Surface temperature of two portable ventilators during simulated use under clinical conditions.

During performance testing of portable ventilators, it was noted that an area on the case of one of the devices, the LTV 1000, was noticeably warm. This investigation examined the case temperatures of this portable ventilator and a portable ventilator currently in the Department of Defense inventory, the Uni-Vent 754, during simulated clinical conditions. Both have an integral method of producing compressed air. The hottest portion of the cases of the LTV 1000 and the Uni-Vent 754 reached temperatures of 39.9 to 46.7 degrees C and 35.4 to 35.9 degrees C, respectively, across a range of simulated clinical conditions. Investigations have found the risk of burns to increase with temperatures greater than 40 degrees C. The cases of these devices are not designed to be in contact with the skin. Personnel should properly position these and other devices during patient transport and not allow contact with the patient's skin.

Dual control modes: combining volume and pressure breaths.

New modes of ventilator operation are often introduced without rhyme or reason. The dual control modes are, however, a response of the manufacturers to clinician desire to provide constant minute volume/tidal volume while operating in the pressure support or pressure control mode. Dual control modes allow the ventilator to control pressure or volume. The ventilator cannot control pressure and volume at the same time, so it is an either/or phenomenon. Dual control modes can operate "within a breath" or on a "breath to breath" basis. Dual control within a breath refers to a technique whereby the ventilator switches from pressure to volume control in the middle of a breath. In the technique of dual control breath to breath, all breaths are pressure control breaths, but the level of pressure varies from breath to breath in an effort to maintain a constant tidal volume.

Adaptive support ventilation.

Adaptive support ventilation (ASV) is a newer form of closed-loop ventilation control available on the Galileo ventilator (Hamilton Medical). ASV provides automated selection of initial ventilator parameters based on measurements of patient lung mechanics and breathing effort. After initiation, ASV will "titrate" ventilator output (mandatory breath rate, tidal volume, inspiratory pressure, inspiratory time, and I to E ratio) to maintain a calculated optimal breathing pattern that ensures delivery of a clinician selected minute ventilation target. ASV may be thought of as an "electronic" ventilator management protocol that may improve the safety and efficacy of mechanical ventilation. Additional clinical investigations regarding the effect of ASV on outcome, ventilator days, and so forth are forthcoming.

Work of breathing characteristics of seven portable ventilators.

Portable ventilators (PVs) are used for patient transport with increasingly frequency. Due to design differences it would not be unexpected to find differences among these ventilators in the imposed work of breathing (WOBI) during spontaneous respiratory efforts. The purpose of this investigation was to compare the WOBI characteristics during spontaneous breathing of seven PVs; Bird Avian, Bio-Med Crossvent 4, Pulmonetics LTV 1000, Hamilton Max, Drägerwerk Oxylog 2000, Impact Uni-Vent 750, and Impact Uni-Vent 754 using a model of spontaneous breathing. Differences between the PVs in regards to the measured parameters increased with increases in simulated breathing demand. WOBI, peak inspiratory pressure, and pressure-time product were consistently less with the LTV 1000 over the range of simulated breathing conditions. During pressure support ventilation these parameters were significantly less with the LTV 1000 compared with the Crossvent 4. Only the WOBI produced by the LTV was consistently lower than the physiologic work of breathing across the simulated spontaneous breathing conditions. Based on these results it is predicted PVs with flow triggering and positive end-expiratory pressure compensation will consistently offer the least WOBI. Clinicians should be aware of these characteristics when using PVs with spontaneous breathing patients.

Prehospital use of continuous positive airway pressure (CPAP) for presumed pulmonary edema: a preliminary case series.

