Sleep during proportional-assist ventilation with load-adjustable gain factors in critically ill patients.

BACKGROUND: Proportional-assist ventilation with load-adjustable gain factors (PAV+) automatically adjusts the flow and volume assist to represent constant fractions of resistance and elastance of the respiratory system, respectively. Resistance and elastance are calculated at random intervals of 4-10 breaths, by applying a 300 ms pause maneuver at the end of selected inspirations. OBJECTIVES: To determine whether the large number of end-inspiratory occlusions during PAV+ operation influences sleep quality in critically ill patients who exhibited good patient-ventilator synchrony during pressure support (PS, baseline). METHODS: One and two nights' polysomnography was performed in sedated (protocol A, n=11) and non-sedated (protocol B, n=9) patients, respectively, while respiratory variables were continuously recorded. In each protocol the patients were ventilated with PAV+ and PS at two levels of assist (baseline and high). RESULTS: In both protocols sleep quality did not differ between the modes of support or the assist levels. In sedated patients sleep efficiency was slightly but significantly higher with PAV+ than with high PS, while it did not differ between modes in non-sedated patients. The two modes of support had comparable effects on respiratory variables. Independent of the mode of support and particularly at high assist, a significant proportion of patients developed periodic breathing during sleep (27% in protocol A and 44% in protocol B). CONCLUSION: In patients exhibiting good patient-ventilator synchrony during PS, the large number of short-term end-inspiratory occlusions with PAV+ operation did not adversely influence sleep quality. With both modes high assist may cause unstable breathing during sleep.

Effect of voluntary respiratory efforts on breath-holding time.

INTRODUCTION: Near the end of a maximal voluntary breath-hold, re-inhalation of the expired gas allows an additional period of breath-holding, indicating that the breaking point does not depend solely on chemical drive. We hypothesized that afferents from respiratory muscle and/or chest wall are significant in breath-holding. METHODS: Nineteen normal adults breathed room air through a mouthpiece connected to a pneumotachograph and were instructed to breath-hold with and without voluntary regular respiratory efforts against an occluded airway. RESULTS: Fifty one trials with and 53 without respiratory efforts were analyzed. The mean number of efforts per minute was 19+/-2.3 and the mean lowest airway pressure (P(aw)) -16.6+/-5.4 cmH(2)O. Breath-holding time (BHT) did not differ without (33.0+/-18.2 s) and with (29.3+/-12.3 s) efforts. In five patients arterial blood gasses were measured before and at the end of breath-holding and they did not differ between trials without and with efforts, indicating similar chemical drive. Our results suggest that afferents from respiratory muscle and/or chest wall are not the major determinants of BHT.

Influence of respiratory efforts on b2-agonist induced bronchodilation in mechanically ventilated COPD patients: a prospective clinical study.

BACKGROUND: Several in vitro studies have shown that at similar tidal volume (VT), bronchodilator delivery to target sites is significantly lower during controlled mechanical ventilation (CMV) than that during simulated spontaneous breathing. However, the influence of active respiratory efforts on the magnitude of b2-agonist induced bronchodilation in mechanically ventilated patients has not been examined. OBJECTIVE: To examine the influence of controlled and assisted modes of ventilatory support on the bronchodilative effect induced by b2-agonists administered with a metered dose inhaler (MDI) and a spacer device in a homogeneous group of mechanically ventilated patients with acute exacerbation of chronic obstructive pulmonary disease (COPD). METHODS: Prospective clinical study. Ten mechanically ventilated patients with acute exacerbation of COPD were prospectively randomized to receive 4 puffs of salbutamol (S, 100 micro g/puff) either with volume-controlled (VC) or pressure-support (PS) ventilation. On PS the pressure level was such that VT was comparable between ventilatory modes. After a 6-h washout period, patients were crossed-over to receive the drug by the alternative mode of ventilation. Static and dynamic airway pressures, minimum (R(int)) and maximum (R(rs)) inspiratory resistance, the difference between R(rs) and R(int) (DeltaR), end-inspiratory static compliance of the respiratory system (C(rs)), intrinsic positive end-expiratory pressure (PEEP(i)) and heart rate (HR) were measured before and at 15, 30, 60, 120, 180 and 240 min after S administration. RESULTS: S caused a significant decrease in dynamic and static airway pressures, PEEP(i), R(int) and R(rs). These changes were not influenced by the ventilatory mode and were evident at 15, 30, 60 and 120 min after S. HR, C(rs) and DeltaR did not change after S administration. CONCLUSIONS: Considering the use of propofol with its presumed bronchodilative properties as a shortcoming of our study, it is concluded that the magnitude of bronchodilation induced by salbutamol delivered by an MDI and a spacer device in mechanically ventilated COPD patients is not affected by the presence or absence of active respiratory efforts.

