neonates with RDS. Infants were randomly assigned to volume-targeted ventilation using IMV (n = 10), synchronized IMV (SIMV; n = 10), or assist/control ventilation (A/C; n = 10) after meeting eligibility requirements and before initial surfactant treatment. Following measurements of arterial blood gases and cardiovascular and respiratory parameters, infants received surfactant. Infants were studied for 6 hr following surfactant treatment. Infants assigned to each mode of ventilation had similar birth weight, gestational age, and Apgar scores at birth, and similar oxygenation indices at randomization. Three patients were eliminated from final data analysis because of exclusionary conditions unknown at randomization. Oxygenation improved significantly following surfactant therapy in all groups by 1 hr after surfactant treatment (P < 0.05). No further improvements occurred with time. Total respiratory rate was lowest (P < 0.05) and variation in tidal volume (V(t)) was least in the A/C group (P < 0. 05). Minute ventilation (V(')(E)), delivered airway pressures, respiratory system mechanics, and hemodynamic parameters were similar in all groups. We conclude that volume-targeted A/C ventilation resulted in more consistent tidal volumes at lower total respiratory rates than IMV or SIMV. Oxygenation and lung mechanics were not altered by synchronization, possibly due to the volume-targeting strategy. Of the modes studied, A/C, a fully-synchronized mode, may be the most efficient method of mechanical ventilator support in neonates receiving surfactant for treatment of RDS.     

Initial synchronized intermittent mandatory ventilation versus assist/control ventilation in treatment of moderate acute respiratory distress syndrome: a prospective randomized controlled trial

Background

Assist/control (A/C) ventilation may induce delirium in patients with acute respiratory distress syndrome (ARDS). We conducted a trial to determine whether initial synchronized intermittent mandatory ventilation with pressure support (SIMV + PS) could improve clinical outcomes in these patients.

Methods

Intubated patients with moderate ARDS were enrolled and we compared SIMV + PS with A/C. Identical sedation, analgesia and ventilation strategies were performed. The co-primary outcomes were early (≤72 h) partial pressure of arterial oxygen to fraction of inspired oxygen (PaO₂/FiO₂) and incidence of delirium. The secondary outcomes were all-cause in-hospital mortality, dosages of analgesics and sedatives, incidence of patient-ventilator asynchrony, and duration of mechanical ventilation and hospital stay.

Results

We screened 2,684 patients and 40 patients were enrolled in our study. In SIMV + PS, early (≤72 h) PaO₂/FiO₂ was greater improved than that at baseline and that in A/C (P<0.05) with lower positive end-expiratory pressure (PEEP) (8.7±3.0 vs. 10.3±3.2, P<0.001) and FiO₂ (58%±18% vs. 67%±19%, P<0.001). We found more SIMV + PS success (defined as SIMV + PS successfully applied without switching to A/C) (100.0% vs. 16.7%, P<0.001), less male (46.3% vs. 85.7%, P=0.015) and pulmonary etiology of ARDS (53.8% vs. 92.9%, P=0.015), and lower PEEP (9.1±3.1 vs. 10.3±3.3, P=0.004) and FiO₂ (58%±19% vs. 71%±19%, P<0.001) in survival patients. However, there were no significant differences in incidence of delirium and mortality, dosages of analgesics and sedatives, incidence of patient-ventilator asynchrony, duration of mechanical ventilation and hospital stay (P>0.05).

Conclusions

In patients with moderate ARDS, SIMV + PS can safely and effectively improve oxygenation, but does not decrease mortality, incidence of delirium and patient-ventilator asynchrony, dosages of analgesics and sedatives, and duration of mechanical ventilation and hospital stay.     

Noninvasive intermittent self ventilation in chronic respiratory insufficiency

Corresponding to the two compartments of the respiratory system (lungs and ventilatory pump), two different forms of respiratory insufficiency can be distinguished on a pathophysiologic basis: disturbances of gas exchange with primary oxygenation failure (hypoxemia) due to pulmonary diseases and reduced ventilation of the lungs (hypoventilation) with primary elevated arterial carbon dioxide partial pressure (hypercapnia) and secondary hypoxemia due to disorders of the ventilatory pump. Different methods can be employed in the diagnosis of respiratory insufficiency, e.g., spirometry, blood gas analysis, nocturnal monitoring with capnography, or transcutaneous pCO2 registration and measurement of mouth occluding pressure. Therapeutic measures for respiratory insufficiency are based on two treatment principles: long-term oxygen therapy to improve hypoxemia and noninvasive ventilation therapy to improve hypercapnia and concomitant hypoxemia. The indication for long term ventilation depends on subjective complaints, objective measurement parameters, disease course, and other disease symptoms. The most frequent indications for home mechanical ventilation are diseases with restrictive ventilatory defects, especially chest wall disorders and neuromuscular disorders.     

