Lung ventilation and the physiology of breathing

This article summarizes the anatomical features of the lungs, airway and thorax pertinent to the physiology of breathing and discusses chemoreceptor detection mechanisms, brainstem centres and relays involved in the control of breathing. We will discuss lung mechanics including spirometry, lung compliance, airway resistance and the role of surfactant and consider how these can be affected by disease states. It is recommended to revise principles of arterial blood gas analysis in addition to this article.     

Lung ventilation and the physiology of breathing

This article summarizes the anatomical features of the lungs, airway and thorax pertinent to the physiology of breathing and discusses chemoreceptor detection mechanisms, brainstem centres and relays involved in the control of breathing. We will discuss lung mechanics including spirometry, lung compliance, airway resistance and the role of surfactant and consider how these can be affected by disease states. It is recommended to revise principles of arterial blood gas analysis in addition to this article.     

Lung ventilation and the physiology of breathing

This article summarizes the anatomical features of the lungs, airway and thorax pertinent to the physiology of breathing and discusses chemoreceptor detection mechanisms, brainstem centres and relays involved in the control of breathing. We will discuss lung mechanics including spirometry, lung compliance, airway resistance and the role of surfactant and consider how these can be affected by disease states. It is recommended to revise principles of arterial blood gas analysis in addition to this article.     

Lung ventilation and the physiology of breathing

This article summarizes the anatomical features of the lungs, airway and thorax pertinent to the physiology of breathing and discusses chemoreceptor detection mechanisms, brainstem centres and relays involved in the control of breathing. We will discuss lung mechanics including spirometry, lung compliance, airway resistance and the role of surfactant and consider how these can be affected by disease states. It is recommended to revise principles of arterial blood gas analysis in addition to this article.     

Measurement of respiratory function: ventilation

Measurement of ventilatory function is often impeded by poor technique when individuals perform tests of respiratory function. Static lung volumes (with the exception of residual volume and other capacities that contain residual volume) can be easily measured with spirometry. Residual volume and functional residual capacity can be measured using helium dilution or body plethysmography, although neither of these techniques are used in daily clinical practice. Dead space (the volume of gas not participating in gas exchange) can be measured using a single-breath nitrogen washout technique, or by application of the Bohr equation. Dynamic volumes measure airflow through the lungs, and often require individuals to perform a forced vital capacity (FVC) manoeuvre. Data generated from such tests include peak expiratory flow rate, forced expiratory capacity in 1 second (FEV₁) and the ratio FEV₁:FVC, which may be used in the diagnosis, assessment and management of respiratory disease. Flow–volume loops provide more detailed analysis of dynamic airflow in the lungs and are often displayed on anaesthetic machines. Measurement of the maximum voluntary ventilation (MVV) provides a global assessment of respiratory system function because it is influenced by airway resistance, respiratory muscle function, ventilation control mechanisms and compliance of lungs or chest wall. An approximation of MVV can be made by multiplying the FEV₁ by 35.     

Continuous non-invasive monitoring of energy expenditure, oxygen consumption and alveolar ventilation during controlled ventilation: validation in an oxygen consuming lung model

Background: We have developed a combined indirect calorimetric and breath-by-breath capnographic device (GEM) for respiratory monitoring: oxygen consumption (V?O₂), carbon dioxide excretion (V?CO₂), respiratory quotient (RQ), energy expenditure (EE), alveolar ventilation (V?_A) and dead space/total ventilation (V_D/V_T). Methods: The device was tested in a lung model in which V?O₂ was achieved by combustion of hydrogen. V?CO₂ was achieved by delivering CO₂ into the single alveolus combustion chamber. V?O₂, V?CO₂, compliance, and anatomical dead space could be varied independently. Results: Measured V?O₂ was 101±3% (SD) of set value at a F₁O₂<0.6 and 101±7% at a F₁O₂>0.6 during 15 hours of testing. The corresponding V?CO₂ values were 99±2% and 102±7%. The GEM could with good accuracy measure accumulated energy expenditure (EE) during simulated unstable patient conditions up to a F₁O₂ of 0.8. At F₁O₂ above 0.8 V?CO₂ and V?O₂ could be estimated using a default RQ value of 0.85. On-line estimated V?_A and V_D/V_T values could be obtained at any F₁O₂ up to 1.0. In a test sequence with stable V?O₂ and V?CO₂ the GEM adequately followed changes in V?_A, induced by changes in anatomical dead space, breathing frequency and compliance. Conclusion: The overall performance of the device is satisfactory and well comparable with any equipment tested. It allows near-continuous non-invasive monitoring of EE, V?O₂, V?CO₂, V?_A, V_D/V_T in ventilated, critically ill patients, providing a rationale for ventilator settings and nutritional support.     

