Is It Safe for Patients With Chronic Hypercapnic Respiratory Failure Undergoing Home Noninvasive Ventilation To Discontinue Ventilation Briefly?

Is It Safe for Patients With Chronic Hypercapnic Respiratory Failure Undergoing Home Noninvasive Ventilation To Discontinue Ventilation Briefly?* Sait Karakurt, MD; Francesco Fanfulla, MD; and Stefano Nava, MD Study objectives: A brief discontinuation (< 1 week) of long-term ventilation may be necessary in patients who are not totally ventilator-dependent in cases of technical problems, intolerable nasal irritation, upper airway congestion, or travel. We examined the incidence, timing, and causes of possible clinical deterioration after a brief withdrawal of ventilation in patients with chronic respiratory failure (CRF) who were well-established on long-term noninvasive mechanical ventilation (NIMV). Study design: Prospective clinical study. Patients: Eleven inpatients in clinically stable condition (COPD, 6 patients; and restrictive thoracic disease [RTD], 5 patients) who had severe CRF (PaCO 2, > 50 mm Hg) and had been receiving NIMV for (mean SD) 19.3 5.3 months were enrolled. Interventions and measurements: Arterial blood gas (ABG) levels, maximal inspiratory pressure (PImax), breathing pattern, dyspnea rating, and life symptoms (measured by a questionnaire) were recorded daily after NIMV withdrawal for 6 days or until the patients showed clinical and/or ABG level deterioration. Pulmonary function tests were performed and neuromuscular drive was measured at the beginning and the end of the study. Results: Five of the 11 patients (45.4%) [COPD, 3 patients; and RTD, 2 patients] were reconnected to a ventilator before the scheduled time because of ABG level deterioration. Despite these changes, none of the patients reported severe worsening of symptoms or other medical complications. The patients whose ABG levels worsened had statistically significant decreases in tidal volume and PImax, suggesting that the development of alveolar hypoventilation was related to respiratory muscle weakness. Conclusions: A brief discontinuation of NIMV in patients who were affected by chronic hypercapnic respiratory failure and were well-established on NIMV is associated with a relatively high incidence of ABG level worsening due to the development of alveolar hypoventilation. If NIMV must be briefly interrupted for clinical reasons, the patient should be monitored closely for abrupt worsening, and prompt technical intervention should be provided if a ventilator fails. (CHEST 2001; 119:1379 1386) Key words: COPD; hypoventilation; noninvasive mechanical ventilation; respiratory insufficiency; restrictive thoracic disease; ventilator failure Abbreviations: ABG arterial blood gas; CRF chronic respiratory failure; NIMV noninvasive mechanical ventilation; P 0.1 neuromuscular drive; PFT pulmonary function test; Pimax maximal inspiratory pressure; RR respiratory frequency; RTD restrictive thoracic disease; Ti inspiratory time; Ttot total breathing cycle time; Vt tidal volume *From the Department of Pulmonary and Critical Care Medicine (Dr. Karakurt), Marmara University, Istanbul, Turkey; the Division of Pneumology (Dr. Fanfulla), Fondazione S. Maugeri, Istituto Recovero e Cura a Corattere Scientifico, Centro Medico di Montescano, Italy; and the Respiratory Intensive Care Unit (Dr. Nava), Fondazione S. Maugeri, Istituto Recovero e Cura a Corattere Scientifico, Centro Medico di Pavia, Italy. Supported in part by Telethon Italy grant No. 1125C. Manuscript received June 20, 2000; revision accepted December 5, 2000. Correspondence to: Stefano Nava, MD, Respiratory Intensive Care Unit, Centro Medico di Pavia, Fondazione S. Maugeri, Via Ferrata 8, 27100 Pavia, Italy; e-mail: snava@fsm.it Long-term mechanical ventilation has been used over the last few decades both as a life-support procedure and as a life-sustaining measure. The former is reserved for patients affected by chronic respiratory failure (CRF) requiring mechanical ventilation for 12 h per day, and the latter for patients with greater respiratory autonomy. In particular, the prolonged application of noninvasive mechanical ventilation (NIMV) has been associated with the reversal of hypoventilation in patients affected by restrictive thoracic diseases (RTDs) such as muscular dystrophies, amyotrophic lateral sclerosis, or chest wall diseases. 1 6 The efficacy of the long-term administration of NIMV to patients with COPD is still controversial, 3,7 10 but it seems that selected populations of severely hypercapnic patients with COPD may show satisfactory clinical and physiologic responses. 3,11 14 Most patients affected by CRF live at home, where they are periodically observed by a specialized team in charge of the home-care program. 15 It has been reported that some technical problems, such as CHEST / 119 / 5/ MAY, 2001 1379