Influence of caloric intake on the respiratory mode during mandatory minute volume ventilation

Mandatory minute volume ventilation has been proposed as a method for weaning patients from ventilators. The purpose of this study was to delineate the influence of caloric intake on spontaneous ventilation in patients receiving mandatory minute volume ventilation. While the value of such ventilation remained unchanged, eight patients were studied at the following three different levels of daily caloric intake: (1) level A, mean of 223 kcal/sq m; (2) level B, mean of 1,380 kcal/sq m; and (3) level C, mean of 2,100 kcal/sq m. We performed gas exchange measurements and a 24-hour recording of ventilation with a monitoring system providing distinction between spontaneous and mechanical cycles. We found that the ventilatory mode was markedly dependent upon the nutritional intake; the percentage of spontaneous ventilation over 24 hours increased from 11 +/- 7 percent (+/- SE) during diet A to 50 +/- 9 percent during diet B and 79 +/- 8 percent during diet C. This increment paralleled the increase in production of carbon dioxide with caloric intake. We suggest therefore that the patient's ability to breathe spontaneously when receiving mandatory minute volume ventilation should be interpreted according to caloric intake.     

Effects of exercise modality on patterns of ventilation and respiratory timing.

Ventilatory patterns and respiratory timing were measured in 14 subjects during cycling (CYC) and treadmill exercise (TM) at similar leg frequencies (fLEG) to determine if mode of exercise affects patterns of ventilation and respiratory timing. Measurements of breathing frequency (fR), tidal volume (VT), expired ventilation (VE), and inspiratory (TI) and expiratory (TE) time were obtained at fLEG of 50, 70, and 90 rev.min-1 (rpm) for CYC and at similar incremental fLEG (strides.min-1; spm) during TM achieved by increasing belt speed at 0% grade. CYC exercise intensity was approximately 50% VO2,max at all fLEG, whereas VO2 increased progressively with TM. fR increased significantly (P < 0.001) with increasing fLEG of TM (20.5 +/- 4.6, 25.4 +/- 5.8, and 36.3 +/- 7.6 breaths.min-1; mean +/- SD), but during CYC fR changed significantly (P < 0.05) only between fLEG of 70 and 90 rpm (25.0 +/- 5.9 vs 28.5 +/- 6.9 breaths.min-1). Both average breath TI and TE obtained by grouping into incremental ranges of fR decreased significantly (P < 0.05) with increasing fR up to 36 breaths.min-1 and the relationships of TI and TE to fR, TI to TE, and central inspiratory drive (VT/TI) to VE were the same for CYC and TM. Group average fR and fLEG were synchronized during TM, but individual subjects did not exhibit a high degree of entrainment. This study shows respiratory timing patterns to be independent of mode of exercise over the range of fR observed when describing patterns by grouping into incremental ranges of fR.     

间断直接脱机与加热湿化给氧法对慢性阻塞性肺疾病患者脱机时间的影响

目的比较不同的脱机与护理方法对慢性阻塞性肺疾病(COPD)机械通气患者脱机时间的影响,为COPD机械通气患者寻找一种有效的脱机与护理方法。方法对符合临床脱机的103例COPD机械通气患者随机分为3组,Ⅰ组采用SIMV PSV通气法,逐步降低通气条件后至完全脱机;Ⅱ组采用间断直接脱机法进行脱机锻炼,脱机期间以持续气道湿化法进行人工气道护理、给氧等,Ⅲ组采用间断直接脱机法进行脱机锻炼,脱机期间使用加热湿化器进行气道加温湿化、给氧护理。比较3组间的脱机时间、48h后脱机失败发生率,以及Ⅱ组与Ⅲ组在脱机锻炼过程中的不同湿化效果和氧合状态。结果3个组脱机时间分别为Ⅰ组(120±43.2)h,Ⅱ组(86±12.6)h,Ⅲ组(50±21.2)h,Ⅲ组的脱机时间明显短于Ⅰ组和Ⅱ组,差异有统计学意义(P<0.05),Ⅱ组的脱机时间短于Ⅰ组(P<0.05),Ⅲ组湿化、氧合效果优于Ⅱ组,差异有统计学意义。结论COPD机械通气患者在脱机过程中采用间断直接脱机法进行脱机锻炼,脱机期间使用加热湿化器进行气道加温湿化、给氧护理,能明显缩短脱机时间,提高脱机的成功率。     

Episodes of hypoxemia during synchronized intermittent mandatory ventilation in ventilator-dependent very low birth weight infants