Effect of inspiratory flow rate on beta2-agonist induced bronchodilation in mechanically ventilated COPD patients.

OBJECTIVES: To test the effect of two different inspiratory flow rates on the bronchodilation induced by beta2-agonists administered by metered dose inhaler (MDI). PATIENTS: Ten patients with acute exacerbation of chronic obstructive pulmonary disease and receiving mechanical ventilation with constant inspiratory flow (V'I). DESIGN: Patients received four puffs of salbutamol (100 microg/puff) with either low V'I (0.6 l/s) or high V'I (1.2 l/s) administered with an MDI adapted to inspiratory limb of the ventilator circuit using an aerosol cloud enhance spacer. After a 6-h washout patients were crossed-over to receive the drug by the alternative mode of administration. MEASUREMENTS AND RESULTS: Static and dynamic airway pressures, intrinsic positive end-expiratory pressure, and minimum and maximum inspiratory resistance values showed a significant decrease after salbutamol. These changes were not affected by the inspiratory flow rate and were evident 15, 30, and 60 min after administration. Heart rate, static end-inspiratory respiratory system compliance, and the difference between minimum and maximum inspiratory resistance were unchanged after salbutamol. CONCLUSIONS: Salbutamol delivered by MDI and spacer device induces significant bronchodilation in mechanically ventilated patients with chronic obstructive pulmonary disease, but the magnitude of the effect is not affected by the inspiratory flow rate. These results do not support flow rate manipulations when bronchodilators are administered during controlled mechanical ventilation.

Acute effects of ventilator settings on respiratory motor output in patients with acute lung injury.

OBJECTIVE: During assisted mechanical ventilation, changes in ventilator settings may acutely affect the respiratory motor output via the mechanoreceptor reflex feedback system, thus interfering with patient management. This feedback system in mechanically ventilated patients with parenchymal lung injury remains largely unexplored. To investigate this, the early response of respiratory motor output to varying ventilator settings was determined in 13 sedated patients with acute lung injury. DESIGN: During assist/control and pressure support (PS) ventilation changes in (1) tidal volume (V(T)) at fixed inspiratory flow (V'(I)), (2) V'(I) at fixed V(T) and (3) PS level were employed and the response of respiratory motor output was followed for two breaths after the change. Respiratory motor output was assessed by total pressure generated by the respiratory muscles (Pmus), computed from esophageal pressure (Pes). RESULTS: Neural expiratory time increased with increasing V(T) and PS, while it remained constant with V'I changes. Neural inspiratory time (T(I)n) increased with decreasing V'(I) and PS, but was not affected by V(T) changes. None of the changes in ventilator settings influenced significantly the rate of rise of Pmus, used as an index of respiratory drive. The changes in respiratory timing resulted in significant changes in breathing frequency, which increased with decreasing V(T) and PS and increasing V'(I). The time integral of Pmus, an index of respiratory effort, increased with increasing T(I)n. These acute responses were not related to the severity of deterioration of respiratory system mechanics. CONCLUSIONS: We conclude that alterations in commonly used ventilator settings induce acute changes in respiratory timing, without affecting the respiratory drive. These changes, probably mediated via mechanoreceptor reflex feedback, are dependent on the type of the alteration in the ventilator settings.