defective and malfunctioning equipment and the improper use of the ventilator by caregivers, might cause a temporary suspension of mechanical ventilation. 16 As a matter of fact, about 60% of patients enrolled in a home ventilatory program will sooner or later experience one of these problems. Furthermore, a brief ( 1 week) interruption of long-term ventilation may be necessary in patients who are not totally ventilator-dependent, in cases of intolerable nasal irritation and upper airway congestion due to respiratory infection, and in the event of travel. 17,18 The brief withdrawal of NIMV may be associated with a worsening in clinical status and may lead to important legal and ethical issues. Hill et al 19 studied the effect of temporary NIMV suspension in six patients affected by RTD and stated that discontinuation of NIMV could be done safely as long as symptoms and gas exchange are closely monitored, since the mean Paco 2 and Pao 2 did not vary significantly for a few days after the suspension of ventilation. Despite their conclusion, two of their six patients (30%) had to be reconnected to the machine between day 1 and 3 due to worsening of symptoms, suggesting that, at least in a subgroup of patients, the suspension of NIMV for even a few hours may be a problem. In this study, we investigated the effect of NIMV withdrawal for 6 days in 11 patients with chronic hypercapnic respiratory failure who had been successfully ventilated (ie, they had improved arterial blood gas [ABG] levels and symptoms) and had been well-established in a home-care program for at least 1 year. The patients were admitted to the hospital, and the primary outcomes evaluated were the incidence and timing of any clinical deterioration. A secondary aim of the study was the identification of simple physiologic indexes that possibly were associated with the clinical response. Patients Materials and Methods Eleven patients in clinically stable condition with severe CRF were included in this study. Clinical stability was defined as a lack of hospital admissions and exacerbations requiring supplemental medical therapy and no variations in ABG levels (ie, no changes 5% in ph, Paco 2, and Pao 2 ) or ventilator settings in the 3 months preceding the study. All the patients had been successfully (see the next section) established on home mechanical ventilation for at least 12 months, and all of them were receiving supplemental oxygen therapy at various flow rates that were kept constant during the daytime and eventually were adjusted during nighttime NIMV, as illustrated below. Six patients were affected by COPD, which was defined according to the American Thoracic Society standard, 20 and five patients were affected by RTD (chest wall disease, four patients; obesity-hypoventilation syndrome, one patient). The individual patient characteristics that are pertinent to the study are shown in Table 1. Patients gave oral informed consent to their participation in the study, which was approved by the Ethics Committee of the S. Maugeri Foundation and was conducted in accordance with the Declaration of Helsinki. Criteria for Home NIMV Enrollment The 11 patients were established on home mechanical ventilation following the flow-chart used in our institution (Fig 1). Briefly, drawing on the clinical and scientific evidence, 12 14 we enrolled only patients with ph 7.35 and Paco 2 50 mm Hg. A prolonged (ie, 3 to 4 weeks) in-hospital trial of nocturnal NIMV was employed thereafter to assess individual responses; if daytime hypercapnia improved ( 6% from baseline), we withdrew ventilation for about 30 days and then reassessed the patient s ABG levels. If the patient had been able to maintain the same level of Paco 2 that was recorded at hospital discharge, we did not Table 1 Demographic and Respiratory Characteristics of the Patients at the Enrollment of the Home NIMV Program* Patient No. Pathology Age, yr/sex Weight, kg MVD, mo FEV 1, % pred FVC, % pred FEV 1 /FVC, % pred RV, ml TLC, ml ph Paco 2, mm Hg Pao 2, mm Hg 1 COPD 58/M 58 24 14 57 20 306 150 7.38 67.8 53.7 2 RTD 57/M 61 26 41 41 82 47 46 7.40 53.8 60.4 3 COPD 69/M 86 15 26 57 36 252 140 7.35 57.7 53.4 4 RTD 65/M 75 12 35 56 58 72 60 7.36 52.1 55.5 5 RTD 28/M 116 16 99 94 85 148 99 7.39 51.6 58.9 6 COPD 70/M 74 22 26 50 53 7.39 52.3 57.0 7 COPD 65/M 66 18 37 43 59 101 69 7.35 63.5 52.6 8 COPD 73/M 58 28 39 71 31 235 140 7.37 55.8 48.7 9 COPD 53/M 64 13 20 63 25 242 124 7.38 50.5 52.6 10 RTD 71/M 73 20 40 55 56 112 76 7.39 59.5 47.5 11 RTD 66/M 44 18 44 49 95 159 98 7.38 54.3 59.6 Mean 61.4 70.5 19.3 37.4 57.8 54.2 167.4 100.2 7.37 56.3 54.5 SD 12.7 18.8 5.3 22.5 14.7 24.8 86.9 37.0 0.02 5.4 4.2 *M male; MVD home mechanical ventilation duration; RV residual volume; TLC total lung capacity; % pred percent predicted. 1380 Clinical Investigations