OBJECTIVE: To describe the prehospital use of a continuous positive airway pressure (CPAP) system for the treatment of acute respiratory failure presumed to be due to cardiogenic pulmonary edema. METHODS: Prospective case-series analysis. Paramedics administered CPAP via face mask at 10 cm H2O to patients believed to be in cardiogenic pulmonary edema and in imminent need of endotracheal intubation (ETI). Data from run sheets and hospital records were analyzed for treatment intervals, vital signs, complications, admitting diagnoses, need for ETI, and mortality. RESULTS: Nineteen patients received prehospital CPAP therapy. Mean duration of therapy was 15.5 minutes. Pre- and post-therapy pulse oximetry was available for 15 patients and demonstrated an increase from a mean of 83.3% to a mean of 95.4%. None of the patients were intubated in the field. Two patients who did not tolerate the CPAP mask required ETI upon arrival in the emergency department (ED); an additional five patients required ETI within 24 hours. There was one death in the series and two additional adverse events (one aspiration pneumonia, one pneumothorax); none of these were attributable to the use of CPAP. The diagnosis of cardiogenic pulmonary edema was corroborated by the ED or in-hospital physician in 13 patients (68%). Paramedics reported no technical difficulties with the CPAP system. CONCLUSION: For patients with acute respiratory failure and presumed pulmonary edema, the prehospital use of CPAP is feasible and may avert the need for ETI. Future controlled studies are needed to assess the utility and cost-effectiveness of prehospital CPAP systems.

Prone positioning and inhaled nitric oxide: synergistic therapies for acute respiratory distress syndrome.

BACKGROUND: Inhaled nitric oxide (INO) and prone positioning have both been advocated as methods to improve oxygenation in patients with acute respiratory distress syndrome (ARDS). This study was designed to evaluate the relative contributions of INO and prone positioning alone and in combination on gas exchange in trauma patients with ARDS. METHODS: Sixteen patients meeting the consensus definition of ARDS were studied. Patients received mechanical ventilation in the supine position, mechanical ventilation plus INO at 1 part per million in the supine position, mechanical ventilation in the PP, and mechanical ventilation in the prone positioning plus INO at 1 part per million. A stabilization period of 1 hour was allowed at each condition. After stabilization,hemodynamic and gas exchange variables were measured. RESULTS: INO and prone positioning both increased PaO2/FIO2 compared with ventilation in the supine position. PaO2/FIO2 increased by 14% during use of INO, and 10 of 16 patients (62%) responded to INO in the supine position. PaO2/FIO2 increased by 33%, and 14 of 16 patients (87.5%) responded to the prone position. The combination of INO and prone positioning resulted in an improvement in PaO2/FIO2 in 15 of 16 patients(94%), with a mean increase in PaO2/FIO2 of 59%. Pulmonary vascular resistance was reduced during use of INO, with a greater reduction in pulmonary vascular resistance seen with INO plus prone positioning (175 +/- 36 dynes x s/cm5 vs. 134 +/- 28 dynes x s/cm5) compared with INO in the supine position (164 +/- 48 dynes x s/cm5 vs.138 +/- 44 dynes x s/cm5). There were no significant hemodynamic effects of INO or prone positioning and no complications were seen during this relative short duration of study. CONCLUSIONS: INO and prone positioning can contribute to improved oxygenation in patients with ARDS. The two therapies in combination are synergistic and may be important adjuncts to mechanical ventilation in the ARDS patient with refractory hypoxemia.

The acute effects of body position strategies and respiratory therapy in paralyzed patients with acute lung injury.

