Effect of weight gain during pregnancy on heart rate variability and hypotension during caesarean section under spinal anaesthesia.

The purpose of this study was to assess the effect of antenatal weight gain on baseline heart rate variability and incidence of hypotension in singleton parturients with a normal pre-pregnancy body mass index, presenting at term for elective caesarean section under spinal anaesthesia. Sixty-six parturients, of ASA physical status 1-2, were allocated to one of three groups according to their weight gain during pregnancy: < 11 kg; 11-16 kg; and > 16 kg. Mean (SD) approximate entropy of baseline heart rate was significantly higher in the < 11 kg group (0.27 (0.11)) compared with the 11-16 kg group (0.14 (0.08)) and the > 16 kg group (0.14 (0.07)) (both p < 0.001). The incidence of hypotension in the < 11 kg group (17/22; 77%) was significantly higher than in the 11-16 kg group (7/22; 32%) (p = 0.006) and the > 16 kg group (8/22; 36%) (p = 0.01). We conclude that weight gain < 11 kg during pregnancy is associated with increased baseline heart rate variability and a higher incidence of hypotension at the time of elective caesarean section under spinal anaesthesia.

Tracheal intubation following training with the GlideScope compared to direct laryngoscopy.

SUMMARY: Tracheal intubation using direct laryngoscopy has a high failure rate when performed by untrained medical personnel. This study compares tracheal intubation following direct laryngoscopy by inexperienced medical students when initially trained by using either the GlideScope, a video assisted laryngoscope, or a rigid (Macintosh) laryngoscope. Forty-two medical students with no previous experience in tracheal intubation were randomly divided into two equal groups to receive training with the GlideScope or with direct laryngoscopy. Subsequently, each medical student performed three consecutive intubations on patients with normal airways that were observed by a anaesthetist who was blinded to the training method. The rates of successful intubation were significantly higher in the Glidescope group after the first (48%), second (62%), and third (81%) intubations compared with the Macintosh group (14%, 14% and 33%; p = 0.043, 0.004 and 0.004, respectively). The mean (SD) times for the first, second, and third successful tracheal intubations were significantly shorter in the Glidescope group (59.3 (4.4) s, 56.6 (7.1) s and 50.1 (4.0) s) than the Macintosh group (70.7 (7.5) s, 73.7 (7.3) s and 67.6 (2.0) s; p = 0.006, 0.003 and 0.0001, respectively). Training with a video-assisted device such as the GlideScope improves the success rate and time for tracheal intubation in patients with normal airways when this is performed by inexperienced individuals following a short training programme.

Effect of suxamethonium vs rocuronium on onset of oxygen desaturation during apnoea following rapid sequence induction.

This study investigates the effect of suxamethonium vs rocuronium on the onset of haemoglobin desaturation during apnoea, following rapid sequence induction of anaesthesia. Sixty patients were randomly allocated to one of three groups. Anaesthesia was induced with lidocaine 1.5 mg.kg(-1), fentanyl 2 microg.kg(-1) and propofol 2 mg.kg(-1), followed by either rocuronium 1 mg.kg(-1) (Group R) or suxamethonium 1.5 mg.kg(-1) (Group S). The third group received propofol 2 mg.kg(-1) and suxamethonium 1.5 mg.kg(-1) only (Group SO). The median (IQR [range]) time to reach S(p)O(2) of 95% was significantly shorter in Group S (358 (311-373 [245-430]) s) [corrected] than in Group R (378 (370-393 [366-420]) s; p = 0.003), and shorter in Group SO (242 (225-258 [189-270]) s) [corrected] than in both Group R (p < 0.001) and Group S (p < 0.001). When suxamethonium is administered for rapid sequence induction of anaesthesia, a faster onset of oxygen desaturation is observed during the subsequent apnoea compared with rocuronium. However, time to desaturation is prolonged whenever lidocaine and fentanyl precede suxamethonium.

Changes in resistances of endotracheal tubes with reductions in the cross-sectional area.