Figure 1. Flow chart used in home mechanical ventilation program. prescribe long-term NIMV. If, on the other hand, overt hypercapnia developed again or if ph decreased significantly during the 4-week observation period, long-term NIMV was prescribed. Table 1 illustrates the time that elapsed from the institution of NIMV, the results of pulmonary function tests (PFTs), and the ABG levels of the patients at enrollment into the home-care program. All the patients received bilevel ventilation with a mean ( SD) inspiratory aid of 15.3 2.1 cm H 2 O and a mean expiratory aid of 2.5 1.0 cm H 2 O with a backup rate of 8 breaths/min. The levels of aid in our clinical practice are individually titrated using the recordings of transdiaphragmatic pressure as follows. Inspiratory positive airway pressure values were titrated in order to achieve a minimum reduction of 50% from the transdiaphragmatic pressure measured during spontaneous breathing, while a level of expiratory positive airway pressure was set to 70 to 80% of the dynamic positive endexpiratory pressure that was recorded during spontaneous breathing. Oxygen saturation was recorded continuously in all patients during the first few nights of NIMV to determine the minimal oxygen flow necessary to achieve an arterial oxygen saturation of 90% for at least 90% of the total sleep time, and most of the patients (9 of 11) also underwent a complete sleep study to exclude the presence of obstructive sleep apnea syndrome (which, indeed, was found to be present in the obese patient). At the time of enrollment into the study, the patients had been receiving NIMV for a mean duration of 19.3 5.3 months (inspiratory positive airway pressure, 14.2 3.6 cm H 2 O; expiratory positive airway pressure, 3.9 1.8 cm H 2 O) and were averaging 6.4 1.1 h of use nightly, as assessed by the ventilator clock. None of the patients used NIMV during the day, except patient 9 who also received ventilation for approximately 2 to 3 h during the day. The patients in our ventilatory home-care program are periodically (usually every 3 months) admitted to our institution for a complete check-up. Measurements ABG levels were obtained from the radial artery and were analyzed using an automatic analyzer (550 Radiometer; ABL; Copenhagen, Denmark). The result of PFTs for static and dynamic lung volumes were recorded with the patient sitting, using a body plethysmograph (MasterLab; Jaeger; Hochberg, Germany). The measurement of maximal inspiratory pressure (Pimax) was obtained at functional residual capacity using a hand-held manometer that was connected to a rigid mouthpiece, with a small air leak to prevent glottic closure, while the patient was wearing a noseclip. The best of three efforts not varying by 5% was chosen for data analysis. 21 Airflow was measured with a heated pneumotachograph (Screenmate box; Jaeger) that was positioned immediately beyond the mouthpiece when the patient was breathing spontaneously or between the ventilator circuit and the facemask when the patient was receiving mechanical ventilation. Airway pressure during mechanical ventilation was measured through tubing connected to a differential pressure transducer ( 300 cm H 2 O; Honeywell; Freeport, IL). Tidal volume (Vt) was obtained by integration of the flow signal. The inspiratory time (Ti), expiratory time, total breathing cycle time (Ttot), respiratory frequency (RR), and duty cycle (Ti/Ttot) were calculated as the average values of 10 consecutive breaths after 5 min of breathing or in the presence of a monotonous breathing pattern. The airway pressure developed in the first 100 ms after the onset of inspiration made after end-expiratory occlusion (ie, CHEST / 119 / 5/ MAY, 2001 1381

