Static Behavior of the Respiratory System

Respiratory-Cardiovascular Interactions During Mechanical Ventilation: Physiology and Clinical Implications

Positive-pressure inspiration and positive end-expiratory pressure (PEEP) increase pleural, alveolar, lung transmural, and intra-abdominal pressure, which decrease right and left ventricular (RV; LV) preload and LV afterload and increase RV afterload. The magnitude and clinical significance of the resulting changes in ventricular function are determined by the delivered tidal volume, the total level of PEEP, the compliance of the lungs and chest wall, intravascular volume, baseline RV and LV function, and intra-abdominal pressure. In mechanically ventilated patients, the most important, adverse consequences of respiratory-cardiovascular interactions are a PEEP-induced reduction in cardiac output, systemic oxygen delivery, and blood pressure; RV dysfunction in patients with ARDS; and acute hemodynamic collapse in patients with pulmonary hypertension. On the other hand, the hemodynamic changes produced by respiratory-cardiovascular interactions can be beneficial when used to assess volume responsiveness in hypotensive patients and by reducing dyspnea and improving hypoxemia in patients with cardiogenic pulmonary edema. Thus, a thorough understanding of the physiological principles underlying respiratory-cardiovascular interactions is essential if critical care practitioners are to anticipate, recognize, manage, and utilize their hemodynamic effects. © 2022 American Physiological Society. Compr Physiol 12:1-24, 2022.

Respiratory and cardiopulmonary limitations to aerobic exercise capacity in adults born preterm

Adults born preterm, regardless of whether they develop bronchopulmonary dysplasia, have underdeveloped respiratory and cardiopulmonary systems. The resulting impaired respiratory and cardiopulmonary systems are inadequate for the challenges imposed by aerobic exercise, which is exacerbated by the presence of bronchopulmonary dysplasia. Thus the respiratory and cardiopulmonary systems of these preterm individuals may be the most influential contributors to the significantly lower aerobic exercise capacity compared with their term born counterparts. The precise underlying cause(s) of the lower aerobic exercise capacity in adults born preterm is not entirely known but could be a number of interrelated parameters including mechanical ventilatory constraints, impaired pulmonary gas exchange efficiency, and excessive cardiopulmonary pressures. Likewise, additional aspects, such as impaired cardiovascular function and altered muscle bioenergetics, may play additional roles in limiting aerobic exercise capacity. Whether or not all or some of these aspects are present in adults born preterm and precisely how they may contribute to the lower aerobic exercise capacity are only beginning to be systematically explored. The purpose of this mini-review is to outline what is currently known about the respiratory and cardiopulmonary limitations during exercise in this population and to identify key areas where additional knowledge will help to advance this area. Additionally, where possible, we highlight the similarities and differences between obstructive lung disease resulting from preterm birth and chronic obstructive pulmonary disease (COPD) as the physiology and pathophysiology of these two forms of obstructive lung disease may not be identical.

Regional physiology of ARDS

The acute respiratory distress (ARDS) lung is usually characterized by a high degree of inhomogeneity. Indeed, the same lung may show a wide spectrum of aeration alterations, ranging from completely gasless regions, up to hyperinflated areas. This inhomogeneity is normally caused by the presence of lung edema and/or anatomical variations, and is deeply influenced by the gravitational forces.

For any given airway pressure generated by the ventilator, the pressure acting directly on the lung (i.e., the transpulmonary pressure or lung stress) is determined by two main factors: 1) the ratio between lung elastance and the total elastance of the respiratory system (which has been shown to vary widely in ARDS patients, between 0.2 and 0.8); and 2) the lung size. In severe ARDS, the ventilatable parenchyma is strongly reduced in size (‘baby lung’); its resting volume could be as low as 300 mL, and the total inspiratory capacity could be reached with a tidal volume of 750–900 mL, thus generating lethal stress and strain in the lung. Although this is possible in theory, it does not explain the occurrence of ventilator-induced lung injury (VILI) in lungs ventilated with much lower tidal volumes. In fact, the ARDS lung contains areas acting as local stress multipliers and they could multiply the stress by a factor ~ 2, meaning that in those regions the transpulmonary pressure could be double that present in other parts of the same lung. These ‘stress raisers’ widely correspond to the inhomogenous areas of the ARDS lung and can be present in up to 40% of the lung.

Although most of the literature on VILI concentrates on the possible dangers of tidal volume, mechanical ventilation in fact delivers mechanical power (i.e., energy per unit of time) to the lung parenchyma, which reacts to it according to its anatomical structure and pathophysiological status. The determinants of mechanical power are not only the tidal volume, but also respiratory rate, inspiratory flow, and positive end-expiratory pressure (PEEP). In the end, decreasing mechanical power, increasing lung homogeneity, and avoiding reaching the anatomical limits of the ‘baby lung’ should be the goals for safe ventilation in ARDS.