Isocapnic high frequency jet ventilation: dead space depends on frequency, inspiratory time and entrainment

Twelve healthy pigs were ventilated with high frequency jet ventilation via a Mallinckrodt HiLo jet tube. The expired gas was led to a conventional ventilator and CO2 analyzer which were used to measure CO2 elimination. There was no bias flow, so that the jet entrained only expired gas, i.e. rebreathing occurred. Frequency was varied between 2 and 11 Hz and the duration of inspiration, as a fraction of the ventilatory cycle (Ti/Ttot), from 5 to 20%. The minute ventilation, Vjet, delivered by the jet ventilator was adjusted to maintain a constant PaCO2. At 2 Hz and a Ti/Ttot of 5%, Vjet was of the same magnitude as ventilation during conventional intermittent positive pressure ventilation, and the total dead space fraction, VD/VT was 0.32. Both increasing frequency at a constant Ti/Ttot, and increasing Ti/Ttot at a constant frequency, increased VD/VT which was maximal (0.8) at 11 Hz and a Ti/Ttot of 20%. When entrainment was blocked, tidal jet volume had to be greatly increased. The continuous measurement of CO2 elimination was found to be useful for maintaining isocapnia when the jet ventilator setting was changed.     

全麻下单肺通气对肺功能的影响

目的　探讨单肺通气状况的最佳呼吸方式。方法　 60例 ASA ～ 级开胸病人单肺麻醉时 ,在分钟通气量设定的条件下 ,随着吸气时间相对延长 (通过降低呼吸频率和吸∶呼比值 )的不同通气条件变化 ,对单肺功能和血气分析的影响进行了临床研究。结果　单肺通气时 ,在呼吸频率 =12次 /分 ,吸∶呼比值 =1∶ 1时 ,通气侧肺顺应性、实际分钟通气量和脉搏血氧饱和度明显改善 ,血气结果最佳 ,与双肺通气相比较无显著性差异 ( P>0 .0 5 ) ;气道压力显著降低 ,与单肺通气其它通气条件相比较有显著性降低 ( P<0 .0 5 )。但过分降低呼吸频率 ( RR<10次 /分 )达到的效果会适得其反。结论　在单肺通气使用麻醉呼吸器时 ,相对降低呼吸频率 ( RR=12次 /分 )、相对延长吸气时间 ( I∶ E=1∶ 1.5 ) ,可达到提高肺顺应性和通气量 ,降低气道压力 ,提高血氧和降低二氧化碳分压的良好效果     

高呼气末正压小潮气量通气对肺功能正常患者血流动力学的影响

目的 观察肺功能正常的患者术后使用不同水平呼气末正压(PEEP)小潮气量通气对血流动力学的影响.方法 102例ASA Ⅰ或Ⅱ级,择期全麻下耳鼻喉科术后患者,随机均分为六组.研究组使用保护性肺通气模式,潮气量5 ml/kg,根据PEEP 0、5、10、15和20 emH2O分为P0、P5、P10、P15和P20.五组.对照组(C组)使用常规机械通气模式,潮气量10 ml/kg.观察保护性肺通气前(T1)及保护性肺通气30 min后(T2)的血流动力学的改变.结果 与C组及T1时比较,T2时加速度指数(ACI)在P0、P5、P10和P15组升高,P20组降低;心脏指数(CI)、左室做功指数(LCWI)、平均动脉压(MAP)在P20组降低(P＜0.05);胸腔液体水平(TFC)P10P15和P20组T2时明显低于T1时(P＜0.05).结论 对肺功能正常患者实施小潮气量的保护性肺通气,PEEP在0和5 cmH2O2水平,对血流动力学无明显影响;当PEEP在10和15 cmH2O冰平时ACI增强和TFC减少,有潜在血流动力学危害;当PEEP在20 cmH2O时CI和MAP均降低.有明显血流动力学波动.     