neuromuscular drive [P 0.1 ]) was obtained, as described in detail by Withelaw et al. 22 Resting dyspnea was quantified using a modified Borg scale, with 0 being the lowest level and 10 the highest level. The patients were asked to answer a brief questionnaire daily about their level of sleepiness and to keep a diary of symptoms. A physician who was unaware of the aims of the study administered the questions. Study Protocol On day 0, the patients ABG levels and breathing patterns were measured during ventilation after a night of receiving NIMV. Then, approximately 3 h later, the measurement of ABG levels and breathing patterns was repeated during spontaneous breathing, and PFTs, Pimax, P 0.1, dyspnea rating, and answers to the questionnaire were recorded (baseline). The patients then discontinued NIMV. All the measurements, with the exceptions of PFTs and P 0.1, were recorded each day at the same hour, except on day 1 when the measurements also were repeated, for reasons of safety, approximately 10 h after discontinuing NIMV (5 pm). On day 7, the recordings were performed in the same order as for day 0, and the trial was concluded. If the patients required early resumption of NIMV before the scheduled time because of interrupting criteria, the last measurements were recorded before the reinstitution of NIMV. At day 7, in all patients surviving without NIMV, nighttime ventilation again was applied. The objective criteria for interrupting the experimental protocol were the presence of one or more of the following: (1) ph 7.35; (2) change in Paco 2 of 6% from baseline; (3) decrease in Pao 2 of 6% from baseline; (4) morning symptom worsening (ie, constant headache, general weakness, or tiredness); and (5) dyspnea of 6 on the Borg scale and/or a respiratory rate of 35 breaths/min. Data Analysis The flow and the pressure signals were fed into a microcomputer through an analog/digital board and were processed with appropriate software (Labdat and Anadat; RHT-InfoDat Inc; Montreal, Quebec, Canada). Results are expressed as mean SD. A t test for dependent samples was used to assess differences between the data collected at baseline and at the end of the study. A t test for independent samples was used to assess differences between the two groups of patients considered (ie, those whose conditions deteriorated and those whose conditions did not). Forward stepwise multiple regression analysis was performed to identify the physiologic variables collected at baseline that best identified patients who then went on to deteriorate clinically. Statistical significance was defined as a two-tailed p value 0.05. Results Figure 2 illustrates the proportion of patients over time who were able to remain stable without receiving NIMV. Five of the 11 patients (45.4%) met one of the five criteria for immediate reconnection to a ventilator before the scheduled time. The reasons for reconnection were the following: an increased Paco 2 level (one patient on day 4); a simultaneous increase in Paco 2 and decrease in ph to 7.35 (two patients on days 3 and 5); and a simultaneous increase in Paco 2 Figure 2. Proportion of patients surviving over time after withdrawal from NIMV. with a dyspnea score of 5 (one patient on day 6). Despite these changes, none of the patients, with the exception of patient 5 who had substantial dyspnea, reported severe worsening of symptoms such as general fatigue or headache, and no other medical complications developed. For subsequent analysis, the patients then were divided into those in whom withdrawal was not followed by deterioration (six patients) and those in whom it was (five patients). The underlying pathologies in the patients whose conditions did not deteriorate were COPD (three patients) and chest wall restrictive disease (three patients), while in the group that needed early reconnection the underlying pathologies were COPD (three patients), chest wall restrictive disease (one patient), and obesity-hypoventilation syndrome (one patient). At enrollment, the two groups of patients (those whose conditions deteriorated and those that did not) had similar ABG levels, PFT results, and other physiologic variables (Table 2). Figure 3 illustrates the individual changes in Paco 2 (top) and ph (bottom) from enrollment to the end of the study. Overall, the mean Paco 2, Pao 2, and ph levels did not vary significantly (Paco 2 variance, 51.6 6.6 to 55.3 10.6 mm Hg; Pao 2 variance, 55.5 5.0 to 56.1 3.7 mm Hg; and ph variance, 7.39 0.02 to 7.37 0.04), but the patients who needed to be reconnected to NIMV (represented by solid lines) clearly showed a significant decrease in ph (p 0.004) and an increase in Paco 2 (p 0.008) but not a decrease in Pao 2 (p 0.25). Table 3 shows the pattern of breathing, dyspnea score, Pimax level, P 0.1 level, and PFT results at enrollment and at the end of the study for the whole group of patients. No statistical significance was observed in the mean data or when the patients were divided into the two groups according to the clinical response to NIMV withdrawal, with the exception of Vt and Pimax levels, which were statistically lower (p 0.05 for both parameters) only in the group 1382 Clinical Investigations