Low flow inflation pressure-time curve in patients with acute respiratory distress syndrome.

OBJECTIVE: In mechanically ventilated patients with ARDS, determination of the lower (LIP) and upper (UIP) inflection points of the static pressure-volume curve (P-V) is crucial for planning ventilatory strategies. Recently, a simple new method was proposed for measuring the P-V curve by inflating the lung with constant low flow [14]. We hypothesized that during low flow inflation LIP and UIP might be determined using the pressure-time curve (P-T) instead of P-V. METHODS: Eleven paralyzed patients with ARDS were studied. During volume control ventilation the patients were allowed to reach passive functional residual capacity (FRC) and then ventilator frequency, inspiratory to total breath duration ratio and tidal volume (VT) were set to 5 breaths/ min, 80% and 500 or 1,500 ml, respectively. With these settings, constant inspiratory flow (V'I) was administered for 9.6 s and ranged, depending on VT, between 0.05 and 0.15 l/s. P-V and P-T were obtained at two levels of positive end-expiratory pressure (PEEP; 0 and 10 cm H2O), with V'I being achieved either fast (< 0.1 s, minimum delay) or slowly (0.4 s, maximum delay). RESULTS: With minimum flow delay for a given experimental condition, the shape of the P-T did not differ from that of P-V. In all cases P-T correctly identified the presence of LIP and UIP, which did not differ significantly between P-T and P-V. With maximum flow delay, compared to P-V, the initial part of P-T was significantly shifted to the left. P-T did not identify the presence of UIP and LIP in one and two cases, respectively. CONCLUSIONS: Provided that constant flow is given relatively fast, P-T accurately determines the shape of P-V, as well as the LIP and UIP. Flow delay causes a leftward shift of the initial part of P-T, masking the presence of LIP and UIP in some cases.

How to set the ventilator in asthma.

All patients with bronchial asthma are at risk of developing severe episodes of airway narrowing that do not respond to the usual medical treatment, a life-threatening situation referred to as status asthmaticus. In some cases, ventilatory failure occurs, necessitating mechanical ventilation to support gas exchange and to unload the respiratory muscles, giving time for other therapeutic interventions to improve the functional status of the patient. Mechanical ventilatory support poses additional risks to the patients, due to interaction between the pathophysiology of the disease and the process of mechanical ventilation. Dynamic hyperinflation, a cardinal feature of the pathophysiology, may cause serious complications during mechanical ventilation. Setting the ventilator, such as to minimize the dynamic hyperinflation, is a key point in the management of mechanically ventilated patients with status asthmaticus. Strategies to reduce dynamic hyperinflation, such as hypoventilation (permissive hypercapnia), increase of expiratory time and promotion of patient-ventilator synchrony are mandatory and significantly decrease the morbidity and mortality of the disease. Continuous monitoring of the effectiveness of these strategies, as well as the functional status of the patient, is crucial in order to limit complications associated with mechanical ventilation and to identify the time that weaning can start.

Use of bronchodilators during non-invasive mechanical ventilation.

Bronchodilators represent one of the most important therapeutic weapons for the treatment of airway obstructive diseases and the inhaled route of administration is very often employed due to the greater drug availability and reduced magnitude of side effects. During acute exhacerbations, it is not unfrequent that the elastic and resistive loads imposed on the ventilatory pump overcome the force sustainable by the respiratory muscles and the patient requires ventilatory assistance, in order to relieve fatigue and to optimize alveolar gas exchange. During these episodes, inhaled bronchodilators, far from being discontinued, sometime must be administered during mechanical ventilation, that, in hypercapnic ventilatory failure can be frequently applied noninvasively with a good rate of success. While in the current literature there are a lot of data about inhaled drug administration during invasive mechanical ventilation, very few data are available on the topic of aerosol therapy during noninvasive mechanical ventilation. With the present paper we want to analyze the rationale, the feasibility and the current data dealing with the administration of inhaled drugs during noninvasive mechanical ventilation.

