Comparison between tube compensation and pressure support ventilation techniques on respiratory mechanics

In the intubated patient, the presence of an endotracheal tube increases the work of breathing during spontaneous breathing. The tube compensation technique was developed as a new ventilator mode that can compensate for that additional the work of breathing. We investigated the respiratory parameters during the pressure support ventilation 0, 5, 10 cmH2O and tube compensation 100% modes of the Puritan Bennett 840 ventilator in ten postoperative patients who had undergone radical surgery for oesophageal cancer. Measurements were performed just before extubation. The tidal volume, respiratory rate and other respiratory parameters were measured with a Ventrak respiratory monitor, and the duty ratio, mean inspiratory flow, and rapid shallow breathing index were calculated. In particular, we performed a comparison between pressure support ventilation 5 cmH2O and tube compensation 100%, because pressure support ventilation 5 cmH2O is the usual ventilating mode before the extubation in our intensive care unit. The tidal volume of pressure support ventilation 10 cmH2O was significantly larger and the respiratory rate was significantly lower than the other three modes. There was no significant difference in the minute volume, tidal volume, and respiratory rate between pressure support ventilation 5 cmH2O and tube compensation 100%. The duty ratio of pressure support ventilation 10 cmH2O was significantly smaller than the other three modes. There was no significant difference in the duty ratio and rapid shallow breathing index between pressure support ventilation 5 cmH2O and tube compensation 100%. It was concluded that the assist levels of pressure support ventilation 5 cmH2O and tube compensation 100% were almost equal for clinical purposes.     

Inspiratory muscle relaxation rate assessed from sniff nasal pressure.

BACKGROUND--Slowing of the maximum relaxation rate (MRR) of inspiratory muscles measured from oesophageal pressure (POES) during sniffs has been used as an index of the onset and recovery of respiratory muscle fatigue. The purpose of this study was to measure MRR at the nose (PNASAL MRR), to investigate its relationship with POES MRR, and to establish whether PNASAL MRR slows with respiratory loading. METHODS--Five normal subjects were studied. Each performed sniffs before and after two minutes of maximal isocapnic ventilation (MIV). In a separate session the subjects performed submaximal sniffs. POES and PNASAL were recorded during sniffs and the MRR (% pressure fall/10 ms) for each sniff was determined. RESULTS--Before MIV mean POES MRR was 8.9 and PNASAL MRR was 9.3. The mean (SD) difference between PNASAL MRR and POES MRR during a maximal sniff was 0.48 (0.34) (n = 64) and during submaximal sniffs was 0.28 (0.46) (n = 526). The subjects showed a mean decrease in sniff POES MRR of 27.4% (range 22.5-36%) after MIV and a similar reduction in sniff PNASAL MRR of 28.5% (range 24.1-41.3%). Both returned to control values within 5-10 minutes. CONCLUSIONS--PNASAL MRR reflects POES MRR over a wide range of sniff pressures, PNASAL MRR of maximal sniffs reflects POES MRR in normal subjects at rest and following MIV, so measurement of PNASAL MRR may be a useful non-invasive method for measuring inspiratory muscle MRR, thereby providing an index of respiratory muscle fatigue.     

Mouth pressure in response to magnetic stimulation of the phrenic nerves.

BACKGROUND--Diaphragm strength can be assessed by the measurement of gastric (TW PGA), oesophageal (TW POES), and transdiaphragmatic (TW PDI) pressure in response to phrenic nerve stimulation. However, this requires the passage of two balloon catheters. A less invasive method of assessing diaphragm contractility during stimulation of the phrenic nerves would be of clinical value. A study was undertaken to determine whether pressure measured at the mouth (TW PM) during magnetic stimulation of the phrenic nerves accurately reflects TW POES, and to investigate the relations between TW PM and TW PDI; and also to see whether glottic closure and twitch potentiation can be avoided during these measurements. METHODS--Eight normal subjects and eight patients with suspected respiratory muscle weakness without lung disease were studied. To prevent glottic closure magnetic stimulation of the phrenic nerves was performed at functional residual capacity during a gentle expiratory effort against an occluded airway incorporating a small leak. TW PDI, TW POES, and TW PM were recorded. Care was taken to avoid potentiation of the diaphragm. RESULTS--In normal subjects mean TW PM was 13.7 cm H2O (range 11.3-16.1) and TW POES was 13.3 cm H2O (range 10.4-15.9) with a mean (SD) difference of 0.4 (0.81) cm H2O. In patients mean TW PM was 9.1 cm H2O (range 0.5-18.2) and TW POES was 9.3 (range 0.7-18.7) with a mean (SD) difference of -0.2 (0.84) cm H2O. The relation between TW PM and TW PDI was less close but was well described by a linear function. In patients with diaphragm weakness (low sniff PDI) TW PM was < 10 cm H2O. CONCLUSIONS--TW PM reliably reflects TW POES and can be used to predict TW PDI in normal subjects and patients without lung disease. TW PM may therefore be a promising non-invasive, non-volitional technique for the assessment of diaphragm strength.     