Table 2 Clinical and Respiratory Parameters at the Beginning of the Study in the Groups of Patients in Whom Withdrawal From Ventilation Was Observed (Deteriorators) or Not (Nondeteriorators) by Clinical Worsening* Parameters Nondeteriorators Deteriorators ph 7.40 0.03 7.38 0.02 Paco 2,mmHg 48.2 5.9 54.4 6.3 Pao 2,mmHg 57.2 2.6 54.1 6.2 FEV 1, % pred 29.6 8.3 43.8 29.0 FVC, % pred 53.4 8.3 61.5 18.5 FEV 1 /FVC, % pred 50.6 21.8 57.2 28.7 Vt, ml 601.2 185.7 575.3 206.7 RR, breaths/min 21.8 5.1 22.7 4.4 Pimax, cm H 2 O 56.6 26.0 54.5 20.0 P 0.1,cmH 2 O 4.0 0.6 4.9 1.8 Vt/Ti, L/s 0.447 0.094 0.593 0.264 Borg scale 1.9 1.0 2.5 1.4 *Values given as mean SD. p 0.05 for all the considered variables. See Table 1 for abbreviations not used in the text. needing early reconnection vs that not needing it (Vt, 602.4 218.9 vs 468.4 180.1 ml; Pimax, 56.6 21.6 vs 41.0 16.3 cm H 2 O, respectively). In an attempt to find any variables (including the level of inspiratory support) that were associated with the occurrence of clinical deterioration, a stepwise regression analysis was performed. As shown in Table 4, the level of the baseline Vt/Ti ratio and the level of the changes in Paco 2 recorded while patients were receiving NIMV and during the baseline measurement during spontaneous breathing were significantly associated with the clinical deterioration and the need to recommence NIMV. Patients with more marked increases in Paco 2 during baseline spontaneous ventilation (expressed as a percentage of the baseline values during spontaneous breathing) and with higher baseline mean inspiratory flow were those who required reconnection to NIMV. Discussion In this prospective study, we have shown that briefly interrupting NIMV in patients with chronic hypercapnic respiratory failure who had been successfully established in a home-care program for 1 year causes clinical and ABG deterioration in 40% of those patients. Sporadic discontinuation from long-term ventilation, either intentional or unintentional, is quite a common problem in clinical practice. It has been shown, for example, by Srinivasan et al 16 that in the Greater Los Angeles Branch of National Medical Homecare, 74% of the ventilatordependent patients and 45% of those receiving nighttime ventilation alone had episodes of ventilator Figure 3. Individual changes in Paco 2 and ph from enrollment to the end of the study. Solid line patients with deterioration; dashed line patients without deterioration. Note the point overlap (bottom). failure. The main causes of failure were related to defective equipment, improper care, damage, or tampering by caregivers and by incorrect use of the machine. The authors claimed that 99% of the problems could be solved in the home and did not have an adverse clinical effect on the large majority of the patients. Interestingly enough, in 140 of the Table 3 Respiratory Parameters at the Beginning and at the End of the Study* Variables Enrollment End of the Study RR, breath, min 22.3 4.4 25.8 6.6 Vt, ml 587.1 188.0 494.1 135.8 P 0.1,cmH 2 O 4.5 1.4 4.6 1.2 Pimax, cm H 2 O 55.4 21.7 44.5 13.9 Borg scale 2.2 1.2 3.2 1.5 Ti, s 1.16 0.26 1.02 0.20 Te, s 1.71 0.74 1.54 0.70 Ttot, s 2.87 0.83 2.57 0.70 Vt/Ti, L/s 0.526 0.292 0.486 0.129 *Te expiratory time. p 0.05 for all the considered variables. Values given as mean SD. CHEST / 119 / 5/ MAY, 2001 1383

Table 4 Forward Stepwise Multiple Regression Analysis* Variables Standardized SE p Value CO 2 0.68 0.23 0.02 Vt/Ti 0.58 0.24 0.04 * CO 2 variation of Paco 2 between spontaneous breathing and mechanical ventilation recorded at the entry of the study (expressed as % of baseline value). r 0.76; SEE (standard error of the estimate) 0.38; p 0.05. 189 cases of ventilator failure a backup ventilator was available at the patient s home, so that ventilation was promptly restored. The choice of allocating a second ventilator to be kept at home is usually made in most countries only if the patient is totally ventilator-dependent, while it is very uncommon if the patient needs only partial ventilatory support, based on the common-sense assumption that these latter patients may discontinue NIMV for a few days without any major discomfort. Technical failure of the machine is not the only cause of brief discontinuation of NIMV or poor compliance. For example, ventilators usually are removed electively from a home for a complete maintenance overhaul at the factory after every 5,000 h of use, and this maintenance may not necessarily be accompanied by the use of a substitute ventilator. More important, the patient may choose voluntarily to suspend NIMV in the case of nasal irritation due to cold or poor humidification, rhinitis or aerophagia, and discomfort from the mask or headgear; these problems have been shown to be present in, respectively, 23%, 13%, and 8% of patients enrolled in a home-care program. 