Initial and Subsequent Dosing of Rectal Acetaminophen in Children

Background

Recent studies have determined that an initial rectal acetaminophen dose of approximately 40 mg/kg is needed in children to achieve target antipyretic serum concentrations. The timing and amount of subsequent doses after a 40-mg/kg dose has not been clarified for this route of administration. Based on the authors' previous pharmacokinetic data, they examined whether a 40-mg/kg loading dose followed by 20-mg/kg doses at 6-h intervals maintain serum concentrations within the target range of 10-20 microg/ml, without evidence of accumulation.

Methods

Children (n = 16) received rectal acetaminophen (40 mg/kg) and up to three additional doses of 20 mg/kg at 6-h intervals. Venous blood samples were taken every 30 min for 4 h, then every 60 min for 4 h, and every 4 h for 16 h. The authors assessed whether their published pharmacokinetic parameters predicted the acetaminophen concentrations in the present study. They also assessed their dosing regimen by determining the fraction of time each individual maintained the target concentration.

Results

All patients received the initial loading dose; 10 of 16 patients received three subsequent doses. Serum concentrations with the initial dose were in the target range 38 +/- 25% of the time. With subsequent dosing, the target range was maintained 60 +/- 29% of the time. The highest serum concentration with initial or subsequent dosing was 38.6 microg/ml. Pharmacokinetic parameters from the earlier study predicted the serum concentrations observed for both initial and subsequent doses.

Conclusions

A rectal acetaminophen loading dose of 40 mg/kg followed by 20-mg/kg doses every 6 h results in serum concentrations centered at the target range of 10-20 microg/ml. There was large interindividual variability in pharmacokinetic characteristics. There was no evidence of accumulation during the 24-h sampling period.

Improvement in aerosol delivery with helium-oxygen mixtures during mechanical ventilation

In mechanically ventilated patients with airway obstruction, helium-oxygen (He-O2) mixtures reduce airway resistance and improve ventilation, but their influence on aerosol delivery is unknown. Accordingly, we determined the effect of various He-O2 mixtures on albuterol delivery from metered-dose inhalers (MDIs) and jet nebulizers in an in vitro model of mechanical ventilation. Albuterol delivery from a MDI was increased when the ventilator circuit contained 80% helium and 20% oxygen (He-O2 80/20) versus O2: 46.7 +/- 3.3 versus 30.2 +/- 1.3 (SE)% of the nominal dose (p < 0.001)-the difference was mainly due to decreased drug deposition in the spacer chamber, mean 39.2% and 55.2%, respectively (p < 0.001). Nebulizer efficiency at a flow rate of 6 L/min was five times lower with He-O2 80/20 than O2, and the amount of nebulized drug was inversely correlated with gas density (r = 0.94, p < 0.0001). When the nebulizer was operated with O2, greater albuterol delivery was achieved when the ventilator circuit contained He-O2 rather than O2. In summary, He-O2 mixtures in the circuit increased aerosol delivery for both MDIs and nebulizers in the mechanically ventilated model by as much as 50%. In conclusion, at appropriate flow rates and concentrations, He-O2 in the ventilator circuit may improve aerosol delivery in mechanically ventilated patients with severe airway obstruction.

Effect of isocapnic hypoxia on variational activity of breathing

In the presence of either hypocapnia or sleep, hypoxia has been shown to induce periodic breathing and increase the total variational activity of breath components. It is not known whether hypoxia induces alterations in breathing variability during wakefulness and in the absence of hypocapnia. To address this issue, we studied nonobtrusively 14 healthy awake subjects before and during the delivery of a hypoxic gas mixture via a plastic hood; the subjects' oxygen saturation decreased from 98 to 79% and end-tidal carbon dioxide tension was kept constant. Compared with air, isocapnic hypoxia increased the gross variability of minute ventilation (V I), tidal volume (VT), inspiratory time (TI), and expiratory time (TE) (all p < 0.004). Isocapnic hypoxia decreased the autocorrelation coefficient at a lag of one breath for TE (p < 0. 008) and V I (p = 0.07), the number of consecutive breath lags having significant autocorrelation coefficients for TE (p = 0.03), and the cycle time of oscillations in V I (p = 0.03). When partitioned, the increase in total variational activity during isocapnic hypoxia was found to result from increases in the random fractions of V I, VT, TI, and TE (all p < 0.05), and the oscillatory fractions of V I, VT, and TE (all p < 0.03). In conclusion, hypoxia induced hidden oscillations in V I, VT, and TE despite wakefulness and an isocapnic state, suggesting that neural responses may have a more important role in the genesis of hypoxia-induced oscillations than previously reported.