The future of mechanical ventilation: lessons from the present and the past

The adverse effects of mechanical ventilation in acute respiratory distress syndrome (ARDS) arise from two main causes: unphysiological increases of transpulmonary pressure and unphysiological increases/decreases of pleural pressure during positive or negative pressure ventilation. The transpulmonary pressure-related side effects primarily account for ventilator-induced lung injury (VILI) while the pleural pressure-related side effects primarily account for hemodynamic alterations. The changes of transpulmonary pressure and pleural pressure resulting from a given applied driving pressure depend on the relative elastances of the lung and chest wall. The term ‘volutrauma’ should refer to excessive strain, while ‘barotrauma’ should refer to excessive stress. Strains exceeding 1.5, corresponding to a stress above ~20 cmH₂O in humans, are severely damaging in experimental animals. Apart from high tidal volumes and high transpulmonary pressures, the respiratory rate and inspiratory flow may also play roles in the genesis of VILI. We do not know which fraction of mortality is attributable to VILI with ventilation comparable to that reported in recent clinical practice surveys (tidal volume ~7.5 ml/kg, positive end-expiratory pressure (PEEP) ~8 cmH₂O, rate ~20 bpm, associated mortality ~35%). Therefore, a more complete and individually personalized understanding of ARDS lung mechanics and its interaction with the ventilator is needed to improve future care. Knowledge of functional lung size would allow the quantitative estimation of strain. The determination of lung inhomogeneity/stress raisers would help assess local stresses; the measurement of lung recruitability would guide PEEP selection to optimize lung size and homogeneity. Finding a safety threshold for mechanical power, normalized to functional lung volume and tissue heterogeneity, may help precisely define the safety limits of ventilating the individual in question. When a mechanical ventilation set cannot be found to avoid an excessive risk of VILI, alternative methods (such as the artificial lung) should be considered.

Toward the modeling of mucus draining from human lung: role of airways deformation on air-mucus interaction

Chest physiotherapy is an empirical technique used to help secretions to get out of the lung whenever stagnation occurs. Although commonly used, little is known about the inner mechanisms of chest physiotherapy and controversies about its use are coming out regularly. Thus, a scientific validation of chest physiotherapy is needed to evaluate its effects on secretions. We setup a quasi-static numerical model of chest physiotherapy based on thorax and lung physiology and on their respective biophysics. We modeled the lung with an idealized deformable symmetric bifurcating tree. Bronchi and their inner fluids mechanics are assumed axisymmetric. Static data from the literature is used to build a model for the lung's mechanics. Secretions motion is the consequence of the shear constraints apply by the air flow. The input of the model is the pressure on the chest wall at each time, and the output is the bronchi geometry and air and secretions properties. In the limit of our model, we mimicked manual and mechanical chest physiotherapy techniques. We show that for secretions to move, air flow has to be high enough to overcome secretion resistance to motion. Moreover, the higher the pressure or the quicker it is applied, the higher is the air flow and thus the mobilization of secretions. However, pressures too high are efficient up to a point where airways compressions prevents air flow to increase any further. Generally, the first effects of manipulations is a decrease of the airway tree hydrodynamic resistance, thus improving ventilation even if secretions do not get out of the lungs. Also, some secretions might be pushed deeper into the lungs; this effect is stronger for high pressures and for mechanical chest physiotherapy. Finally, we propose and tested two a dimensional numbers that depend on lung properties and that allow to measure the efficiency and comfort of a manipulation.

The application of esophageal pressure measurement in patients with respiratory failure

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

Respiratory system dynamical mechanical properties: modeling in time and frequency domain

The mechanical properties of the respiratory system are important determinants of its function and can be severely compromised in disease. The assessment of respiratory system mechanical properties is thus essential in the management of some disorders as well as in the evaluation of respiratory system adaptations in response to an acute or chronic process. Most often, lungs and chest wall are treated as a linear dynamic system that can be expressed with differential equations, allowing determination of the system's parameters, which will reflect the mechanical properties. However, different models that encompass nonlinear characteristics and also multicompartments have been used in several approaches and most specifically in mechanically ventilated patients with acute lung injury. Additionally, the input impedance over a range of frequencies can be assessed with a convenient excitation method allowing the identification of the mechanical characteristics of the central and peripheral airways as well as lung periphery impedance. With the evolution of computational power, the airway pressure and flow can be recorded and stored for hours, and hence continuous monitoring of the respiratory system mechanical properties is already available in some mechanical ventilators. This review aims to describe some of the most frequently used models for the assessment of the respiratory system mechanical properties in both time and frequency domain.