Effect of synchronized intermittent mandatory ventilation on respiratory workload in infants after cardiac surgery

BACKGROUND: Synchronized intermittent mandatory ventilation (SIMV) is commonly used in infants and adults. However, few investigations have examined how SIMV reduces respiratory workload in infants. The authors evaluated how infants' changing respiratory patterns when reducing SIMV rate increased respiratory load. The authors also investigated whether SIMV reduces infant respiratory workload in proportion to the rate of mandatory breaths and which rate of SIMV provides respiratory workloads similar to those after tracheal extubation. METHODS: When 11 post-cardiac surgery infants aged 2-11 months were to be weaned with SIMV, the authors randomly applied five levels of mandatory breathing: 0, 5, 10, 15, and 20 breaths/min. All patients underwent ventilation with SIMV mode: pressure control ventilation, 16 cm H2O; inspiratory time, 0.8 s; triggering sensitivity, 0.6 l/min; and positive endexpiratory pressure, 3 cm H2O. After establishing steady-state conditions at each SIMV rate, arterial blood gases were analyzed, and esophageal pressure, airway pressure, and airflow were measured. Inspiratory work of breathing, pressure-time products, and the negative deflection of esophageal pressure were calculated separately for assisted breaths, for spontaneous breaths, and for total breaths per minute. Measurements were repeated after extubation. RESULTS: As the SIMV rate decreased, although minute ventilation and arterial carbon dioxide tension were maintained at constant values, spontaneous breathing rate and tidal volume increased. Work of breathing, pressure-time products, and negative deflection of esophageal pressure increased as the SIMV rate decreased. Work of breathing and pressure-time products after extubation were intermediate between those at a SIMV rate of 5 breaths/min and those at 0 breaths/min. CONCLUSION: When the load to breathing was increased progressively by decreasing the SIMV rate in post-cardiac surgery infants, tidal volume and spontaneous respiratory rate both increased. In addition, work of breathing and pressure-time products were increased depending on the SIMV rate.     

Effect of Synchronized Intermittent Mandatory Ventilation on Respiratory Workload in Infants after Cardiac Surgery

Background: Synchronized intermittent mandatory ventilation (SIMV) is commonly used in infants and adults. However, few investigations have examined how SIMV reduces respiratory workload in infants. The authors evaluated how infants' changing respiratory patterns when reducing SIMV rate increased respiratory load. The authors also investigated whether SIMV reduces infant respiratory workload in proportion to the rate of mandatory breaths and which rate of SIMV provides respiratory workloads similar to those after tracheal extubation.

Methods: When 11 post-cardiac surgery infants aged 2-11 months were to be weaned with SIMV, the authors randomly applied five levels of mandatory breathing: 0, 5, 10, 15, and 20 breaths/min. All patients underwent ventilation with SIMV mode: pressure control ventilation, 16 cm H2O; inspiratory time, 0.8 s; triggering sensitivity, 0.6 l/min; and positive end-expiratory pressure, 3 cm H2O. After establishing steady-state conditions at each SIMV rate, arterial blood gases were analyzed, and esophageal pressure, airway pressure, and airflow were measured. Inspiratory work of breathing, pressure-time products, and the negative deflection of esophageal pressure were calculated separately for assisted breaths, for spontaneous breaths, and for total breaths per minute. Measurements were repeated after extubation.