Real-time studies of the interactions between epidermal growth factor and its receptor during endocytic trafficking

The interactions of growth factors with cell surface receptors regulate fundamental cell processes, such as growth, differentiation and transformation. Understanding the nature of these interactions at the molecular level is of fundamental importance in cell biology. This is not only from the point of view of basic science, but also because of the repercussions such knowledge might have in understanding the mode of action of drugs in cells. Receptor mediated endocytosis has been implicated in the downregulation of the mitogenic signal. However, no data are thus far available on how growth factor/receptor interactions might control endocytic trafficking. Here we show that information on modes of binding and receptor conformational changes can be obtained using time-resolved fluorescence methods. We have found that fluorescent probes bound to epidermal growth factor (EGF) show dynamic fluorescence quenching when EGF is bound to internalising EGF receptors (EGFR). We propose that this dynamic quenching takes place because EGF-bound probes interact with tryptophan residues in the extracellular domain of the EGF-EGFR complex. Real-time accumulation of fluorescent decays has also allowed us to follow the time course of a conformational change in EGFR occurring during endocytosis, and correlate this information with endosomal trafficking and EGFR recycling.

Assessment of neural inspiratory time in ventilator-supported patients

Neural inspiratory time (TI) is a measurement of fundamental importance in studies of patient-ventilator interaction. The measurement is usually based on recordings of flow, esophageal pressure (Pes), and transdiaphragmatic pressure (Pdi), but the concordance of such estimates of neural TI with a more direct measurement of neural activity has not been systematically evaluated. To address this issue, we studied nine ventilator-supported patients in whom we employed esophageal electrode recordings of the diaphragmatic electromyogram (EMG) as the reference measurement of neural TI. Comparison of the indirect estimates of neural TI duration, based on flow, Pes, and Pdi against the reference measurement, revealed a mean difference (bias) ranging from -54 to 612 ms during spontaneous breathing and from -52 to 714 ms during mechanical ventilation; the respective precisions (standard deviations of the differences) ranged from 79 to 175 ms and from 74 to 221 ms. Because an indirect estimate of neural TI duration could be identical to that of the reference measurement and yet be displaced in time, this lag or lead was quantified as the phase angle of neural TI onset. Flow-based estimates of the onset of neural TI displayed a systematic lag, which may be explained at least in part by concurrent intrinsic positive end-expiratory pressure. In conclusion, the indirect estimates of the onset and duration of neural TI in ventilator-dependent patients displayed poor agreement with the diaphragmatic EMG measurement of neural TI.

How is mechanical ventilation employed in the intensive care unit? An international utilization review

A 1-d point-prevalence study was performed with the aim of describing the characteristics of conventional mechanical ventilation in intensive care units ICUs from North America, South America, Spain, and Portugal. The study involved 412 medical-surgical ICUs and 1,638 patients receiving mechanical ventilation at the moment of the study. The main outcome measures were characterization of the indications for initiation of mechanical ventilation, the artificial airways used to deliver mechanical ventilation, the ventilator modes and settings, and the methods of weaning. The median age of the study patients was 61 yr, and the median duration of mechanical ventilation at the time of the study was 7 d. Common indications for the initiation of mechanical ventilation included acute respiratory failure (66%), acute exacerbation of chronic obstructive pulmonary disease (13%), coma (10%), and neuromuscular disorders (10%). Mechanical ventilation was delivered via an endotracheal tube in 75% of patients, a tracheostomy in 24%, and a facial mask in 1%. Ventilator modes consisted of assist/control ventilation in 47% of patients and 46% were ventilated with synchronized intermittent mandatory ventilation, pressure support, or the combination of both. The median tidal volume setting was 9 ml/kg in patients receiving assist/control and the median setting of pressure support was 18 cm H(2)O. Positive end-expiratory pressure was not employed in 31% of patients. Method of weaning varied considerably from country to country, and even within a country several methods were in use. We conclude that the primary indications for mechanical ventilation and the ventilator settings were remarkably similar across countries, but the selection of modes of mechanical ventilation and methods of weaning varied considerably from country to country.

Does inhaled albuterol improve diaphragmatic contractility in patients with chronic obstructive pulmonary disease?