18 Bach and Alba 17 reported that 16 of 30 patients receiving chronic nasal NIMV had to switch to a body ventilator or mouth NIMV because of episodes of upper respiratory infections and nasal congestion. Indeed, since most of these patients are not home-bound, they also may want to take a short holiday or to visit relatives without taking their ventilator. The problem of briefly discontinuing NIMV in partially ventilator-dependent patients never has been assessed in detail, although possible deterioration of the patient s health could have not only clinical but also legal and ethical implications. The only study dealing with this issue was done by Hill and coworkers. 19 They withdrew NIMV from six patients with restrictive thoracic disease who had been established in a home-care program for at least 2 months. Their overall mean results showed no major ABG level deterioration at the end of the trial, although Paco 2 increased from 54 to 56 mm Hg and Pao 2 decreased from 76 to 69 mm Hg, while most of the patients had more dyspnea at rest, increased daytime somnolence, more morning headaches, and less daytime energy, suggesting that the efficacy of NIMV may depend more on the improvement of nocturnal hypoventilation than on resting of the ventilatory muscle. Because of the reported symptoms, two of the six patients needed to be reconnected to NIMV long before the scheduled 1-week withdrawal period had been completed. Indeed, the first patient required reinstitution of breathing support on the first day of withdrawal, and the other patient required it on the third day. Unfortunately, individual ABG data were not reported, so that we do not know whether the clinical worsening also was accompanied by a deterioration in gas exchange. In the present investigation, we found a slightly lower incidence of the failure to remain without NIMV than did Hill et al 19 (45.4% vs 30%, respectively) and the first evidence of ABG level deterioration at day 3 without ventilation, in the absence, however, of a clear worsening in the clinical status. These differences between the two studies may well be explained by the different characteristics of the patients enrolled (restrictive thoracic disease in the study by Hill et al 19 and a mixed population in ours), the different durations of long-term NIMV (12.7 vs 19.3 months, respectively), and a switch from a volume ventilator to a pressure ventilator in four of the six patients in the study performed by Hill et al 19 in the month before the study began, while our patients used their usual pressure ventilator. Furthermore, our selection criteria of enrollment in the NIMV program included close monitoring and observation of the patients for 3 weeks while in the hospital. This was not performed in the study by Hill et al 19 and highlights the possible limitations of applying our findings to other populations of patients in whom NIMV has been initiated using different criteria. Last, and perhaps most important, our symptom questionnaire was focused on resting dyspnea, sleeplessness, and morning headaches, while the American study 19 also focused on the assessment of energy and fatigue. Despite these possible differences, the common point of both studies was that a relatively high incidence of clinical problems develops a few days after NIMV discontinuation. This suggests that the temporary interruption of NIMV in stable patients with hypercapnia may not be safe in all patients, and, if ventilation is suspended for any reason, close monitoring of symptoms and gas exchange should be done, preferably in a more protected environment than the home. Prompt intervention in the case of ventilator failure is mandatory. This is readily achievable for patients who are provided with a second back-up ventilator, but this may 1384 Clinical Investigations

be an unnecessary expense for patients who are not totally ventilator-dependent. Our data also suggest that every effort should be made by the home-care team and the caregiver to avoid the development of problems that could interfere with the daily use of NIMV. In particular, great care should be taken to minimize the problem of nose irritation, either by adequately protecting the bridge of the nose, varying the site of mask friction, or periodically changing the type or size of the mask. 23 Adequate humidification and heating also should be considered as measures to avoid rhinitis and abnormal increases in nasal resistances. 