BACKGROUND: Routine turning of critically ill patients is a standard of care. In recent years, specialized beds that provide automated turning have been introduced. These beds have been reported to improve lung function, reduce hospital-acquired pneumonia, and facilitate secretion removal. This trial was designed to measure the physiological effects of routine turning and respiratory therapy in comparison with continuous lateral rotation (CLR). METHODS: The study was a prospective, quasi-experimental, random assignment, trial with patients serving as their own controls. Paralyzed, sedated patients with acute respiratory distress syndrome were eligible for study. Patients were randomized to receive four turning and secretion management regimens in random sequence for 6 h each over a period of 24 h: (1) routine turning every 2 h from the left to right lateral position; (2) routine turning every 2 h from the left to right lateral position including a 15-min period of manual percussion and postural drainage (P&PD); (3) CLR with a specialized bed that turned patients from left to right lateral position, pausing at each position for 2 min; and (4) CLR with a specialized bed that turned patients from left to right lateral position pausing at each position for 2 min, and a 15-min period of percussion provided by the pneumatic cushions of the bed every 2 h. RESULTS: Nineteen patients were entered into the study. There were no statistically significant differences in the measured cardiorespiratory variables. There was a tendency for the ratio of partial pressure of arterial oxygen to fractional inspired oxygen concentration (PaO2/FIO2) to increase (174 +/- 31 versus 188 +/- 36; P = 0.068) and for the ratio of deadspace to tidal volume (Vd/Vt) to decrease (0.62 +/- 0.18 versus 0.59 +/- 0.18; P = 0.19) during periods of CLR, but these differences did not achieve statistical significance. There were statistically significant increases in sputum volume during the periods of CLR. The addition of P&PD did not increase sputum volume for the group as a whole. However, in the four patients producing more than 40 ml of sputum per day, P&PD increased sputum volume significantly. The number of patient turns increased from one every 2 h to one every 10 min during CLR. CONCLUSION: The acute effects of CLR are undoubtedly different in other patient populations (spinal cord injury and unilateral lung injury). The link between acute physiological changes and improved outcomes associated with CLR remain to be determined.

Prone positioning for acute respiratory distress syndrome in the surgical intensive care unit: who, when, and how long?

BACKGROUND: We evaluated the effects of prone positioning (PP) on surgery and trauma patients with acute respiratory distress syndrome (ARDS). METHODS: Patients with ARDS were studied. Exclusion criteria were contraindications to PP. Patients were evaluated in the supine position and after being turned to the PP. After 6 hours, patients were returned to the supine position for 3 hours. One hour after each position change, arterial and mixed venous blood was drawn and analyzed for blood gases and pH, and hemodynamics were measured. RESULTS: Over 20 months, 27 patients met the criteria, and 20 of the patients were entered into the study. On day 1, 18 of 20 patients (90%) responded with an increase in PaO(2) during PP. On day 2, 16 of 17 patients (94%) responded; on day 3, 15 of 16 patients responded (94%); on day 4, 11 of 13 patients responded (85%); on day 5, 8 of 8 patients responded (100%); and on day 6, 4 of 5 patients responded (80%). Pao(2)/Fio(2) and Qs/Qt were significantly improved (P<.05) during PP. There were 91 periods of PP, lasting 10.3+/-1.2 hours. Of 91 changes to PP, 78 changes (86%) resulted in an improvement in Pao(2)/Fio(2) of more than 20%. CONCLUSIONS: PP improves oxygenation in ARDS for 6 days with few complications.

Effects of body temperature on accuracy of continuous cardiac output measurements.

Intermittent measurement of cardiac output is routine in the critically ill surgical patient. A new catheter allows real-time continuous measurement of cardiac output. This study evaluated the impact of body temperature variation on the accuracy of these measurements compared to standard intermittent bolus thermodilution technique. This prospective study in a university hospital surgical intensive care unit included 20 consecutive trauma patients. Data were collected with pulmonary artery catheters, which allowed both continuous (COC) and bolus (COB) thermodilution measurements. The catheter was placed through either the subclavian or internal jugular vein. Measurements for COB were performed using a bolus (10 cm3) of ice-cold saline with a closed-injectate delivery system at end-expiration. Computer-generated curves were created on a bedside monitor, and the average of three measurements within 10% of one another was used as COB. COC was determined as the average of the displayed CO before and after thermodilution CO measurements. Body temperature was measured from the pulmonary artery catheter and was grouped as < or =36.5 degrees C, 36.6-38.4 degrees C, and > or =38.5 degrees C. COB and COC were compared for agreement by plotting the mean of the differences (COB - COC) between the methods. The differences were plotted against the average of each pair and analyzed with linear regression. One hundred seventy-eight paired measurements were made over a period of 1 to 3 days. CO ranged from 3.7 to 15.5 L/min. Eighty-one percent of measurements were at a temperature of 36.5-38.4 degrees C. Approximately 7% of measurements were at a temperature below 36.5 degrees C and 11.2% were in patients with a core temperature above 38.5 degrees C. Correlation between the two techniques was 0.96, 0.91, and 0.82 for temperatures of < or =36.5 degrees C, 36.6-38.4 degrees C, and > or = 38.5 degrees C, respectively. In conclusion, the COC measurements correlate well with COB in trauma patients with a core temperature < or =38.5 degrees C. The accuracy degraded at higher temperatures, which may be related to the smaller signal-to-noise ratio at elevated body temperatures.

