Respiratory Care

STUDY QUESTION: Do the nebulizer-drive characteristics of mechanical ventilators influence aerosol delivery? MATERIALS & METHODS: Using a test bench protocol incorporating an ETT and test lung, we measured the aerosol generated by two different jet nebulizers charged with radiolabeled (technetium pertechnetate, ^99mTc) saline solution. A low-resistance absolute filter was used to collect the radioaerosol delivered to the distal tip of the ETT, the quantity of which was expressed as a percentage of the nebulizer charge (inhaled mass%). Airlife and Hudson jet medication nebulizers were selected for testing, and four contemporary ventilators (BEAR 1000, Hamilton Veolar, Infrasonics Adult Star, and Puritan-Bennett 7200ae) were set at f = 15/min, V_T = 1,000 mL, square inspiratory flow pattern, inspiratory flowrate = 45 L/min (I:E = 1:2; inspiratory time fraction = 33%) for all experiments. Humidification was not used because it has been previously shown to reduce aerosol delivery. Serial measurements over time of filter radioactivity were made with each nebulizer until cumulative radioactivity reached a plateau; the results at each interval were graphed and expressed as a percentage of the activity initially placed in the nebulizer. In addition to aerosol delivery, we measured the pressure generated in the nebulizer-drive line proximal to the nebulizer during inspiration and the gas flow through the nebulizer, using a pneumotachograph and recorder. RESULTS: Aerosol delivery to the ETT filter varied over a wide range with values from 3.2% to 3.6% for the Hamilton and 9.6% to 14.7% for the other ventilators. In parallel with these observations, driving pressure and flow varied in a similar manner, with values measured for the Hamilton lower than the other brands. The time to complete nebulization appeared related to driving pressure, with the high pressure ventilators completing the treatment in one third the time (30 vs 90 min). CONCLUSION: The choice of mechanical ventilator has an important influence upon the efficiency of jet nebulization within a ventilator circuit. Driving pressure and flow to the nebulizer can significantly affect nebulizer performance and treatment duration. Therefore, in designing a protocol to deliver a nebulized drug to a ventilated patient, the choice of ventilator must also be taken into account together with previously identified variables that include choice of nebulizer, ventilator settings, and humidification.

BACKGROUND: Intrinsic PEEP (PEEP_t) is frequently present during mechanical ventilation when expiratory times (t_e) are short. Some authors have begun to describe PEEP₁, as a potentially therapeutic mechanism to be manipulated during pressure-controlled inverse ratio ventilation (PCIRV) much like ventilator-applied PEEP (PEEP_appl). Other clinicians warn against PEEP, because of its potential side effects. Efforts to limit PEEP, or to apply it therapeutically have been hindered by the complications associated with the many ventilator adjustments required to effect a desired change in PEEP_t. STUDY QUESTION: Does Equation 12 allow adequate prediction of the t_e necessary to achieve a desired PEEP₁ once an initial PEEP₁ has been measured? P E E P 1 = P E E P t o t − P E E P a p p l METHOD: Using the exponential relationship between pressure and time during lung deflation, we developed a single, composite-function, logarithmic equation to predict the te necessary to achieve a desired PEEP, after the initial level of PEEP has been measured. P E E P 1 = ( P p l a t − P E E P a p p l ) e t e R C . t e = ln P p l a t ( n e w ) − P E E P a a p l P E E P I ( d e s i r e d ) × t e ( i n i t i a l ) ln P p l a t ( i n i t i a l ) − P E E P a p p l P E E P I ( i n i t i a l ) To test Equation 12 we used a test lung attached to a mechanical ventilator and recording pressure monitor attached to the alveolar port of the test lung. We used the pressure recordings to measure and set te. Two series of measurements were taken. RESULTS: In both series, the limits of agreement between the measured PEEP, and the predicted PEEP_t were determined. The predicted value for PEEP demonstrated only a slight bias, underestimating measured (or actual) PEEP, by 0.79 (Series 1) and 0.39 (Series 2). IN CONCLUSION: In the laboratory, this technique appears to work well. More work is required to validate the equations with different ventilators and on various groups of patients.

