Respiratory elastic load compensation in anesthetized patients with kyphoscoliosis

Methods and Applications in Respiratory Physiology: Respiratory Mechanics, Drive and Muscle Function in Neuromuscular and Chest Wall Disorders

Individuals with neuromuscular and chest wall disorders experience respiratory muscle weakness, reduced lung volume and increases in respiratory elastance and resistance which lead to increase in work of breathing, impaired gas exchange and respiratory pump failure. Recently developed methods to assess respiratory muscle weakness, mechanics and movement supplement traditionally employed spirometry and methods to evaluate gas exchange. These include recording postural change in vital capacity, respiratory pressures (mouth and sniff), electromyography and ultrasound evaluation of diaphragmatic thickness and excursions. In this review, we highlight key aspects of the pathophysiology of these conditions as they impact the patient and describe measures to evaluate respiratory dysfunction. We discuss potential areas of physiologic investigation in the evaluation of respiratory aspects of these disorders.

Hysteresivity of the lung and tissue strip in the normal rat: effects of heterogeneities

We measured lung impedance in rats in closed chest (CC), open chest (OC), and isolated lungs (IL) at four transpulmonary pressures with a optimal ventilator waveform. Data were analyzed with an homogeneous linear or an inhomogeneous linear model. Both models include tissue damping and elastance and airway inertance. The homogeneous linear model includes airway resistance (Raw), whereas the inhomogeneous linear model has a continuous distribution of Raw characterized by the mean Raw and the standard deviation of Raw (SDR). Lung mechanics were compared with tissue strip mechanics at frequencies and operating stresses comparable to those during lung impedance measurements. The hysteresivity (eta) was calculated as tissue damping/elastance. We found that 1) airway and tissue parameters were different in the IL than in the CC and OC conditions; 2) SDR was lowest in the IL; and 3) eta in IL at low transpulmonary pressure was similar to eta in the tissue strip. We conclude that eta is primarily determined by lung connective tissue, and its elevated estimates from impedance data in the CC and OC conditions are a consequence of compartment-like heterogeneity being greater in CC and OC conditions than in the IL.

Immediate response to inspiratory resistive loading in anesthetized patients with kyphoscoliosis: spirometric and neural effects

In kyphoscoliosis (KS), lung volumes are reduced, respiratory elastance and resistance are increased, and breathing pattern is rapid and shallow, attributes that may contribute to defense of tidal volume (VT) in the face of inspiratory resistive loading. The control of ventilation of 12 anesthetized patients about to undergo corrective spinal surgery was compared to that of 11 anesthetized patients free of cardiothoracic disease during quiet breathing and the first breath through one of three linear resistors. Mean forced vital capacity (FVC) of the KS group was 48% that of the controls (C). Passive elastance (Ers) and active elastance and resistance (E'rs and R'rs, respectively) were computed according to previously described techniques (Behrakis PK, Higgs BD, Baydur A, Zin WA, Milic-Emili J (1983) Active inspiratory impedance in halothane-anesthetized humans. J Appl Physiol 54: 1477-1481). Baseline tidal volume VT, inspiratory duration Tl, expiratory duration TE, duration of total breathing cycle TT, and inspiratory duty cycle TI/TT were significantly reduced, while VE was slightly decreased in the KS. Ers, E'rs, and R'rs, were, respectively, 72, 69, and 89% greater in the KS. Driving pressure (Pmus) was derived from the equation of motion, using active values of respiratory elastance. With resistive loading, there was greater prolongation of TI in the C, while percent reduction in VT and minute ventilation VE was less in KS. Compensation in both groups was achieved through three changes in the Pmus waveform. (1) Peak amplitude increased. (2) The duration of the rising phase increased. (3) The rising Pmus curve became more concave to the time axis. These changes were most marked with application of the highest resistance in both groups. Peak driving pressure and mean rate of rise of Pmus were greater in the KS. Increased intrinsic impedance, Pmus, and differences in changes in neural timing in anesthetized kyphoscoliotics contribute to modestly greater VT defense, compared to that of anesthetized subjects free of cardiorespiratory disease.

Total respiratory system, lung, and chest wall mechanics in sedated-paralyzed postoperative morbidly obese patients

OBJECTIVE

To study the relative contribution of the lung and the chest wall on the total respiratory system mechanics, gas exchange, and work of breathing in sedated-paralyzed normal subjects and morbidly obese patients, in the postoperative period.

SETTING

Policlinico Hospital, University of Milan, Italy.

METHODS

In ten normal subjects (normal) and ten morbidly obese patients (obese), we partitioned the total respiratory mechanics (rs) into its lung (L) and chest wall (w) components using the esophageal balloon technique together with airway occlusion technique, during constant flow inflation. We measured, after abdominal surgery, static respiratory system compliance (Cst,rs), lung compliance (Cst,L), chest wall compliance (Cst,w), total lung (Rmax,L) and chest wall (Rmax,w) resistance. Rmax,L includes airway (Rmin,L) and "additional" lung resistance (DR,L). DR,L represents the component due to viscoelastic phenomena of the lung tissue and time constant inequalities (pendelluft). Functional residual capacity (FRC) was measured by helium dilution technique.

RESULTS

We found that morbidly obese patients compared with normal subjects are characterized by the following: (1) reduced Cst,rs (p < 0.01), due to lower Cst,L (55.3 +/- 15.3 mL x cm H2O-1 vs 106.6 +/- 31.7 mL x cm H2O-1; p < 0.01) and Cst,w (112.4 +/- 47.4 mL x cm H2O-1 vs 190.7 +/- 45.1 mL x cm H2O-1; p < 0.01); (2) increased Rmin,L (4.7 +/- 3.1 mL x cm H2O x L-1 x s; vs 1.0 +/- 0.8 mL x cm H2O x L-1 x s; p < 0.01) and DR,L (4.9 +/- 2.6 mL x cm H2O x L-1 x s; vs 1.5 +/- 0.8 mL x cm H2O x L-1 x s; p < 0.01); (3) reduced FRC (0.665 +/- 0.191 L vs 1.691 +/- 0.325 L; p < 0.01); (4) increased work performed to inflate both the lung (0.91 +/- 0.25 J/L vs 0.34 +/- 0.08 J/L; p < 0.01) and the chest wall (0.39 +/- 0.13 J/L vs 0.18 +/- 0.04 J/L; p < 0.01); and (5) a reduced pulmonary oxygenation index (PaO2/PAO2 ratio).

CONCLUSION

Sedated-paralyzed morbidly obese patients, compared with normal subjects, are characterized by marked derangements in lung and chest wall mechanics and reduced lung volume after abdominal surgery. These alterations may account for impaired arterial oxygenation in the postoperative period.