Ventilators and weaning modes.

Although new ventilator modes have become available to facilitate weaning, there is little evidence that these have improved weaning outcomes. Knowledge based computer weaning systems have also been described, but these are in their infancy, and their role is unclear. Prospective, randomized clinical studies are required to examine whether such modalities are superior to existing approaches.

Prolonged use of heat and moisture exchangers does not affect device efficiency or frequency rate of nosocomial pneumonia.

OBJECTIVE: To determine whether use of a single heat and moisture exchanger (HME) for < or =120 hrs affects efficiency, resistance, level of bacterial colonization, frequency rate of nosocomial pneumonia, and cost compared with changing the HME every 24 hrs. DESIGN: Prospective, controlled, randomized, unblinded study. SETTING: Surgical intensive care unit at a university teaching hospital. PATIENTS: A total of 220 consecutive patients requiring mechanical ventilation for >48 hrs. INTERVENTIONS: Patients were randomized to one of three groups: a) hygroscopic HME (Aqua+) changed every 24 hrs (HHME-24); b) hydrophobic HME (Duration HME) changed every 120 hrs (HME-120); and c) hygroscopic HME (Aqua+) changed every 120 hrs (HHME-120). Devices in all groups could be changed at the discretion of the staff when signs of occlusion or increased resistance were identified. MEASUREMENTS AND MAIN RESULTS: Daily measurements of inspired gas temperature, inspired relative humidity, and device resistance were made. Additionally, daily cultures of the patient side of the device were accomplished. The frequency rate of nosocomial pneumonia was made by using clinical criteria. Ventilatory support variables, airway care, device costs, and clinical indicators of humidification efficiency (sputum volume, sputum efficiency) were also recorded. Prolonged use of both hygroscopic and hydrophobic devices did not diminish efficiency or increase resistance. There was no difference in the number of colony-forming units from device cultures over the 5-day period and no difference between colony-forming units in devices changed every 24 hrs compared with devices changed after 120 hrs. The average duration of use was 23+/-4 hrs in the HHME-24 group, 73+/-13 hrs in the HME-120 group, and 74+/-9 hrs in the HHME-120 group. Mean absolute humidity was greater for the hygroscopic devices (30.4+/-1.1 mg of H2O/L) compared with the hydrophobic devices (27.8+/-1.3 mg of H2O/L). The frequency rate of nosocomial pneumonia was 8% (8:100) in the HHME-24 group, 8.3% (5:60) in the HME-120 group, and 6.6% (4:60) in the HHME-120 group. Pneumonia rates per 1000 ventilatory support days were 20:1000 in the HHME-24 group, 20.8:1000 in the HME-120 group, and 16.6:1000 in the HHME-120 group. Costs per day were $3.24 for the HHME-24 group, $2.98 for the HME-120 group, and $1.65 for the HHME-120 group. CONCLUSIONS: Changing the hydrophobic or hygroscopic HME after 3 days does not diminish efficiency, increase resistance, or alter bacterial colonization. The frequency rate of nosocomial pneumonia was also unchanged. Use of HMEs for >24 hrs, up to 72 hrs, is safe and cost effective.

The effects of passive humidifier dead space on respiratory variables in paralyzed and spontaneously breathing patients.

