The literature dealing with the magnitude, mechanism and effects of reduced FRC in the perioperative period is reviewed. During general anaesthesia FRC is reduced by approximately 20%. The reduction is greater in the obese and in patients with COPD. The most likely mechanism is the loss of inspiratory muscle tone of the muscles acting on the rib cage. Gas trapping is an additional mechanism. Lung compiance decreases and airways resistance increases, in large part, due to decreased FRC. The larynx is displaced anteriorly and elongated, making laryngoscopy and intubation more difficult. The change in FRC creates or increases intrapulmonary shunt and areas of low ventilation to perfusion. This is due to the occurrence of compression atelectasis, and to regional changes in mechanics and airway closure which tend to reduce ventilation to dependent lung zones which are still well perfused. Abdominal and thoracic operations tend to increase shunting further. Large tidal volume but not PEEP will improve oxygenation, although both increase FRC. Both FRC and vital capacity are reduced following abdominal and thoracic surgery in a predictable pattern. The mechanism is the combined effect ofincisional pain and reflex dysfunction of the diaphragm. Additional effects of thoracic surgery include pleural effusion, cooling of the phrenic nerve and mediastinal widening. Postoperative hypoxaemia is a function of reduced FRC and airway closure. There is no real difference among the various methods of active lung expansion in terms of the speed of restoration of lung function, or in preventing postoperative atelectasis/pneumonia. Epidural analgesia does not influence the rate of recovery of lung function, nor does it prevent atelectasis/pneumonia.

/pdf/perioperative-functional-residual-capacity-3v4ehcwttm.pdf

Perioperative functional residual capacity.

Thesemodificationsoftherespiratoryfunctionoccur early after surgery and are more often transient andcould lead to ARF. The clinical result (severity of theARF) is the product of perioperative-related ventilatoryimpairment and severity of the preoperative pulmonarycondition. Maintenance of adequate oxygenation in thepostoperative period is of major importance, especiallywhen pulmonary complications such as ARF occur. Al-though invasive endotracheal mechanical ventilation hasremained the cornerstone of ventilatory strategy for manyyearsforsevereARF,severalstudieshaveshownthatmor-tality associated with pulmonary disease is largely relatedto complications of postoperative reintubation and me-chanicalventilation.Therefore,majorobjectivesforanes-thesiologists are first to prevent the occurrence of postop-erative complications and second to ensure oxygenadministration and carbon dioxide removal while avoid-ing intubation if ARF occurs. Noninvasive ventilation(NIV) does not require an artificial airway (endotrachealtube or tracheotomy), and its use is well established toprevent ARF occurrence (prophylactic treatment) or totreat ARF to avoid reintubation (curative treatment) (fig.1). Studies show that patient-related risk factors, such aschronic obstructive pulmonary disease, age older than 60yr, American Society of Anesthesiologists class of II orhigher, obesity, functional dependence, and congestiveheart failure, increase the risk for postoperative pulmo-nary complications.

Postoperative noninvasive ventilation.

Respiratory muscles are the only skeletal muscles vital to life. Surgical procedures can affect the respiratory muscles by a number of pathophysiological mechanisms including thoracoabdominal mechanics, reflexes, neuromechanical coupling, and loss of muscular integrity. Impairment of respiratory muscle function after surgery may lead to postoperative complications such as hypoventilation, hypoxia, atelectasis, and infections, some of which may be life threatening. In this paper we review surgery that impairs the function of the respiratory muscles—namely cardiac, thoracic, and upper abdominal surgery. Therapeutic interventions or protective measures for respiratory muscles during or after each type of surgery are also briefly addressed. Lung transplantation and lung volume reduction surgery are discussed, since both have been shown to improve the function of respiratory muscles, and possible pathophysiological mechanisms are reviewed. The preoperative assessment of respiratory muscle function is also considered. Furthermore, when postoperative pulmonary complications occur it is worth considering whether the cause could be dysfunction of the respiratory muscles and thus a proper assessment of their function is needed.