BACKGROUND AND OBJECTIVES: Partial obstruction of endotracheal tubes due to accumulation of secretions and mucus plugs can increase the tube resistance and subsequently impose increased resistive load on the patient. This study was performed to determine the changes in the resistance of endotracheal tubes of sizes 7.5, 8.0 and 8.5 mm with different degrees and locations of endotracheal tube narrowing. METHODS: Reductions of 10%, 25%, 50% and 75% in the endotracheal tube's cross-sectional areas were created at different sites along the axes of the tube connected to an artificial lung. While ventilating with a constant inspiratory flow, a 1 s end-inspiratory occlusion manoeuvre was applied and the resulting plateau pressure was determined. The resistance was calculated as (peak airway pressure--plateau pressure)/peak inspiratory flow. RESULTS: Significant increases in the endotracheal tube's resistances were observed as the tube's cross-sectional area reduction was increased from 25% to 50% and from 50% to 75% for the 7.5 mm endotracheal tube, from 25% to 50% for the 8.0 mm endotracheal tube, and from 50% to 75% for the 8.5 mm endotracheal tube. Changes of the endotracheal tube resistances were not affected by the site of cross-sectional area reductions along the axes of the tubes. CONCLUSIONS: For endotracheal tubes of sizes 7.5, 8.0 and 8.5 mm, significant changes in the tubes resistances are observed when the partial obstructions of the tubes exceed certain critical values. The location of the partial obstruction did not affect the changes in the endotracheal tube resistances.

Supplementation of pre-oxygenation in morbidly obese patients using nasopharyngeal oxygen insufflation.

During apnoea following induction of anaesthesia, morbidly obese patients may suffer a rapid decrease in oxygen saturation. This study compares pre-oxygenation alone with pre-oxygenation followed by nasopharyngeal oxygen insufflation on the onset of desaturation occurring during the subsequent apnoea. A randomised controlled trial was performed in 34 morbidly obese patients undergoing gastric bypass or gastric band surgery. Seventeen patients received nasopharyngeal oxygen supplementation following pre-oxygenation (Study group, body mass index = 41.8 (6.9) kg.m(-2)), and the other 17 patients received pre-oxygenation alone (Control group, body mass index = 42.7 (5.4) kg.m(-2)). Time from the onset of apnoea until S(p)o(2) fell to 95% was compared between the two groups with a cut-off of 4 min. In the control group, the S(p)o(2) fell from 100% to 95% during the subsequent apnoea in 145 (27) s, with a significantly negative correlation (r(2) = 0.66, p < 0.05) between the time to desaturation to 95% and the body mass index. In the study group, the S(p)o(2) was maintained in 16 of 17 patients at 100% for 4 min when apnoea was terminated. In conclusion, nasopharyngeal oxygen insufflation following pre-oxygenation in morbidly obese patients delays the onset of oxyhaemoglobin desaturation during the subsequent apnoea.

Nasopharyngeal oxygen insufflation following pre-oxygenation using the four deep breath technique.

This paper evaluates the effectiveness of nasopharyngeal oxygen insufflation following preoxygenation using the four deep breath technique within 30 s, on the onset of haemoglobin desaturation during the subsequent apnoea. Thirty ASA I or II patients were randomly allocated to one of two groups. In the study group (n = 15), pre-oxygenation was followed by insufflation of oxygen at a flow of 5 l.min(-1) via a nasopharyngeal catheter commenced at the onset of apnoea. In the control group, pre-oxygenation was not followed by nasopharyngeal oxygen insufflation (n = 15). In the control group, SpO2 fell to 95% within a mean (SD) apnoea time of 3.65 (1.15) min, whereas in the study group, SpO2 was maintained in all patients at 100% throughout the 6 min of apnoea, at which point apnoea was terminated and positive pressure ventilation commenced. We conclude that nasopharyngeal oxygen insufflation following pre-oxygenation using the four deep breath technique can delay the onset of haemoglobin desaturation for a significant period of time during the subsequent apnoea.

Mean alveolar pressure during constant-flow and constant-pressure inflation of diseased lungs.