BACKGROUND: Passive humidifiers have gained acceptance in the intensive care unit because of their low cost, simple operation, and elimination of condensate from the breathing circuit. However, the additional dead space of these devices may adversely affect respiratory function in certain patients. This study evaluates the effects of passive humidifier dead space on respiratory function. METHODS: Two groups of patients were studied. The first group consisted of patients recovering from acute lung injury and breathing spontaneously on pressure support ventilation. The second group consisted of patients who were receiving controlled mechanical ventilation and were chemically paralyzed following operative procedures. All patients used 3 humidification devices in random order for one hour each. The devices were a heated humidifier (HH), a hygroscopic heat and moisture exchanger (HHME) with a dead space of 28 mL, and a heat and moisture exchanger (HME) with a dead space of 90 mL. During each measurement period the following were recorded: tidal volume, minute volume, respiratory frequency, oxygen consumption, carbon dioxide production, ratio of dead space volume to tidal volume (VD/VT), and blood gases. In the second group, intrinsic positive end-expiratory pressure was also measured. RESULTS: Addition of either of the passive humidifiers was associated with increased VD/VT. In spontaneously breathing patients, VD/VT increased from 59 +/- 13 (HH) to 62 +/- 13 (HHME) to 68 +/- 11% (HME) (p < 0.05). In these patients, constant alveolar ventilation was maintained as a result of increased respiratory frequency, from 22.1 +/- 6.6 breaths/min (HH) to 24.5 +/- 6.9 breaths/min (HHME) to 27.7 +/- 7.4 breaths/min (HME) (p < 0.05), and increased minute volume, from 9.1 +/- 3.5 L/min (HH) to 9.9 +/- 3.6 L/min (HHME) to 11.7 +/- 4.2 L/min (HME) (p < 0.05). There were no changes in blood gases or carbon dioxide production. In the paralyzed patient group, VD/VT increased from 54 +/- 12% (HH) to 56 +/- 10% (HHME) to 59 +/- 11% (HME) (p < 0.05) and arterial partial pressure of carbon dioxide (PaCO2) increased from 43.2 +/- 8.5 mm Hg (HH) to 43.9 +/- 8.7 mm Hg (HHME) to 46.8 +/- 11 mm Hg (HME) (p < 0.05). There were no changes in respiratory frequency, tidal volume, minute volume, carbon dioxide production, or intrinsic positive end-expiratory pressure. DISCUSSION: These findings suggest that use of passive humidifiers with increased dead space is associated with increased VD/VT. In spontaneously breathing patients this is associated with an increase in respiratory rate and minute volume to maintain constant alveolar ventilation. In paralyzed patients this is associated with a small but statistically significant increase in PaCO2. CONCLUSION: Clinicians should be aware that each type of passive humidifier has inherent dead space characteristics. Passive humidifiers with high dead space may negatively impact the respiratory function of spontaneously breathing patients or carbon dioxide retention in paralyzed patients. When choosing a passive humidifier, the device with the smallest dead space, but which meets the desired moisture output requirements, should be selected.

Positive end-expiratory pressure and response to inhaled nitric oxide: changing nonresponders to responders.

BACKGROUND: Inhaled nitric oxide (INO) has been shown to improve oxygenation in two thirds of patients with acute respiratory distress syndrome (ARDS). Failure to respond to INO is multifactorial. We hypothesized that the addition of positive end expiratory pressure (PEEP) might modify the response to INO in patients who had previously failed to respond to INO. METHODS: Patients with ARDS who failed to respond to INO at 1 ppm (PaO2 increase of < 20%) were selected. Each patient underwent a PEEP trial using an improvement in static lung compliance as the end point. One hour after the new PEEP level was reached, hemodynamic and blood gas values were obtained. INO was then reinstituted at 1 ppm, and hemodynamic and blood gas variables were obtained 1 hour later. RESULTS: Six of nine patients demonstrated an increase in PaO2/FIO2 (161 +/- 27 to 186 +/- 29) with a mean increase in PEEP of 3.7 cm H2O. Each patient responding to PEEP further improved PaO2/FIO2 (186 +/- 29 to 223 +/- 36) with INO at 1 ppm. The three patients who failed to improve after the PEEP increase also failed to respond to a second trial of INO. There were no changes in cardiac output or systemic vascular resistance. Pulmonary artery pressures decreased slightly (39 +/- 5 vs 38 +/- 7 vs 35 +/- 9 mm Hg). Pulmonary vascular resistance decreased significantly after reintroduction of INO (298 +/- 131 vs 310 +/- 122 vs 249 +/- 105 dynes/sec/cm-5) in patients who responded positively. CONCLUSIONS: The response of ARDS patients to INO can be improved if optimum alveolar recruitment is achieved by the addition of PEEP. PEEP and INO have a synergistic effect on PaO2/FIO2. Patients who fail to respond to INO may benefit from an optimum PEEP trial.