Results: As the SIMV rate decreased, although minute ventilation and arterial carbon dioxide tension were maintained at constant values, spontaneous breathing rate and tidal volume increased. Work of breathing, pressure-time products, and negative deflection of esophageal pressure increased as the SIMV rate decreased. Work of breathing and pressure-time products after extubation were intermediate between those at a SIMV rate of 5 breaths/min and those at 0 breaths/min.     

Sildenafil may facilitate weaning in mechanically ventilated COPD patients: a report of three cases

We report three cases of mechanically ventilated chronic obstructive pulmonary disease patients who were intubated due to an exacerbation of their disease and who presented with repeated spontaneous breathing trial failures. Patients were given 50 mg of sildenafil through the nasogastric tube, under close monitoring of haemodynamic and ventilatory parameters. After sildenafil, pulmonary artery pressure, pulmonary artery occlusion pressure, the respiratory frequency to tidal volume ratio and the P(a)CO2-P(ET)CO2 (arterial minus end-tidal carbon dioxide pressure) decreased. Cardiac output increased in two of the patients, while all of them were successfully extubated. This is the first report of successful extubation after sildenafil use.     

Effect of continuous positive airway pressure (CPAP) in patients with chronic obstructive pulmonary chsease (COPD) depending on intrinsic PEEP levels

The application of continuous positive airway pressure (CPAP) is known to reduce inspiratory work of breathing in intubated patients with chronic obstructive pulmonary disease (COPD). This effect is caused by a decrease in elastic work related to a reduction in intrinsic PEEP. The aim of this study was to relate the decrease in inspiratory work due to CPAP to the intrinsic PEEP levels obtained during spontaneous breathing without positive pressure. Ten intubated patients with COPD who had been ventilated for acute respiratory failure were studied. Intrinsic PEEP was determined during tracheal occlusions performed at end-expiration when the patient was breathing without positive airway pressure. Inspiratory work was computed during breathing through a circuit with a CPAP of 0.5 kPa and the same circuit without positive pressure. Intrinsic PEEP-levels ranged from 0.26 to 1.31 kPa. Compared to spontaneous breathing without positive pressure, CPAP reduced the total inspiratory work per liter of ventilation (Wltot) from 1.42±0.48 to 1.24±0.50 J·1^-1 (means±SD P <0.01). This decrease was found to be related to the intrinsic PEEP-levels: the largest reductions were found in the patients with an intrinsic PEEP-level close to the CPAP-level applied. In intubated patients with COPD, the decrease in Wltot due to a CPAP of 0.5 kPa was found to be related to the intrinsic PEEP-levels present when no positive airway pressure was applied. The intrinsic PEEP measured during tracheal occlusions could be used to estimate the effect of CPAP in these patients.     

Accuracy of end-tidal carbon dioxide monitoring using the NBP-75 microstream capnometer. A study in intubated ventilated and spontaneously breathing nonintubated patients

Arterial carbon dioxide partial pressure measurements using the NBP-75 microstream capnometer were compared with direct PaCO2 values in patients who were (a) not intubated and spontaneously breathing, and (b) patients receiving intermittent positive pressure ventilation of the lungs and endotracheal anaesthesia. Twenty ASA physical status I-III patients, undergoing general anaesthesia for orthopaedic or vascular surgery were included in a prospective crossover study. After a 20-min equilibration period following the induction of general anaesthesia, arterial blood was drawn from an indwelling radial catheter, while the end-tidal carbon dioxide partial pressure was measured at the angle between the tracheal tube and the ventilation circuit using a microstream capnometer (NBP-75, Nellcor Puritan Bennett, Plesanton, CA, USA) with an aspiration flow rate of 30 mL min(-1). Patients were extubated at the end of surgery and transferred to the postanaesthesia care unit, where end-tidal carbon dioxide was sampled through a nasal cannula (Nasal FilterLine, Nellcor, Plesanton, CA, USA) and measured using the same microstream capnometer. In each patient six measurements were performed, three during mechanical ventilation and three during spontaneous breathing. A good correlation between arterial and end-tidal carbon dioxide partial pressure was observed both during mechanical ventilation (r = 0.59; P = 0.0005) and spontaneous breathing (r = 0.41; P = 0.001); while no differences in the arterial to end-tidal carbon dioxide tension difference were observed when patients were intubated and mechanically ventilated (7. 3 +/- 4 mmHg; CI95: 6.3-8.4) compared to values measured during spontaneous breathing in the postanesthesia care unit, after patients had been awakened and extubated (6.5 +/- 4.8 mmHg; CI95: 5. 2-7.8) (P = 0.311). The mean difference between the arterial to end-tidal carbon dioxide tension gradient measured in intubated and non-intubated spontaneously breathing patients was 1 +/- 6 mmHg (CI95: -11-+13). We conclude that measuring the end-tidal carbon dioxide partial pressure through a nasal cannula using the NBP-75 microstream capnometer provides an estimation of arterial carbon dioxide partial pressure similar to that provided when the same patients are intubated and mechanically ventilated.     