INTRODUCTION: An important goal in managing mechanical ventilation is optimizing key variables such as mean alveolar pressure (PA). PURPOSE: Determine the effects of lung mechanics on PA during constant-flow inflation (CFI) and constant-pressure inflation (CPI) in models of nonhomogenous lung disease. METHODS: We postulated a mathematical lung model consisting of 2 parallel lung units with the airways and the chest wall. Analysis was performed for step inputs of inspiratory flow and pressure while maintaining the same tidal volume, respiratory frequency, and positive end-expiratory pressure. The analysis was performed for purely restrictive, purely obstructive, and mixed lung disease. RESULTS: When simulating purely restrictive lung disease, PA was always higher in healthy alveoli than in diseased alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always greater during CPI than during CFI. For purely obstructive lung diseases, PA was always lower in diseased alveoli than in healthy alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always greater during CPI than during CFI. For mixed lung diseases with equal time constants, PA was always higher in diseased alveoli than in healthy alveoli, with constant inspiratory flow or constant inspiratory pressure. However, PA in each alveoli was always the same during CPI as during CFI. CONCLUSIONS: For the same tidal volume, the mean alveolar pressure in different alveoli depends on the type of disease. The difference in mean alveolar pressure between a normal and a diseased alveolus depends on the difference in time constant between those alveoli, regardless of the mode of ventilation.

Preoxygenation: comparison of maximal breathing and tidal volume breathing techniques.

BACKGROUND: Preoxygenation with tidal volume breathing for 3-5 min is recommended by Hamilton and Eastwood. This report compares tidal volume preoxygenation technique with deep breathing techniques for 30-60 s. METHODS: The study was conducted in two parts on patients undergoing elective coronary bypass grafting. In the first group (n = 32), each patient underwent all of the following preoxygenation techniques: the traditional technique consisting of 3 min of tidal volume breathing at an oxygen flow of 5 l/min; four deep breaths within 30 s at oxygen flows of 5 l/min, 10 l/min, and 20 l/min; and eight deep breaths within 60 s at an oxygen flow of 10 l/min. The mean arterial oxygen tensions after each technique were measured and compared. In the second group (n = 24), patients underwent one of the following techniques of preoxygenation: the traditional technique (n = 8), four deep breaths (n = 8), and eight deep breaths (n = 8). Apnea was then induced, and the mean times of hemoglobin desaturation from 100 to 99, 98, 97, 96, and 95% were determined. RESULTS: In the first group of patients, the mean arterial oxygen tension following the tidal breathing technique was 392+/-72 mm Hg. This was significantly higher (P<0.05) than the values obtained following the four deep breath technique at oxygen flows of 5 l/min (256+/-73 mm Hg), 10 l/min (286+/-69 mm Hg), and 20 l/min (316+/-67 mm Hg). In contrast, the technique of eight deep breaths resulted in a mean arterial oxygen tension of 369+/-69 mm Hg, which was not significantly different from the value achieved by the traditional technique. In the second group of patients, apnea following different techniques of preoxygenation was associated with a slower hemoglobin desaturation in the eight-deep-breaths technique as compared with both the traditional and the four-deep-breaths techniques. CONCLUSION: Rapid preoxygenation with the eight deep breaths within 60 s can be used as an alternative to the traditional 3-min technique.

Inspiratory pressure/maximal inspiratory pressure: does it predict successful extubation in critically ill infants and children?

OBJECTIVE: To evaluate the accuracy of the initial negative inspiratory pressure (PI) to maximal negative inspiratory pressure (PImax) ratio in predicting extubation outcome for intubated infants and children. DESIGN: A prospective study. SETTING: Pediatric intensive care unit. PATIENTS: A sample of 50 stable intubated pediatric patients who were judged clinically ready for extubation. METHODS: Using a one-way valve, PI and PImax were measured in all patients, after which the < or = ratio PI/PImax was calculated and its accuracy in predicting extubation outcome evaluated. MEASUREMENTS AND RESULTS: A total of 39 patients (78%) were successfully extubated and 11 patients (22%) were not. The mean PI/PImax ratio was not significantly different between extubation successes (0.36 +/- 0.14) and failures (0.45 +/- 0.1) (P > 0.05). The cut-off value of 0.3 for PI/PImax identified in adult patients did not discriminate between extubation success and failure in children. Furthermore, a discriminatory cut-off value other than 0.3 could not be identified for infants and children. CONCLUSION: The PI/PImax ratio cannot be used to predict extubation outcome in pediatric patients. Indices that predict extubation outcome in adults should not be extrapolated to infants and children before testing and validation.