Locating of the required key-variables to be employed in a ventilation management decision support system.

The aim of the paper is to identify the key physiological variables and ventilator settings involved in ventilation management, and required for an appropriate Clinical Decision Support System (CDSS). Based on the results of a questionnaire designed for the purpose of the research, 70 hours of physiological and ventilation data were recorded. Recorded data were classified by clinicians into three major lung pathologies and were further statistically analyzed for identifying strong relationships between monitored and controlled ventilator parameters. Correlation analysis was evaluated by Intensive Care Unit (ICU) clinicians. Based on the evaluators' majority voting the number and type of participating variables in a CDSS was drastically decreased. The number and type of monitored variables ranged from a single one to six, depending on the patient's lung pathology, and the controlled ventilator setting. Evaluation results were successfully applied to Neural Network models for providing suggestions on Tidal Volume and the Fraction of inspired Oxygen.

Lung emptying in patients with acute respiratory distress syndrome: effects of positive end-expiratory pressure.

The pattern of lung emptying was studied in 10 mechanically-ventilated patients with acute respiratory distress syndrome. At four levels of positive end-expiratory pressure (PEEP) (0, 5, 10 and 15 cmH2O) tracheal (Ptr) and airway pressures (Paw), flow (V') and volume (V) were continuously recorded. Tidal volume was set between 0.5-0.6 L and V'/V curves during passive expiration were obtained. Expired volume was divided into five equal volume slices and the time constant (taue) and effective deflation compliance (Crs(eff)) of each slice was calculated by regression analysis of V/V' and postocclusion V/Ptr relationships, respectively. In each slice, the presence or absence of flow limitation was examined by comparing V'/V curves with and without decreasing Paw. For a given slice, total expiratory resistance (Rtot) (consisting of the respiratory system (Rrs), endotracheal tube (Rtube) and ventilator circuit (Rvent)) was calculated as the taue/Crs(eff) ratio. In the absence of flow limitation Rrs was obtained by subtracting Rtube and Rvent from Rtot, while in the presence of flow limitation Rrs equaled Rtot. The taue of the pure respiratory system (taue(rs)) was calculated as the product of Rrs and Crs(eff). At zero PEEP, taue(rs) increased significantly towards the end of expiration (52+/-31%) due to a significant increase in Rrs (46+/-36%). Application of PEEP significantly decreased Rrs at the end of expiration and resulted in a faster and relatively constant rate of lung emptying. In conclusion, without positive end-expiratory pressure the respiratory system in patients with acute respiratory distress syndrome deflates with a rate that progressively decreases, due to a considerable increase in expiratory resistance at low lung volumes. Application of positive end-expiratory pressure decreases the expiratory resistance, probably by preventing airway closure, and as a result modifies the pattern of lung emptying.

Bronchodilator delivery by metered-dose inhaler in mechanically ventilated COPD patients: influence of flow pattern.