The most important function of the respiratory muscles is breathing since they are the motor arm of the respiratory system. Breathing, a lifelong task, is borne mainly by the inspiratory muscles, especially the diaphragm. The other inspiratory muscles contribute little to quiet breathing but are recruited when higher levels of ventilation are required, such as in exercise and obstructive or restrictive lung disease. The expiratory muscles are not usually used during breathing except at high levels of ventilatory effort. A second function of the respiratory muscles is to perform explosive manoeuvres such as coughing and vomiting. The respiratory muscles also have a role as stabilisers of the thorax and abdomen since they take part in the formation of the thoracic and abdominal walls. Respiratory muscle function depends on the function of the respiratory centres, …

/pdf/surgery-and-the-respiratory-muscles-4rywbtt9p5.pdf

Surgery and the respiratory muscles

Continuous positive airway pressure effect on functional residual capacity, vital capacity and its subdivisions.

Postoperative pulmonary complications are responsible for significant increases in hospital cost as well as patient morbidity and mortality; respiratory muscle dysfunction represents a contributing factor. Upper airway dilator muscles functionally resist the upper airway collapsing forces created by the respiratory pump muscles. Standard perioperative medications (anesthetics, sedatives, opioids, and neuromuscular blocking agents), interventions (patient positioning, mechanical ventilation, and surgical trauma), and diseases (lung hyperinflation, obesity, and obstructive sleep apnea) have differential effects on the respiratory muscle subgroups. These effects on the upper airway dilators and respiratory pump muscles impair their coordination and function and can result in respiratory failure. Perioperative management strategies can help decrease the incidence of postoperative respiratory muscle dysfunction. Such strategies include minimally invasive procedures rather than open surgery, early and optimal mobilizing of respiratory muscles while on mechanical ventilation, judicious use of respiratory depressant anesthetics and neuromuscular blocking agents, and noninvasive ventilation when possible.

/pdf/postoperative-respiratory-muscle-dysfunction-pathophysiology-41okog2jhm.pdf

Postoperative respiratory muscle dysfunction: pathophysiology and preventive strategies.

Diaphragm dysf~ction induced by upper abdominal surgery. Role of postoperative pain. G. Simonneau, A. Vivien, R. Sartene, F. Kunstlinger, K. Samii, Y. Noviant and P. Duroux (Service de Pneumologie, Hopital Antoine Beclere, 157, rue de la Porte de Trivaux, 92141-Clamart, France), Amer. Rev. resp. Dis., 128 (1983) 899-903. The effects of upper abdominal surgery on diaphragmatic function were studied in 5 patients. During quiet tidal breathing, diaphragmatic function was assessed by measuring the volume displacement of the abdomen within the tidal volume (V,,/V,) and the ratio of abdominal and transdiaphragmatic pressure changes (AP,,/AP,,). This function was also studied during forced inspiratory efforts by measuring maximal transdiaphragmatic pressure (Pdi ,,,) and diaphragmatic displacement (DD). V,,/V, and AP,,/AP,i decreased significantly on the first postoperative day. Diaphragmatic dysfunction also occurred during maximal inspiratory efforts as shown by the significant fall in PdimaX and DD on the first and third postoperative days. On the first postoperative day, opiate epidural analgesia did not modify V,/Vr, APatJAP,i, and Pdimax. These parameters spontaneously returned towards control values on the seventh postoperative day. The authors conclude that upper abdominal surgery induces a marked diaphragmatic dysfunction lasting about 1 week and that it is not suppressed by postoperative pain relief. The mechanism of this dysfunction remains to be determined. It may be the main cause of the postoperative pulmonary restrictive pattern.

Surgery Diaphragm dysfunction induced by upper abdominal surgery. Role of postoperativepain: Amer. Rev. resp. Dis., 128 (1983) 899–903

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Surgery Diaphragm dysfunction induced by upper abdominal surgery. Role of postoperativepain: Amer. Rev. resp. Dis., 128 (1983) 899–903