Mechanical ventilators optimized for pediatric use decrease work of breathing and oxygen consumption during pressure-support ventilation.

OBJECTIVES: a) To investigate whether the patient work of breathing needed to trigger inspiration is affected by the type of ventilator delivering pressure-support ventilation for mechanically ventilated pediatric patients. b) To determine whether changes in oxygen consumption (VO2) trend with changes in work of breathing and would thus be helpful in tracking work of breathing. DESIGN: Prospective study. SETTING: Pediatric intensive care unit at a university hospital. PATIENTS: Nine mechanically ventilated patients (2 to 75 months of age). INTERVENTIONS: While maintaining a constant pressure-support ventilation level, patients were alternately supported with the Siemens Servo 900C, the Bird VIP, and the Newport Wave E200 ventilators in random order. MEASUREMENTS AND MAIN RESULTS: Work of breathing, defined as the integral of the pressure-volume curve corresponding to negative pressure, was calculated with a pulmonary monitoring system. VO2 was measured with a metabolic cart. Patient distress levels were assessed using the COMFORT scale, a behavioral scoring system. Mean values (20 breaths/patient) for measured variables with each ventilator were compared using analysis of variance and Scheffé tests, with p < .05 indicating statistical significance. The lowest VO2 (103 +/- 35 mL/min/m2) and work of breathing (24 +/- 15 g.cm/m2) were achieved with the Bird VIP ventilator and were significantly (p < .05) lower than those values obtained with either the Siemens Servo 900C (VO2 147 +/- 33 mL/min/m2; work of breathing 49 +/- 18 g.cm/m2) or the Newport Wave E200 (VO2 122 +/- 33 mL/min/m2; work of breathing 35 +/- 15 g.cm/m2). Also, the values of work of breathing and VO2 obtained using the Newport Wave E200 were significantly (p < .05) lower than those values obtained using the Servo 900C. No change in behavioral distress occurred when the ventilators were changed. In all patients, there was a clear similarity in the trends of VO2 and work of breathing. CONCLUSIONS: We conclude that VO2 and work of breathing may be reduced significantly using the latest generation of mechanical ventilators optimized for infant and pediatric use. Because work of breathing is less with the Bird VIP than the other two ventilators tested, leading to a corresponding decrease in VO2, we suggest that the Bird VIP better adapts the patient to the ventilator and may facilitate weaning from ventilatory support. We also suggest that changes in VO2 might be helpful in tracking changes in work of breathing.

PEEP does not improve pulmonary mechanics in infants with bronchiolitis.

Positive end-expiratory pressure (PEEP) may improve pulmonary mechanics, work of breathing, and gas exchange in some patients with respiratory failure. These beneficial effects do not occur consistently, however, and side effects, such as gas trapping due to expiratory flow limitation, may be exacerbated. We determined the effects of PEEP (0, 3, 6, and 9 cm H2O applied in random order) on the expiratory airway resistance and static compliance of nine infants mechanically ventilated for acute bronchiolitis. We also noted the presence of inadvertent PEEP (PEEPi) to determine its influence on the response to applied PEEP. Applied PEEP at any level failed to consistently improve passive expiratory airway resistance or increase compliance from baseline (PEEP = 0 cm H2O, resistance = 92 +/- 32 cm H2O/L/s; compliance = 0.71 +/- 0.19 ml/cm H2O/kg). Increases in end-expiratory lung volumes ranged from 18 to 40% of the tidal volume at maximal PEEP. Although all infants had PEEPi (5 +/- 2 cm H2O), PEEPi had no influence on the response of mechanics to applied PEEP other than that peak inspiratory pressures increased when PEEP > PEEPi. We conclude that the routine use of PEEP in infants with bronchiolitis does not consistently improve passive expiratory pulmonary mechanics and may increase the risk of barotrauma from gas trapping.