Search

During conventional mechanical ventilation, fixed set pressure, flow, and tidal volume result in a mismatch between patient and ventilator inspiratory time and in a patient's inability to adapt to changing ventilatory demand. Synchrony between the patient and ventilator improves neuromuscular coupling and the ability to adapt to increased ventilatory demand or loading. The sensation of dyspnea prevents ineffective inspiratory efforts and attenuates periodic breathing during sleep.

Patient-ventilator synchrony is important in the management of the ventilator-dependent patient. Factors inherent to the patient and the ventilator influence patient-ventilator synchrony. Detection of patient-ventilator synchrony may require monitoring of airway pressure and flow waveforms.

The time course of ventilatory failure, pump failure, and diaphragm peripheral fatigue was determined during the application of external inspiratory resistive loads (IRL) in anesthetized rabbits. Pump failure is defined as the inability of the diaphragm to sustain the expected force under IRL. To assess contractile fatigue, transdiaphragmatic pressures (Pdi) generated by bilateral phrenic nerve stimulation at 75 Hz (Pdi-75) and 20 Hz (Pdi-20) were measured. The amplitude of evoked diaphragm electromyographic (EMG) signals was measured to assess neurotransmission failure. The rate of rise of spontaneous diaphragm EMG was used as an index of respiratory drive. Ventilation was evaluated together with arterial blood gases. During IRL the rate of rise of spontaneous diaphragm EMG increased, and there was a progressive hypercapnic acidosis and hypoxemia, indicating ventilatory failure. In contrast, Pdi-75 and Pdi-20 were stable until the time of respiratory arrest (apnea), when they decreased by 34 and 45%, respectively. The amplitude of evoked diaphragm EMG signals remained unchanged throughout the IRL and decreased only slightly at the time of apnea. We conclude that IRL induces progressive ventilatory failure long before any contractile fatigue of the diaphragm or pump failure occurs. This suggests that ventilatory failure is due to central fatigue, whereas pump failure (apnea) is attributable to multiple factors.

Lung elastance and resistance increase in the supine posture. To evaluate the effects of change in posture on regional lung mechanics at different lung volumes, lung elastance and resistance were measured at graded volume subdivisions and three esophageal levels at seated and supine body positions, using the esophageal balloon technique. Volumes were adjusted to be the same in both postures. In general, lung elastance (both static and dynamic) tended to be higher in supine posture and uniform at all lung volumes, except at 80% vital capacity, where it increased sharply. The ratio of dynamic to static lung elastance was slightly higher at the cephalad esophageal level, where regional flow rates and relative volume expansion are lower. Lung resistance varied inversely with lung volume but was higher at corresponding volume subdivisions in the supine posture. It decreased at more cephalad esophageal levels, where volume expansion and flow are less. Thus, the increase in regional flow at low volume subdivisions (most marked in the supine position) also contributed to higher lung resistance at these volumes. These findings are explained on the basis of a combination of Newtonian physics as well as nonlinear viscoelastic properties of the lung as applied to regional flow and volume expansion.

Airway occlusion pressure (P0.1) and the ratio of breathing frequency (f) to tidal volume (VT) (f/Vt) are good predictors of weaning outcome. However, the specificity of f/VT in predicting weaning success is relatively low. We postulated that the product of P0.1 and f/VT (P0.1*f/VT) would better predict weaning outcome than either variable alone. In 45 male patients, we prospectively evaluated P0.1*f/VT, P0.1, and f/VT in predicting weaning outcome. The threshold values of each variable were determined from published data. The sensitivity, specificity, and positive and negative predictive values in detecting weaning success, and the area under the receiver operating characteristic (ROC) curves were calculated. Ten (22%) of the 45 patients failed weaning. P0.1*f/VT yielded the highest specificity and positive and negative predictive values. P0.1*f/VT, P0.1, and f/VT were all highly sensitive (0.97); but they were less specific, 0.60 for P0.1*f/Vt and 0.40 for P0.1 and f/VT. The areas under the ROC curves for P0.1*f/VT, P0.1, and f/VT were not significantly different. We conclude that P0.1*f/VT has equivalent sensitivity as P0.1 and f/VT. P0.1 slightly improves the specificity of f/VT in predicting weaning success.

The objective of this study was to compare the effects of continuous positive airway pressure (CPAP), pressure support ventilation (PS), and T-piece on the pressure-time product (PTP) during weaning from mechanical ventilation. The PTP is an estimate of the metabolic work or oxygen consumption of the respiratory muscles. We studied 10 intubated patients recovering from acute respiratory failure of various etiologies. A modified continuous flow (flow-by) CPAP of 0 and 5 cm H2O (CPAP-0 and CPAP-5, respectively), PS of 5 cm H2O (PS-5), and T-piece were applied in random order for 30 min each. In the last 5 min of the 30-min periods, we measured the esophageal pressure and transdiaphragmatic pressure-time products--PTP(es) and PTP(di), cm H2O.s/min, respectively-multiplied by respiratory frequency. Breathing pattern, total lung resistance (RL), quasi-static lung compliance (CL), intrinsic positive end-expiratory pressure (PEEPi), end-expiratory transpulmonary pressure (Ptpexp), arterial blood gases, blood pressure, and heart rate were also measured. In comparison to T-piece, CPAP-5 decreased PTP(es) 40% (p less than 0.01) and PTP(di) 43% (p less than 0.02), whereas PS-5 decreased PTP(es) 34% (p less than 0.01) and PTP(di) 38% (p less than 0.05). The decrease in PTP(es) with CPAP-5 was associated with a significant reduction in RL, and to a less extent in PEEPi relative to airway pressure. The contribution of the decrease in PEEPi to the reduction in PTP(es) amounted to 36%. With PS-5, respiratory system mechanics and PEEPi were not significantly different compared with T-piece. With CPAP-0, PTP tended to be lower than with T-piece.(ABSTRACT TRUNCATED AT 250 WORDS)

To our knowledge, the effects of corrective spinal surgery on total respiratory mechanics and its components in anesthetized patients with kyphoscoliosis have not been previously reported in detail. We studied 13 patients with kyposcoliosis; their mean (+/- SD) age was 24.7 +/- 2.1 years; eight underwent anterior and posterior spinal fusions (AF and PF, respectively) two weeks apart (group A), four underwent PF alone (group B), and one had a three-stage procedure. Mean total respiratory elastance (Ers), static and dynamic lung elastance (Est,L and Edyn,L, respectively), chest wall elastance (Ew), and lung resistance (RL) were derived according to previously described methodology. In group A, Ers and Ew increased by 39 percent and 58 percent, respectively, following AF and by 20 percent and 129 percent following PF, while Est,L and Edyn,L did not change or declined following PF. Lung resistance increased 19 percent and 41 percent by the end of AF and PF, respectively, in group A. In group B, Ew more than doubled, resulting in a 39 percent increase in Ers. Increases in Ers, Ew, and respiratory flow resistance observed at the time of spinal corrective surgery for kyphoscoliosis may result from rib cage trauma and changes in airway caliber related to microatelectasis and uneven distribution of mechanical properties within the lungs. Spinal correction results in immediate and short-term deterioration of respiratory mechanics measured under anesthesia.