Nasal ventilation to facilitate weaning in patients with chronic respiratory insufficiency.

BACKGROUND: The non-invasive technique of nasal intermittent positive pressure ventilation (NIPPV) has an established role in providing domiciliary nocturnal ventilatory support in patients with chest wall disorders, neuromuscular disease, and chronic obstructive lung disease. NIPPV was used to simplify ventilatory management and assist the return of spontaneous breathing in patients with chronic respiratory insufficiency who had failed to wean from conventional intermittent positive pressure ventilation (IPPV). METHODS: A trial of NIPPV was carried out in 22 patients with weaning difficulties. Nine patients had chest wall disorders or primary lung disease, six had neuromuscular conditions, and seven had cardiac disorders with additional pulmonary disease. Conventional IPPV via an endotracheal tube or tracheostomy had been continued postoperatively in nine patients and 13 had been ventilated after acute cardiorespiratory decompensation. RESULTS: Conventional IPPV had been continued for a median of 31 days (range 2-219). Eighteen patients were successfully transferred to NIPPV and discharged home a median of 11 days (range 8-13) after starting this type of ventilation. Sixteen patients remain well 1-50 months after hospital discharge and 10 of these continue on domiciliary nocturnal NIPPV. Seven patients have returned to work. CONCLUSION: NIPPV can be used to facilitate the return of spontaneous breathing and to reduce the need for intensive care accommodation in patients with an acute exacerbation of chronic respiratory insufficiency that requires intubation and IPPV.     

Weaning from mechanical ventilation: an update

Many patients admitted to the intensive care unit have respiratory failure and thus require mechanical ventilation. Weaning patients from mechanical ventilation after their primary disease process has been treated can be difficult in approximately 30% of patients. Inadequacies in pulmonary gas exchange and in the performance of the respiratory muscle pump are the most common causes for failure to wean. Assessing whether a patient can be weaned from mechanical ventilation involves two major factors: (1) examining the patient for evidence of an increase in the work of breathing, and (2) measuring spontaneous breathing variables. Although different modalities have been used in weaning patients from mechanical ventilation, none has been shown to be more successful than repeated trials of spontaneous breathing.     

Endotracheal tube versus face mask with and without continuous positive airway pressure (CPAP)

Various ways of delivering continuous positive airway pressure (CPAP) have been extensively studied, with little attention, however, being paid to the effects of an intubation tube compared with breathing through a face mask, with or without CPAP. Pulmonary and cardiovascular variables were measured while 12 patients recovering from coronary artery bypass grafting were spontaneously breathing at ambient airway pressure, then at 7.4 mmHg (1 kPa) CPAP, and again at ambient pressure just before extubation. The same stages were repeated immediately after extubation, with patients breathing through a tight-fitting face mask. Arterial oxygen tension (Pao₂, mean ± s.d.) was better when the patients were breathing at ambient pressure through a face mask (11.7±2.8 kPa) than when they were intubated (10.6±2.4 kPa, P < 0.05). Compared with ambient pressure, CPAP (7.4 mmHg) (1 kPa) increased Pao₂ in both modes (13.4 ± 3.5 kPa with mask, and 12.6 ± 3.5 kPa when intubated, n.s.). The best arterial oxygen saturation was measured during CPAP with a face mask (96± 1%). Cardiac output remained unchanged in all the breathing modes. After coronary artery bypass grafting, spontaneous breathing with a face mask resulted in better Pao₂ than breathing through an endotracheal tube, both with and without 7.4 mmHg (1 kPa) CPAP. This study indicates that unnecessary delay in extubation should be avoided.     