INTRODUCTION: Many respiratory care programs rely heavily on observation instruments to assess student performance in the clinical setting. In addition, the practice of using volunteer clinical instructors (CI) to assess student performance is commonplace. The combination of these variables raises questions about the re-liability of this method of multiobserver assessment. The purpose of this study was to determine whether a training program using videotapes prepares CI to ac-curately record clinical performance on checklist instruments as measured by an interobserver agreement ≥ 0.85. METHODS & SUBJECTS: In a randomized con-trol group posttest-only design, CI were assigned to a control group (n = 17) or an experimental group (n = 17). The CI in the experimental group attended a session during which they viewed segments of videotapes showing tracheal aspiration (TA) and chest physiotherapy (CPT) performed according to commonly accepted stan-dards of care. The videotapes had been developed by the American Association for Respiratory Care (AARC) for use by supervisory personnel responsible for evalu-ating clinical performance. Experimental group CI reviewed checklist instruments and procedural specifications for TA and CPT that corresponded to the video-tapes. The dichotomously scored checklist instruments identified tasks essential to the correct performance of TA and CPT. The instruments were developed by the investigators and pilot tested by an expert panel to establish interrater reliability and face and content validity. One week later, both groups attended a posttest ses-sion during which segments of the TA and CPT videotapes with incorrectly per-formed or omitted tasks were shown. The CI completed the checklist instruments as they viewed the videotapes. RESULTS: Data were statistically analyzed using one-tailed t tests for independent samples (a = 0.05) and chi-square tests (α = 0.05). Mean interobserver agreement was greater in the experimental group than in the control group for both TA (0.885 vs 0.672) and CPT (0.926 vs 0.680), and the dif-ference was significant for both procedures (p < 0.001 for t tests and p < 0.00001 for chi-square). Calculation of effect size demonstrated practical significance. CONCLUSION: The experimental group was more accurate than the control group in using the checklist instruments to record clinical performance shown in the videotapes. Educators may consider developing similarly designed training programs for preparing CI to use checklist instruments to evaluate clinical per-formance.

INTRODUCTION: Peak flow meters (PFMs) are commonly used in the hospital and in the home to evaluate the response to bronchodilator therapy. The purpose of this study was to evaluate the agreement between four commercially available PFMs and a calibrated pneumotachometer. MATERIALS & METHODS: Four PFM brands were tested (Pulmo-Graph, Pocket, Assess, and Mini-Wright). Manufacturers supplied four of each brand for evaluation. Each device was evaluated when new and again after 200 uses, at an altitude of 150 m above sea level. Peak flow readings measured by these devices were compared to peak flow readings measured simultaneously using a calibrated pneumotachometer. Peak flows generated by 2 healthy subjects ranged from 80-800 L/min. Statistical analysis consisted of calculations of mean (SD), bias, and ANOVA. RESULTS: A significant difference was observed in the bias among the four peak flow meter brands (p < 0.001). The bias changed after 200 uses for the Assess (p = 0.046) and for the Pulmo-Graph (p < 0.001) but not for the Mini-Wright (p = 0.558) or the Pocket (p = 0.267). A significant difference was also observed in the bias among the four units tested for each brand (p <; 0.()1 in each case). The Assess indicated flows less than did the pneumotachometer at low to medium flows, but was in agreement ±10% of the pneumotachometer at high flows. The Mini-Wright indicated flows greater than the pneumotachometer at low flows, less than the pneumotachometer at high flows, and was in agreement ±HI% of the pneumotachometer at medium flows. The Pulmo-Graph indicated flows less than the pneumotachometer at high flows, and was in agreement ±10% of the pneumotachometer at low to medium flows. The Pocket was in agreement±10% of the pneumotachometer over the entire flow range. CONCLUSION: Because we observed differences in the bias of the PFM brands evaluated, we recommend that when serial measurement of peak flow is used to guide management of reversible airways disease, the same device always be used in a given patient.

BACKGROUND: Capnometry offers a continuous measurement of the end-tidal carbon dioxide tension (PetCO2) that is thought to predict arterial carbon dioxide tension (PaCO2) and to be useful in monitoring and adjusting mechanical ventilation in the Intensive Care Unit (ICU). METHODS: We designed a study to examine the relationship between the PaCO2 and the PetCO2 in surgical ICU patients. We pooled data from our previous experiments in the ICU setting. In addition to evaluating the correlation between the PaCO2 and the PetCO2, we compared the sequential changes in PaCO2 and PetCO2 (APaCO2 and APetCO2). To be clinically useful as a predictor of PaCO2, the APetCO2 should very closely approximate the APaCO2. RESULTS: A total of 693 pairs of PaCO2 and PetCO2 measurements were included in the analysis. The PetCO2 did demonstrate a statistical correlation with the PaCO2. However, in a large number of individuals (51/80), there was a poor correlation between the two variables, and 31.3% of the APetCO2S did not predict APaCO2. CONCLUSION: Although capnometry may be useful for other reasons, we conclude from the data presented that the continuous measurement of PetCO2 cannot reliably replace measurement of PaCO2 in the ICU setting.