We tested the hypothesis that the decrease in dyspnea in patients with COPD with inhaled albuterol is in part due to increased diaphragmatic contractility. Eleven patients with COPD inhaled albuterol or placebo in a double-blind randomized manner. Subsequently, dyspnea was measured while patients breathed through inspiratory resistors, and diaphragmatic contractility was quantified during maximal inspiratory efforts and after twitch stimulation of the phrenic nerves. Albuterol produced a decrease in dyspnea (5 +/- 2 to 4 +/- 2 [SD] Borg units, p < 0.01), and increases in maximal transdiaphragmatic pressure (92.4 +/- 37.2 to 102.8 +/- 37.2 cm H(2)O, p < 0.03) and potentiated twitch transdiaphragmatic pressures (21.6 +/- 7.1 to 25.2 +/- 7.6 cm H(2)O, p < 0.02). The decrease in dyspnea correlated with the increases in maximal and twitch transdiaphragmatic pressures: r = -0.64 (p = 0. 04) and r = -0.65 (p = 0.04), respectively. Compared with placebo, albuterol produced an increase in inspiratory capacity (1.87 +/- 0. 71 to 2.26 +/- 0.74 L, p = 0.002), which accounted for the increases in maximal and twitch transdiaphragmatic pressures. The decrease in dyspnea correlated with the increase in inspiratory capacity (r = -0. 62, p = 0.04), but not with the increase in FEV(1) (r = -0.13, p = 0. 72). In conclusion, albuterol relieves dyspnea and enhances respiratory muscle output in patients with COPD primarily by improving the length-tension relationship of the diaphragm rather than by improving its contractility.

Influence of ventilator settings in determining respiratory frequency during mechanical ventilation

During mechanical ventilation, changes in inspiratory flow and tidal volume (VT) have been shown to alter respiratory frequency (f ). However, the changes in flow and VT have been accompanied by alteration in ventilator inspiratory time (TI,(vent)), and it is not clear which variable is the primary determinant. To address this issue, we employed four protocols in 15 healthy volunteers receiving assist-control ventilation. When VT was fixed and flow was delivered at 30, 60, and 90 L/min, f increased as a function of the increase in flow and the decrease in TI,(vent). When flow was held constant and VT was changed among 0.5, 1.0, and 1.5 L, f increased as a function of the decreases in VT and TI,(vent). When flow was increased from 60 to 90 L/min and these changes were balanced with VT settings of 1.0 and 1.5 L to maintain a constant TI,(vent), f did not change. When flow and VT were held constant and TI,(vent) was varied by the application of inspiratory pauses (0 to 2 s), f decreased as a function of the increase in TI,(vent) (p < 0.001). In conclusion, the imposed ventilator inspiratory time during mechanical ventilation can determine f independently of delivered inspiratory flow and VT.

Can diaphragmatic contractility be assessed by twitch airway pressures in patients with chronic obstructive pulmonary disease?

In healthy subjects and in patients without lung diseases, twitch airway pressure (Paw(tw)) responses to phrenic nerve stimulation can be used to predict twitch esophageal pressure (Pes(tw)) and twitch transdiaphragmatic pressure (Pdi(tw)), thus overcoming the need for placement of esophageal and gastric balloons. The aim of this study was to determine whether measurements of Paw(tw) combined with simple maneuvers could be used to predict Pes(tw), and possibly Pdi(tw), in patients with severe chronic obstructive pulmonary disease (COPD) (n = 12). Stimulations delivered at relaxed FRC produced a correlation coefficient (r) between Paw(tw) and Pes(tw) of 0.44 (p < 0.001) and of 0.62 (p < 0.001) during stimulations while patients performed a gentle exhalation from FRC. Stimulations performed during a gentle inhalation produced a good correlation between Paw(tw) and Pes(tw) (r = 0.92, p < 0.001); however, the limits of agreement between Paw(tw) and Pes(tw) were wide. Correlations between Paw(tw) and Pdi(tw) during the three experimental conditions were weak. In conclusion, during a gentle inspiratory effort in patients with severe COPD the correlation between Paw(tw) and Pdi(tw) was weak, whereas the correlation between Paw(tw) and Pes(tw) was good, but it was not sufficient to allow the prediction of Pes(tw) from Paw(tw) in all patients.