Evaluation of the spontaneous breathing trial in burn intensive care patients

Background

The extubation failure rate in our burn patients is 30%.

Objective

To evaluate the influence of the 30 min spontaneous breathing trial on extubation outcome in burn patients.

Methods

A prospective, observational study in a burn intensive care unit. All adult patients requiring mechanical ventilation for >24 h and meeting the inclusion criteria underwent a 30 min spontaneous breathing trial (SBT). Extubation was undertaken after a successful SBT.

Results

Of 49 planned extubations, 9 failed (18%), much lower than the 30% extubation failure rate identified prior to the implementation of the SBT. The duration of ventilation was significantly shorter (p = 0.04) in the patients who passed a SBT and those who failed extubation were significantly older (p = 0.003). The logistic regression analysis identified that age independently predicted extubation outcome. Patients who failed extubation, after a successful SBT, had a significantly longer duration of ventilation (p = 0.0001) and ITU length of stay (p = 0.001).

Conclusions

The incidence of extubation failure was much lower and the duration of ventilation significantly shorter in patients who were extubated after a successful SBT. These findings support the use of the SBT in burn patients. Age independently predicts extubation outcome in burn patients who have passed a SBT.     

Automatic tube compensation (ATC)

Automatic tube compensation (ATC) is a new option to compensate for the non-linearly flow-dependent pressure drop across an endotracheal or tracheostomy tube (ETT) during inspiration and expiration. ATC is based on a closed-loop working principle. ATC is not a true ventilatory mode but rather a new option which can be combined with all conventional ventilatory modes. ATC compensates for the tube-related additional work of breathing. As of yet, ATC has been associated with certain benefits for the tracheally intubated spontaneously breathing patient. Among these, reduced work of breathing, preservation of the natural "noisy" breathing pattern, enhanced synchronization between the patient and the ventilator, and improvement of respiratory comfort seem to be most important. Moreover, sufficient spontaneous breathing with ATC alone, i.e. without any additional ventilatory assist, might help to predict more accurately readiness for extubation in the last phase of weaning from mechanical ventilation. Furthermore, it has been shown in patients with acute lung injury that ATC unloaded the inspiratory muscles and increased alveolar ventilation without adversely affecting cardiorespiratory function. It is the purpose of this article to describe the working principle of ATC and to give a review of the actual scientific discussion concerning ATC.     

Bi-level positive airway pressure ventilation reduces the oxygen cost of breathing in long-standing post-polio patients on invasive home mechanical ventilation

BACKGROUND: Today, patients with chronic respiratory failure are commonly treated with non-invasive bi-level positive airway pressure ventilation, supporting spontaneous breathing. However, in conformity with previous clinical routine, many post-polio patients with chronic respiratory failure are still treated with invasive (i.e. via a tracheostomy) controlled mechanical ventilation (CMV). The aim of the study was to investigate the effect of invasive bi-level positive airway pressure ventilation on the work of breathing compared with that during the patients' ordinary CMV and spontaneous breathing without mechanical support. METHODS: Nine post-polio patients on invasive (tracheostomy) nocturnal CMV were investigated. Work of breathing was analysed by assessing differences in oxygen consumption (VO2) using indirect calorimetry. Hereby, the oxygen cost of breathing during the various ventilatory modes could be estimated and related to one another. Data on energy expenditure were also obtained. RESULTS: The oxygen cost of breathing decreased by approximately 15% during bi-level positive airway pressure ventilation compared with CMV and spontaneous breathing. There was no difference between predicted (Harris-Benedict equation) and measured energy expenditure. CONCLUSION: Invasive bi-level positive airway pressure ventilation reduces the oxygen cost of breathing in long-standing tracheostomized post-polio patients, compared with CMV. Furthermore, the Harris-Benedict equation provides a reasonable prediction of energy expenditure in this group of patients.     