24 This study was designed primarily to record any clinical deterioration subsequent to NIMV suspension and not the physiologic mechanisms underlying such an event. Nevertheless, we observed that in the subjects who needed to be reconnected to the ventilator, Vt and Pimax decreased significantly. This is not surprising since, in these clinically stable patients, variations in Paco 2 are mainly determined by alveolar ventilation rather than by CO 2 production. If the RR is kept constant, alveolar ventilation is almost totally dependent on changes in Vt. As a matter of fact, Vt was decreased only in the patients whose gas exchange levels deteriorated. Concomitantly, a decrease in Pimax indirectly suggests that the mechanism of reduced Paco 2 clearance may, at least in part, be due to the theory of ventilatory muscle rest. 7 The removal of periodic resting of the diaphragm and other inspiratory muscles was probably associated with a quick worsening of their inotropic characteristics. If this is true, then, hypothesizing that the inspiratory resistive and elastic loads are constant, the decrease in Pimax leads to a load/capacity (ie, the pressure generated per breath over the maximal pressure generated) imbalance. The patients then may choose to behave like wise fighters, reducing the pressure generated per breath, and therefore the Vt, and becoming more hypercapnic, rather than approaching close to the so-called fatigue threshold. 25 Our results also seem to suggest that patients with a more pronounced increase in Paco 2 briefly after NIMV suspension (ie, 3 h) and an increase in the mean resting inspiratory flow pattern may be more prone to developing clinical deterioration a few days after disconnection from NIMV. Therefore, we suggest considering the Paco 2 response occurring shortly after NIMV suspension as a possible indicator of alveolar hypoventilation development in the case of forced withdrawal, since the recording of the Vt/Ti ratio is sometimes impractical. Indeed, our observations support the theory, already suggested by other authors, 10,13 that the individual responses to long-term NIMV may differ and, therefore, also that the removal of NIMV may result in two subsets of patients (ie, those who deteriorate and those who do not). Further larger prospective studies on this issue are needed to confirm this hypothesis. The present study has some inherent limitations. First, there was not a control group, and, therefore, the study was not randomized. Keeping in mind that each patient served as his or her own control subject, the random withholding of NIMV for several weeks was not accepted by our local ethics committee, since the patients had more severe ABG abnormalities and symptoms before the establishment of the home-care program than after the enrollment (Table 1). The results of the study also support the idea that NIMV removal, even for a brief period, may be dangerous. Another limitation was the lack of assessment of the sleep architecture, which might have afforded a better understanding of the mechanism of ABG deterioration during the daytime. We have emphasized already that the primary outcome of the study was to determine the incidence of problems linked to NIMV discontinuation rather than the underlying physiologic mechanisms. However, it has been suggested that the resetting of respiratory centers due to an improvement of nocturnal hypoventilation and sleep quality should increase the P 0.1 (the index of neuromuscular drive) in the case of a rise in Paco 2. As a matter of fact, P 0.1 remained unchanged, even in the group that showed clinical worsening. The third limitation was the selection of a heterogeneous group of patients. It has been claimed that patients affected by restrictive thoracic disease respond better to the chronic administration of NIMV than do COPD patients. However, the enrollment criteria for this study clearly stated that only patients who had been successfully receiving ventilation (in terms of ABG) for at least 1 year were eligible for inclusion in the study, so we were confident of selecting only the category of patients who were responders to ventilation. Indeed, the proportion of patients with COPD and RTD whose gas exchange levels deteriorated after the brief withdrawal was similar. In conclusion, we have shown that a brief discontinuation of NIMV in patients who have chronic hypercapnic respiratory failure and are well-established on NIMV is associated with a relatively high incidence of ABG level and symptom worsening. The deterioration in ABG level seems to be related to the development of alveolar hypoventilation and may due to the mechanism of chronic fatigue of the respiratory muscle. If NIMV must be briefly interrupted for clinical reasons, close monitoring should be performed to pick up signs of clinical worsening CHEST / 119 / 5/ MAY, 2001 1385

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