Elimination of apparatus dead space — a simple method for improving CO2 removal without increasing airway pressure

During mechanical ventilation the apparatus dead space can be eliminated by insufflating through one lumen of a tracheal double-lumen tube and allowing expiration through the other. In six intravenously anesthetized pigs, this technique resulted in an 18% (1 kPa) decrease in PaCO2 compared with insufflating through both lumens (32 ml rebreathing volume). Oxygenation, airway pressures, and tidal volumes were unchanged. Flushing the trachea with fresh gas during the expiratory phase did not improve the efficiency of ventilation. It is concluded that elimination of apparatus dead space improves CO2 removal without increasing airway pressures and tidal volumes, and it is suggested that minimization of apparatus dead space should be tried before more advanced ventilatory modes are considered.     

Liquid ventilation: an adjunct for respiratory management

Although significant advances in respiratory care have reduced mortality of patients with respiratory failure, morbidity persists, often resulting from iatrogenic mechanisms. Mechanical ventilation with gas has been shown to initiate as well as exacerbate underlying lung injury, resulting in progressive structural damage and release of inflammatory mediators within the lung. Alternative means to support pulmonary gas exchange while preserving lung structure and function are therefore required. Perfluorochemical (PFC) liquids are currently used clinically in a number of ways, such as intravascular PFC emulsions for volume expansion/oxygen carrying/angiography and intracavitary neat PFC liquid for image contrast enhancement or vitreous fluid replacement. As a novel approach to replace gas as the respiratory medium, liquid assisted ventilation (LAV) with PFC liquids has been investigated as an alternative respiratory modality for over 30 years. Currently, there are several theoretical and practical applications of LAV in the immature or mature lung at risk for acute respiratory distress and injury associated with mechanical ventilation.     

Measurement of respiratory function: gas exchange and its clinical applications

Gas exchange is the main function of the lungs. Lungs have a large reserve for gas exchange. Oxygen and carbon dioxide diffuse along their partial pressure gradient across the alveolar–capillary membrane. Alveolar ventilation and pulmonary circulation are closely matched to provide efficient gas exchange in the lungs. Hypoxaemia often results from mismatch in ventilation–perfusion. Gas exchange can be impaired in various disease states. Measurement of the diffusing capacity for carbon monoxide (DLCO) provides estimation of the gas exchange function. A low DLCO indicates an impairment of oxygen transfer across the alveolar–capillary membrane. Based on the lung function tests one can assess the risks of perioperative pulmonary complications. Anaesthesia and surgery adversely affect pulmonary function, many of which adverse effects can be prevented.     

Measurement of respiratory function: gas exchange and its clinical applications

Gas exchange is the main function of the lungs. Lungs have a large reserve for gas exchange. Oxygen and carbon dioxide diffuse along their partial pressure gradient across the alveolar–capillary membrane. Alveolar ventilation and pulmonary circulation are closely matched to provide efficient gas exchange in the lungs. Hypoxaemia often results from mismatch in ventilation–perfusion. Gas exchange can be impaired in various disease states. Measurement of the diffusing capacity for carbon monoxide (DLCO) provides estimation of the gas exchange function. A low DLCO indicates an impairment of oxygen transfer across the alveolar–capillary membrane. Based on the lung function tests one can assess the risks of perioperative pulmonary complications. Anaesthesia and surgery adversely affect pulmonary function, many of which adverse effects can be prevented.     

Intermittent high frequency ventilation. Clinical evaluation of a new mode of ventilation

In this first clinical study of a new mode of ventilation, called intermittent high frequency ventilation (IHFV), a comparison was made in ten patients after myocardial revascularization, between normal ventilation, high frequency ventilation (HFV) and IHFV at 15, 60, 100 and 160 breaths per min (b.p.m.). During IHFV the ventilation was interrupted for 3 s 7 times per min. The measured mean trapped gas volumes were 219 ml at 60 b.p.m. and 716 ml at 160 b.p.m. At 160 b.p.m. during IHFV, the mean, mean airway pressures, pulmonary artery (PAP) and capillary wedge pressures and the Paco2 were decreased, while the cardiac output increased. When the Paco2 was maintained constant, IHFV also allowed a reduction in the mean peak and mean airway pressures. This new mode of ventilation can prevent sustained increases in the lung volume and provide a reduction in intrapulmonary pressures. It also permits the measurement of the trapped gas volume and end-tidal CO2 gas concentrations several times a minute for the clinical management of patients during high frequency ventilation.     