Respiratory care

PURPOSE OF THE REVIEW: Neurosurgical patients frequently develop respiratory complications, adversely affecting neurologic outcome and survival. The review summarizes current literature and management of respiratory complications associated with brain injury. MAJOR FINDINGS: Respiratory complications are commonly associated with traumatic brain injury and subarachnoid haemorrhage. Lung-protective ventilation with reduced tidal volumes improves outcome in acute lung injury, and should be applied to neurosurgical patients in the absence of increased intracranial pressure. Weaning from the mechanical ventilation should be initiated as soon as possible, although the role of neurological status in the weaning process is not clear. Prevention of pneumonia and aspiration improves survival. In patients with difficult weaning, early bedside percutaneous tracheostomy should be considered. FURTHER INVESTIGATIONS: Further studies are warranted to elucidate an optimal oxygenation and ventilation in brain-injured patients, weaning strategies, predictors of the failed weaning and extubation, respiratory support in patients with difficulties to wean, and early tracheostomy.     

A randomized trial of automated versus conventional protocol-driven weaning from mechanical ventilation following coronary artery bypass surgery

Objective: The Siemens servo 300 A ventilator has an automode function that allows automated weaning of patients from mechanical ventilation. Spontaneous breathing triggers the ventilator. After two spontaneously triggered breaths, the ventilator automatically changes from mandatory mechanical ventilation to spontaneous ventilation. If spontaneous breathing or triggering does not occur, the Siemens servo 300 A ventilator changes from spontaneous ventilation back to mandatory mechanical ventilation. We compared the effects of automated versus conventional protocol-driven weaning on the time until extubation in patients undergoing coronary artery bypass graft (CABG) surgery. In addition, we studied the effects of the mode of weaning on hemodynamic and physiologic parameters. Methods: Twenty consecutive male patients without respiratory disease scheduled for CABG at the University Hospital of Regensburg were entered into the study. Patients were randomized to postoperative ventilation with the Siemens 300 A/automode ventilator (group A, n = 10) or with the Siemens 300 ventilator (group B, n = 10). All patients were weaned from ventilation according to a standardized protocol. Results: On average, patients in group A were younger and had lower pulmonary artery pressure (PAP) and higher cardiac output compared to patients in group B. However, patients in group A had longer ischemic and bypass times compared to patients in group B. Postoperative use of analgesia and sedation were similar in both groups. Time from tracheal intubation until extubation was 2 h shorter in patients assigned to automode ventilation compared to patients assigned to conventional ventilation (mean time group A 7.9 h, group B 10.0 h; p = 0.069). Peak airway pressure was reduced by 2 cm H₂O at the beginning of spontaneous ventilation in group A compared to group B. After extubation, cardiac index showed a greater increase in patients assigned to group A compared to those in group B. Conclusions: Automode ventilator weaning trended toward more rapid extubation than did conventional protocol-driven ventilation in conjunction with a standardized weaning protocol. Physiologic and hemodynamic factors were better in patients using automode ventilation compared to patients using conventional ventilation. Automode ventilation was well tolerated and did not induce significant adverse effects.     

Augmentierte Spontanatmung

Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hyercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devices; it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory ventilation (IMV) allows spontaneous breathing of the patient and augments alveolar ventilation by periodically applying positive airway pressure tidal volumes, which are generated by the respirator. Potential advantages include lower mean airway pressure (P_AW), as compared with controlled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fatigue of the respiratory muscles and CO₂ retention. During pressure support ventilation (PSV), inspiratory alterations of P_AW or gas flow (trigger) are detected by the respirator, which delivers a gas flow to maintain P_AW at a fixed value (usually 5–20?cm H₂O) during inspiration. PSV may be combined with other modalities of respiratory therapy such as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of oxygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveolar ventilation is not guaranteed during PSV; thus, close observation of the patient is mandatory to avoid serious respiratory complications. Continuous positive airway pressure breathing (CPAP) maintains P_AW above atmospheric pressure throughout the respiratory cycle, which may increase functional residual capacity and decrease the effort of breathing. CPAP has been conceptually designed for the augmentation of spontaneous breathing and requires the intact central and peripheral regulation of the respiratory system. Airway pressure release ventilation (APRV) improves alveolar ventilation by intermittent release of P_AW, which is kept above atmospheric pressure by means of a high-flow CPAP system. The opening of an expiratory valve for 1–2?s induces a decreased P_AW and lung volume, which increases rapidly to pre-exhalation values after closure of the valve due to the high gas flow within the circuit (90–100?l/min). APRV may improve haemodynamics and V_A/Q distribution as compared with conventional mechnical ventilation. Biphasic positive airway pressure (BIPAP) is characterized by the combination of spontaneous breathing and time-regulated, pressure-controlled mechanical ventilation. During the respiratory cycle the ventilator generates two alternating CPAP levels, which can be modified with regard to time and pressure. As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved V_A/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.     