Distinct patterns of asynchrony, and episodes of hypoxemia, may occur in a spontaneously breathing preterm infant during conventional intermittent mandatory ventilation (IMV) on traditional time-cycled, pressure-limited ventilators. Synchronized IMV (SIMV) and assist/control ventilation are frequent modes of patient-triggered ventilation used with infant ventilators. The objective of this study was to use computerized pulse oximetry to quantify the occurrence of episodes of hypoxemia (oxygen desaturation) during SIMV vs. IMV, in preterm infants < or = 1,250 g who required mechanical ventilation at > or = 14 days of age. We performed a randomized, crossover study with each infant being randomized to IMV or SIMV (Infant Star ventilator) for initial testing for a 1-hr period. Patients were subsequently tested on the alternate modality after a stabilization period of 10 min at the same ventilator and fractional inspired oxygen concentration (FiO2) settings. Pulse oximetry data were obtained with a Nellcor N-200 monitor, a microcomputer, and a software program (SatMaster). An investigator blinded to the randomized assignment evaluated all measurements. Eighteen very low birth weight (VLBW) infants with a birth weight of 777 +/- 39 g (mean +/- SEM) and gestational age 25.1 +/- 0.3 weeks were studied. The average pulse oximeter oxygen saturation (SaO2) was higher on SIMV than IMV (P < 0.01). During SIMV, these infants had significantly fewer episodes of hypoxemia (duration of episodes of oxygen desaturation as a percentage of scorable recording time) to 86-90% SaO2 (P < 0.01), 81-85% SaO2 (P < 0.01), and 76-80% SaO2 (P < 0.05) when compared to IMV. There was also a significant decrease in percentage of time of desaturation to SaO2 < 90% (P = 0.002), < 85% SaO2 (P = 0.003), and < 80% SaO2 (P = 0.02) during SIMV vs. IMV. Our preliminary findings indicate that the use of SIMV in a population of VLBW ventilator-dependent infants (> or = 14 days of age) results in better oxygenation and decreased episodes of hypoxemia as compared to IMV.     

Time course of ventilatory drive in patients with kyphoscoliosis before and during intermittent ventilation

The aim of this retrospective study was to investigate the mouth-occlusion-pressure under CO2-stimulation in five patients (3 women, 2 men) suffering from kyphoscoliosis of different aetiology whilst being under noninvasive ventilation for respiratory muscle fatigue. Ten months after initiation of noninvasive ventilation we could demonstrate a marginal improvement of respiratory muscle strength but the P0.1 CO2-decreased slightly. We conclude that this decrease might be due to an increase in tidal volume allowing for a reduction in respiratory rate.     

Tidal volume and respiratory timing derived from a portable ventilation monitor

STUDY OBJECTIVES: To determine the accuracy of a portable magnetometer designed to measure tidal volume (VT), inspiratory time (TI), and expiratory time (TE). PARTICIPANTS: Fourteen healthy subjects. DESIGN: Subjects breathed over a sixfold range of VTs while at rest (sitting and standing) and during treadmill exercise. We then compared VT, TI, and TE measured by magnetometry (VTmag, TImag, and TEmag) with VT, TI, and TE measured by spirometry (VTspiro, TIspiro, and TEspiro, respectively). SETTING: Pulmonary function and exercise physiology laboratories. MEASUREMENTS AND RESULTS: The sternal-umbilical distance and the anteroposterior displacements of the rib cage and abdomen were measured with two pairs of magnetometer coils. VT was calculated from the sum of these three signals, and was simultaneously measured using a spirometer or flow meter. A total of 1,111 breaths were analyzed for the resting condition, and 1,163 breaths were analyzed for the exercise condition. We found that VTmag was highly correlated with VTspiro at rest (R2 = 0.90) and during exercise (R2 = 0.79) for pooled data. The slope of this relationship approached the line of identity. The mean percentage differences between VTmag and VTspiro were 10.1 +/- 6.6% at rest and 13.5 +/- 8.6% with exercise. By Bland-Altman analysis, the mean differences between VTmag and VTspiro were 38 mL at rest with changes in posture, and 182 mL during exercise. TImag and TIspiro values and TEmag and TEspiro values also were highly correlated (R2 = 0.97 and R2 = 0.95, respectively) for pooled data. CONCLUSION: A portable magnetometer system can give useful measures of VT, TI, and TE over a wide range of VTs in sitting, standing, and exercising subjects.     

Measuring exhaled volume with continuous positive airway pressure and intermittent mandatory ventilation. Techniques and rationale.

When patients breathe spontaneously through a ventilator circuit, a fall in airway pressure during the inspiratory cycle may increase inspiratory effort. A system of delivery which incorporates a distensible reservoir bag and delivers a constant flow of gas that is two or three times the patient's minute volume will prevent a significant drop in inspiratory airway pressure. Unfortunately, the constant flow of gas mixes with the patient's exhaled gas and makes continuous monitoring of exhaled volumes difficult. Two modifications of circuits are described which allow accurate continuous measurement of volume. One of these circuits allows analysis of the concentrations of expired gases. When spontaneous ventilation occurs, tidal volume and minute ventilation demonstrate an intact connection between the ventilator and the patient, continuously indicate the patient's ability to sustain independent ventilation, and give early warning of a change in respiratory status.