In mechanically ventilated patients the flow pattern during bronchodilator delivery by metered-dose inhaler (MDI) could be a factor that might influence the effectiveness of this therapy. In order to test this the effect of two different inspiratory flow patterns on the bronchodilation induced by beta2-agonists administered via MDI and spacer in a group of mechanically ventilated patients with chronic obstructive pulmonary disease (COPD) was examined. Eighteen mechanically ventilated patients with COPD, were prospectively randomized to receive two (n=8, protocol A) or six (n=10 protocol B) puffs salbutamol (100 microg x puff(-1)) either under pressure control (decelerating flow pattern) or under volume control (square wave flow pattern). With both modes, tidal volume and inspiratory time were identical. Salbutamol was administered via an MDI adapted to the inspiratory limb of the ventilator circuit using an aerosol cloud-enhancer spacer. After a 6-h washout, patients were crossed over to receive the same dose of salbutamol (200 or 600 microg, respectively in protocols A and B) by the alternative mode of administration. Static and dynamic airway pressures, minimum (Rint) and maximum (Rrs) inspiratory resistance and the difference between Rrs and Rint (deltaR) were measured before and at 15, 30 and 60 min after salbutamol. Independent of the dose, salbutamol caused a significant decrease in dynamic and static airway pressures, Rint and Rrs. These changes were not influenced by the inspiratory flow pattern and were evident at 15, 30 and 60 min after salbutamol. It is concluded that salbutamol delivered via metered dose inhaler and spacer device, induces significant bronchodilation in mechanically ventilated patients with chronic obstructive pulmonary disease, the magnitude of which is not affected by the inspiratory flow/time profile.

Effect of different levels of pressure support and proportional assist ventilation on breathing pattern, work of breathing and gas exchange in mechanically ventilated hypercapnic COPD patients with acute respiratory failure.

BACKGROUND: Proportional assist ventilation (PAV) has been shown to maintain better patient-ventilator synchrony than pressure support ventilation (PSV); however, its clinical advantage regarding invasive ventilation of COPD patients has not been clarified. OBJECTIVES: To compare the effect of PAV and PSV on respiratory parameters of hypercapnic COPD patients with acute respiratory failure (ARF). METHODS: Nine intubated hypercapnic COPD patients were placed on the PAV or PSV mode in random sequence. For each mode, four levels (L1-L4) of support were applied. At each level, blood gases, flow, tidal volume (VT), airway pressure (Paw), esophageal pressure (Pes) (n = 7), patient respiratory rate (fp), ventilator rate (fv), missing efforts (ME = fp - fv) were measured. RESULTS: We found increases in ME with increasing levels of PSV but not with PAV. PO2 and VT increased whereas PCO2 decreased significantly with increasing levels of PSV (p < 0.05). With PAV, PCO2 decreased and VT increased significantly only at L4 whereas PO2 increased from L1 to L4. Runaways were observed at L3 and L4 of PAV. The pressure-time product (PTP) was determined for effective and missing breaths. The mean total PTP per minute (of effective plus missing breaths) was 160 +/- 57 cm H2O/s.min in PSV and 194 +/- 60 cm H2O/s.min in PAV. CONCLUSION: We conclude that in COPD patients with hypercapnic ARF, with increasing support, PSV causes the appearance of ME whereas PAV develops runaway phenomena, due to the different patient-ventilator interaction; however, these do not limit the improvement of blood gases with the application of both methods.

Effect of varying the pressurisation rate during noninvasive pressure support ventilation.

The aim of the study was to assess the effects of varying the pressurisation rate during noninvasive pressure support ventilation on patients' breathing pattern, inspiratory effort, arterial blood gases, tolerance to ventilation and amount of air leakage. A total of 15 chronic obstructive pulmonary disease patients recovering from an acute episode of hypercapnic acute respiratory failure were studied during four randomised trials with different levels of pressurisation rate. No significant changes were observed in breathing pattern and arterial blood gases between the different runs. The pressure time product of the diaphragm, an estimate of its metabolic consumption, was significantly lower with all pressurisation rates than with spontaneous breathing, but was significantly lowest with the fastest rate. However, air leak, assessed by the ratio between expired and inspired tidal volumes, increased and the patients' tolerance of ventilation, measured using a standardised scale, was significantly poorer with the fastest pressurisation rate. In chronic obstructive pulmonary disease patients recovering from an episode of acute hypercapnic respiratory failure and ventilated with noninvasive pressure support ventilation, different pressurisation rates resulted in different reductions in the pressure time product of the diaphragm; this reduction was greater with the fastest rate, but was accompanied by significant air leaks and poor tolerance.