Pressure support ventilation decreases inspiratory work of breathing during general anesthesia and spontaneous ventilation.

Spontaneous ventilation may offer advantages over controlled mechanical ventilation (CMV), but increase in work of breathing may diminish its usefulness. During general anesthesia, respiratory depression and increased work of breathing often preclude spontaneous ventilation, and patients then receive CMV. We compared the inspiratory work of breathing of anesthetized patients who breathed with pressure support ventilation (PSV) with that associated with a demand gas flow and a standard anesthesia circle system. We studied nine consenting patients who underwent general inhaled anesthesia with or without regional supplementation. An anesthesia/ventilator system (Siemens 900D, Solna, Sweden) provided PSV (5 cm H2O) or demand gas flow during spontaneous inspiration. Gas flow during demand breathing and PSV was initiated when inspiration produced a 2-cm H2O reduction in airway pressure. An anesthesia machine (Dr?ger Narkomed 3, Telford, Pa.) provided a gas flow rate of 6 L/min through a standard semiclosed circle system. Airway pressure, airway gas flow rate, and esophageal pressure were continuously transduced, and data or signals were conveyed to a computer. Tidal volume and respiratory rate were computed from the flow curve. The inspiratory work of breathing was calculated as the integral of the area subserved by a plot of esophageal pressure and tidal volume during inspiration. Heart rate and mean arterial blood pressure were recorded, and arterial blood was sampled for gas tension and pH analysis. No differences were found in pHa, Paco2, Pao2, tidal volume, respiratory rate, heart rate, or mean arterial blood pressure among the three modes of ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)     

PEEP and CPAP following open-heart surgery in infants and children.

The cardiorespiratory effects of 5 cm H2O end-expiratory pressure were studied in 22 infants and children an hour after open-heart surgery during mechanical ventilation with positive end-expiratory pressure (PEEP) and prior to endotracheal extubation approximately 15 hours later during spontaneous breathing (CPAP). Thermodilution cardiac output determinations and respiratory airflow, volume and pressure recordings were made to assess the effects of airway pressure changes on the respiratory waveform and oxygen delivery. Neither PEEP nor CPAP had a significant effect on cardiac output, intrapulmonary shunting, oxygen consumption, or oxygen utilization. Patients who had had pulmonary hypertension preoperatively did not behave differently from those without pulmonary hypertension when removed from ventilatory supprot. Expiratory airflow was significantly prolonged when positive end-expiratory pressure existed during both controlled and spontaneous respiration. During CPAP, this "expiratory braking" was associated with an increase in tidal volume and decreases in respiratory rate and minute volume. Because of the lack of improvement in cardiopulmonary function in this group of patients, and the possibility of untoward effects from sustained end-expiratory pressure, PEEP and CPAP might properly be reserved as temporary supportive techniques should respiratory function be compromised.     

Gastro-oesophageal reflux with the laryngeal mask during day-case gynaecological laparoscopy

We have evaluated if breathing spontaneously via a laryngeal mask airway is associated with a higher risk of gastro-oesophageal reflux compared with positive pressure ventilation via a laryngeal mask airway in 40 patients undergoing day-case gynaecological laparoscopy. Patients were allocated randomly to receive either positive pressure ventilation or breathe spontaneously via a laryngeal mask airway. Using continuous oesophageal pH monitoring, three patients in the ventilated group and one in the spontaneous breathing group had gastrooesophageal reflux (P = 0.29). We found no evidence to suggest that breathing spontaneously via a laryngeal mask airway increased the risk of gastro-oesophageal reflux compared with positive pressure ventilation in this group of patients.