Pigs are not a reliable experimental model in the study of the haemodynamic and respiratory effects of CO2 pneumoperitoneum

BACKGROUND: Haemodynamic and respiratory effects of a CO2 pneumoperitoneum (intra-abdominal pressure = 12 mmHg) associated to a head-up position(15 degrees ) were studied in 20 pigs using a Swan-Ganz catheter and the Single Breath Test for CO2. The pneumoperitoneum induced a moderate rise in mean arterial pressure (+17%) (P<0.001) without any variation in heart rate, cardiac output and systemic vascular resistances. RESULTS: The following respiratory effects were observed: an increase in PaCO2 (+20%) (P<0.001), PE'CO2 (+31%) (P<0.001), expired volume of CO2 (+28%) (P<0.001), arterial to end-tidal CO2 gradient (+80%) (P<0.001) and alveolar dead space (+40%) (P<0.001) occured. Alveolar ventilation remained stable. Finally and contrary to healthy human patient, intraperitoneal CO2 insufflation in pig induced slight haemodynamic changes and major respiratory modifications. CONCLUSION: Thus, our data do not support the conclusion that the pig is a reliable experimental model for studying the pathophysiology of CO2 pneumoperitoneum-induced changes in haemodynamic and respiratory parameters, in human patients.     

Respiration: ventilation

Anaesthetists and intensivists directly manipulate pulmonary function, in particular ventilation. A sound and thorough working knowledge of applied pulmonary physiology of ventilation is essential to the safe conduct of anaesthesia and intensive care medicine. This article discusses pulmonary anatomy, gas exchange in the lung, the mechanics of ventilation, airway resistance, elastance and compliance, the work of breathing and ventilation/perfusion relationships including hypoxic pulmonary vasoconstriction. General anaesthesia has profound effects on the respiratory system including the ventilatory response to hypercapnia and hypoxia, upper airway muscle function, lung volumes and ventilation/perfusion matching. Many surgical procedures are facilitated by one-lung ventilation. When utilizing one-lung ventilation a key aim for the anaesthetist is to maintain adequate alveolar ventilation while minimizing the amount of shunt through the non-ventilated lung. A detailed understanding of one-lung ventilation is therefore vital if a logical approach to management is to be adopted.     

An analysis of flow volume curves during artificial ventilation

Expiratory flow-volume curves during artificial ventilation (FV-av) were analyzed in 48 patients undergoing general anesthesia. They were divided into 4 groups according to preoperative respiratory disorders; obstructive type (group 1), restrictive type (group 2), small airway disease (group 3) and normal control (group 4). Expiratory flow rates and volumes during artificial ventilation were plotted on an X-Y recorder to calculate V¨₅₀/V¨₂₅, mean time constant ratio (MTCR), obstructive index (OI) and slope ratio (SR). FV-av values were compared among groups. FV-av values in groups 2 were significantly higher than those in group 4. The values in group 1 and those in group 3 were not significantly different from those in group 4. FV-av values may reflect restrictive respiratory dysfunctions but they are not sensitive enough to detect obstructive lung disease.(Ochi G, Kojo H, Yorozuya T, et al.: An analysis of flow volume curves during artificial ventilation. J Anesth 7: 120–123, 1993)     

低潮气量治疗在机械通气肺结核术后合并呼吸衰竭患者中的应用

目的 探讨低潮气量维持通气对肺结核术后合并呼吸衰竭患者的应用价值。方法 在有效的抗结核、抗感染治疗的基础上，对32例肺结核术后合并呼吸衰竭患者进行低潮气量（6～8ml／kg）机械通气治疗，观察疗效及并发症。结果 呼吸衰竭治愈31例，死亡1例，治愈率96．9％，无明显并发症。结论 低潮气量维持通气对肺结核术后合并呼吸衰竭患者的治疗是安全的，且疗效显著。