Preferential pulmonary retention of (S)-albuterol after inhalation of racemic albuterol

The (R)-enantiomer of racemic albuterol produces bronchodilation, whereas the (S)-enantiomer may increase airway reactivity. After oral or intravenous administration of racemic albuterol, the (R)- enantiomer is metabolized several times faster than the (S)-enantiomer; however, enantiomer disposition after inhaling racemic albuterol with a metered-dose inhaler (MDI) is not known. Accordingly, 10 healthy subjects inhaled racemic albuterol with a MDI alone and with a MDI and holding chamber. We measured plasma levels of unchanged (R)- and (S)-albuterol before and up to 4 h after inhalation of racemic albuterol, and determined the unchanged R/S ratio in urine before and at 0.5, 4, 8, and 24 h later. The disposition of albuterol's enantiomers with a MDI and holding chamber was similar to that with a MDI alone. The area under the curve (AUC) of the plasma levels over time was significantly lower for the (S)- than for the (R)-enantiomer-395.5 +/- 141.0 (SE) versus 882.7 +/- 126.4 ng. ml(-)(1). min (p < 0.05)-indicating preferential retention of (S)-albuterol in the lung. The R/S ratio in urine at 0. 5 h after albuterol was > 1, reflecting the higher plasma level of the (R)-enantiomer. In conclusion, preferential retention of the (S)- compared with the (R)-enantiomer in the lung could lead to accumulation of the (S)-enantiomer after long-term use of racemic albuterol.

Effect of spontaneous breathing trial duration on outcome of attempts to discontinue mechanical ventilation. Spanish Lung Failure Collaborative Group

The duration of spontaneous breathing trials before extubation has been set at 2 h in research studies, but the optimal duration is not known. We conducted a prospective, multicenter study involving 526 ventilator-supported patients considered ready for weaning, to compare clinical outcomes for trials of spontaneous breathing with target durations of 30 and 120 min. Of the 270 and 256 patients in the 30- and 120-min trial groups, respectively, 237 (87.8%) and 216 (84.8%), respectively, completed the trial without distress and were extubated (p = 0.32); 32 (13.5%) and 29 (13.4%), respectively, of these patients required reintubation within 48 h. The percentage of patients who remained extubated for 48 h after a spontaneous breathing trial did not differ in the 30- and 120-min trial groups (75.9% versus 73.0%, respectively, p = 0.43). The 30- and 120-min trial groups had similar within-unit mortality rates (13 and 9%, respectively) and in-hospital mortality rates (19 and 18%, respectively). Reintubation was required in 61 (13.5%) patients, and these patients had a higher mortality (20 of 61, 32.8%) than did patients who tolerated extubation (18 of 392, 4.6%) (p < 0.001). Neither measurements of respiratory frequency, heart rate, systolic blood pressure, and oxygen saturation during the trial, nor other functional measurements before the trial discriminated between patients who required reintubation from those who tolerated extubation. In conclusion, after a first trial of spontaneous breathing, successful extubation was achieved equally effectively with trials targeted to last 30 and 120 min.

Reconciling in vitro and in vivo measurements of aerosol delivery from a metered-dose inhaler during mechanical ventilation and defining efficiency-enhancing factors

We attempted to resolve the discrepancies in reported data on aerosol deposition from a chlorofluorocarbon (CFC)-propelled metered-dose inhaler (MDI) during mechanical ventilation, obtained by in vivo and in vitro methodologies. Albuterol delivery to the lower respiratory tract was decreased in a humidified versus a dry circuit (16.2 versus 30.4%, respectively; p < 0.01). In 10 mechanically ventilated patients, 4.8% of the nominal dose was exhaled. When the exhaled aerosol was subtracted from the in vitro delivery of 16.2% achieved in a humidified ventilator circuit, the resulting value (16.2 - 4.8 = 11.4%) was similar to in vivo estimates of aerosol deposition. Having reconciled in vitro with in vivo findings, we then evaluated factors influencing aerosol delivery. A lower inspiratory flow rate (40 versus 80 L/min; p < 0.001), a longer duty cycle (0.50 versus 0.25; p < 0.04), and a shorter interval between successive MDI actuations (15 versus 60 s; p < 0.02) increased aerosol delivery, whereas use of a hydrofluoroalkane (HFA)-propelled MDI decreased aerosol delivery compared with the CFC-propelled MDI. A MDI and actuator combination other than that designed by the manufacturer altered aerosol particle size and decreased drug delivery. In conclusion, aerosol delivery in an in vitro model accurately reflects in vivo delivery, providing a means for investigating methods to improve the efficiency of aerosol therapy during mechanical ventilation.