Techniques for automated feedback control of mechanical ventilation.

Mechanical ventilators have become more sophisticated with the advent of microprocessor control. Advances in monitoring have also improved our ability to harmonize patient-ventilator interaction. The next obvious step in this technologic progression is to turn over some decision making to the ventilator. In the jargon of today, we are "closing the loop.'' Ventilators have used closed-loop control for simple tasks for the last decade. Newer closed-loop processes include modes that increase or decrease support based on a single-monitored variable. An example is the automated control of pressure support to maintain a deired tidal volume. More sophisticated closed-loop techniques, such as proportional assist ventilation and adaptive support ventilation, not only monitor multiple input variables but also use closed-loop control of several variables. This article reviews the closed-loop ventilation modes currently available to clinicians.

Changes in respiratory mechanics after tracheostomy.

OBJECTIVE: To determine the effects of tracheostomy on respiratory mechanics and work of breathing (WOB). DESIGN: A before-and-after trial of 20 patients undergoing tracheostomy for repeated extubation failure. SETTING: Surgical intensive care unit at a university teaching hospital and a level I trauma center. PATIENTS: A consecutive sample of 20 patients who met extubation criteria (Pa(O2), >55 mm Hg; pH >7.30; and respiratory rate, <30/min on room air continuous positive airway pressure after 20 minutes) but failed extubation on 2 occasions were eligible for the study. INTERVENTIONS: Respiratory mechanics, lung volumes, and WOB were measured before and after tracheostomy. MAIN OUTCOME MEASURES: Patients in whom extubation fails often progress to unassisted ventilation after tracheostomy. The study hypothesis was that tracheostomy would result in improved pulmonary function through changes in respiratory mechanics. RESULTS: Data are given as means +/- SDs. After tracheostomy, WOB per liter of ventilation (0.97+/-0.32 vs. 0.81+/-0.46 J/L; P<.09), WOB per minute (8.9+/-2.9 vs. 6.6+/-1.4 J/min; P<.04), and airway resistance (9.4+/-4.1 vs. 6.3+/-4.5 cm H20/L per second; P<.07) were reduced compared with breathing via an endotracheal tube. These findings, however, do not fully explain the ability of patients to be liberated from mechanical ventilation after tracheostomy. In 4 patients who were extubated before tracheostomy, WOB was significantly greater during extubation than when breathing through an endotracheal or tracheostomy tube (1.2+/-0.19 vs. 0.81+/-0.24 vs. 0.77+/-0.22 J/L). CONCLUSIONS: We believe that the rigid nature of the tracheostomy tube represents reduced imposed WOB compared with the longer, thermoliable endotracheal tube. The clinical significance of this effect is small, although as respiratory rate increases, the effects are magnified. In patients in whom extubation failed, WOB may be elevated because of incomplete control of the upper airway. Future studies should evaluate the cause of increased WOB after extubation.

The effects of inadequate humidity.

The use of heated humidification in adults does not appear to be an important factor in maintaining body temperature in adults. Heat and moisture losses certainly can be reduced with heated humidification, but the contribution to temperature regulation appears small. The use of an HME reduces heat loss form the respiratory tract, but this loss is minute compared with losses from the skin, fluid administration, and the operative site. In neonates, the use of heated humidification during surgery may help contribute to thermal balance owing to the unique issues of temperature regulation and control in these patients.

Humidification in the intensive care unit.

In summary, current data indicate that body temperature cannot be controlled efficiently by changing inspired gas temperature. Inspired gas temperature should therefore be maintained at 32 degrees C to 34 degrees C for intubated patients and other efforts should be made to optimize body temperature.