Continuous recordings of mixed venous oxygen saturation during weaning from mechanical ventilation and the ramifications thereof

To define the importance of hemodynamic performance and global tissue oxygenation in determining weaning outcome, we recorded mixed venous oxygen saturation (SvO2) continuously in eight ventilator-supported patients who failed a trial of spontaneous breathing and 11 patients who tolerated a trial and were successfully extubated. Immediately before the weaning trial, SvO2 was not statistically different in the two groups (p = 0.28). On discontinuation of the ventilator, SvO2 fell progressively in the failure group (p < 0.01), whereas it did not change in the success group. During the trial of spontaneous breathing, O2 demand was similar in the two groups, but it differed in the manner with which it was met. The success group demonstrated an increase in cardiac index (p < 0.05) and O2 transport (p < 0.02). The failure group did not increase O2 transport, partly because of elevations in right- and left-ventricular afterload, but, instead, increased O2 extraction ratio (p < 0.02) with a consequent fall in SvO2. In turn, the low SvO2 combined with greater venous admixture (p < 0.0006) led to rapid arterial desaturation (p < 0.006) and a relative decrease in O2 being supplied to the tissues. In conclusion, ventilator-supported patients who failed a trial of spontaneous breathing developed a progressive decrease in SvO2 caused by the combination of a relative decrease in convective O2 transport and an increase in O2 extraction by the tissues.

Cycling of inspiratory and expiratory muscle groups with the ventilator in airflow limitation

Research on patient-ventilator interactions has largely focused on inspiratory events, with little attention paid to expiration. We sought to determine the importance of the timing and magnitude of expiratory muscle activity in causing patient-ventilator dyssynchrony. Our study was done with healthy subjects receiving pressure support in whom we induced airflow limitation with a Starling resistor. The timing and magnitude of expiratory muscle activity were obtained by wire electromyographic recording of the activity of the transversus abdominis muscle, and were compared with the cycling of the ventilator and inspiratory muscle activity as determined from a flow tracing and diaphragmatic electromyogram (EMG), respectively. Induction of airflow limitation produced significant phase differences in the cycling of the subjects' expiratory muscle group and that of the machine. Some inspiratory efforts failed to trigger the ventilator, owing in part to an increase in elastic recoil consequent to the commencement of expiratory efforts before the termination of mechanical inflation. A delay in relaxation of the expiratory muscles did not interfere with the success of subsequent inspiratory efforts to trigger the ventilator. We also investigated the accuracy of two approaches for distinguishing between the contributions of expiratory muscle activity and elastic recoil to intrinsic positive end-expiratory pressure (PEEPi): the expiratory increase in gastric pressure (Pga) correlated better with transversus abdominis electromyographic activity (r = 0.7 to 0.95) than did the early inspiratory decrease in Pga (r = 0.04 to 0.53). In conclusion, the continuation of mechanical inflation into neural expiration was associated with failure of the subsequent inspiratory attempt to trigger the ventilator.

A comparison of three modes of ventilation with the use of an adult circle system in an infant lung model

We examined the efficiency of an adult circle system with adult bellows to deliver minute ventilation (VE) to an infant test lung model. A Narkomed 2B system (North American Drager, Telford, PA) using three modes of ventilator setup were used: A = time-cycled, volume-controlled using bellows excursion to control delivered volume; B = time-cycled, pressure-controlled using inspiratory pressure limit adjustment to control delivered volume; C = time-cycled, pressure-controlled using the inspiratory flow adjustment to control delivered volume. VE was measured with two compliances (normal and low) and four endotracheal tube (ETT) sizes (2.5-, 3.0-, 3.5-, and 4.0-mm inner diameter). VE was measured at peak inspiratory pressures (PIP) of 20, 30, 40 or 50 cm H2O while respiratory rate (RR) was held constant at 20 breaths/min. VE was measured as RR was set at 20, 30, 40, or 50 breaths/min while target PIP was held constant at 20 cm H2O. Data were analyzed using the multiple regression technique. With the low compliance model, VE was nearly identical regardless of the ventilator setup. With the normal compliance model, minor differences in VE were observed, especially at the highest RR and PIP. VE was dependent on RR, PIP, and lung compliance. Overall, the ventilator setup resulted in minor changes in VE. Very high PIPs were required to deliver VE to the low compliance model. ETT size did not affect VE when lung compliance was low; however, smaller ETT size was a factor when test lung compliance was normal, decreasing delivered VE at higher PIP and RR. We conclude that with a Narkomed 2B adult circle system VE is dependent on PIP, RR, and lung compliance, but not on mode of ventilator setup.

IMPLICATIONS

The results of this laboratory investigation indicate that when an adult circle system is used during infant anesthesia, the ventilation delivered depends primarily on the respiratory rate, peak inspiratory pressure, and the compliance of the lung being ventilated, rather than on the specific mode of ventilator setup.