Blunted respiratory-related evoked potential in awake obstructive sleep apnoea subjects: a NEP technique study

OBJECTIVE

Respiratory-related evoked potentials (RREP) elicited by transmural pressure in obstructive sleep apnoea (OSA) subjects have reported conflicting data. Different features of pressure stimuli and/or in the timing of stimuli application seem to account for these contradictory results. The negative expiratory pressure (NEP) technique, highly reproducible in terms of rise time and pressure values, allows to minimize the methodological confounding factors. We determined whether the afferent activity from the upper airway (UA) is altered in OSA subjects.

METHODS

RREP potentials were examined in 10 OSA and in 12 non-apnoeic awake subjects by means of the NEP technique.

RESULTS

All controls showed a cortical response to all pressure stimuli. All OSA subjects showed responses to -5 and -10 cmH(2)O whereas six of them showed no responses to -1 cmH(2)O. The amplitude of the P22, N45 and P85 components of the RREP was significantly reduced in OSA with respect to the controls in response to both the -5 and -10 cmH(2)O stimuli. We found no significant differences in latencies.

CONCLUSIONS

Awake OSA subjects had a raised threshold to pressure stimuli and blunted respiratory-related evoked potentials.

SIGNIFICANCE

These data indicate a deficit in afferent activity in the UA.

Postnatal Changes in Pulmonary Mechanics and Energetics of Infants with Respiratory Distress Syndrome following Surfactant Treatment

BACKGROUND

Postnatal alterations in pulmonary mechanics, energetics and functional residual capacity (FRC) describe the structural maturation of the preterm respiratory system.

OBJECTIVE

To evaluate longitudinal changes in pulmonary function in infants with respiratory distress syndrome (RDS) treated with oxygen, positive pressure ventilation and synthetic surfactant (Exosurf).

METHODS

Serial pulmonary function tests were performed in surfactant-treated infants [mean +/- SD birth weight (BW) = 1,112 +/- 276 g, gestational age (GA) = 29 +/- 3 weeks] at postnatal ages: <3 days, 1, 2, 3, 4 and 6-8 weeks until term postmenstrual age (PMA). Tidal volume, pulmonary compliance (C(L)), pulmonary resistance (R(T)) and flow-resistive work were analyzed following simultaneous measurements of airflow and transpulmonary pressure signals. Serial FRC measurements were made in a randomly selected group.

RESULTS

Prior to 28 weeks' PMA, C(L) was unchanged irrespective of GA. At age 1 week the likelihood ratio (LR) for bronchopulmonary dysplasia (BPD) based on C(L), R(T) and GA was predicted to be >90% for those with BW <750 g (LR >100) as compared to <10% probability (LR = 0.3) for infants >1,500 g. Significant linear increase in C(L) to PMA was evident >28 weeks' PMA (r = 0.86, p < 0.01) at 0.17 ml/cm H2O/kg/week. By term PMA, mean C(L) was 2.60 +/- 0.07 ml/cm H2O. Improvements in FRC of preterm infants with RDS who recovered occur at a more rapid rate ( approximately 25 ml/kg) compared to those who developed BPD ( approximately 20 ml/kg).

CONCLUSIONS

Slow but incremental postnatal pulmonary improvement, minimal <28 weeks' PMA, were comparable for all infants. Along with diminished FRC, these changes reflect persistent deleterious effects of positive pressure ventilation, alveolar hyperoxia and unrecognized pulmonary overdistension.

Effect of posture change on control of ventilation

To clarify the control mechanism of ventilation during posture change, ventilatory parameters, PETCO2, and ventilatory response to CO2 were examined in 11 healthy male subjects at supine (0 degrees) and 75 degrees head-up tilt positions. Minute expiratory ventilation (V.E), tidal volume (VT), respiratory frequency (f), end-tidal and transcutaneous PCO2 and CO2 output (V.CO2), and ventilatory response to CO2 were measured during a steady state condition. V.E (V.A) and VT increased significantly at 75 degrees tilt with significant decrease in PETCO2 from 40.1 mmHg (0 degrees) to about 36.1 mmHg (75 degrees). Transcutaneous PCO2 also decreased during tilt, by 3.3 mmHg. Physiological dead space (VD/VT) and V.CO2, however, remained unchanged, and ventilatory equivalent (V.E/V.CO2, V.A/V.CO2) increased significantly. The CO2-ventilatory response curve shifted upward (or leftward) without significant change in the response slope. At 75 degrees tilt, EMG activity of gastro-cnemius muscle increased. These findings suggested that PETCO2 decreased because of increased V.E (V.A) with a leftward shift of CO2-ventilatory response curve. Various signals such as afferents from lower extremities might have net stimulatory effects on a CO2-ventilation control system to reset the controlled level of PETCO2 to a lower range, but without significant change in CO2-ventilatory response during upright position.

Intractable cough associated with the supine body position. Effective therapy with nasal CPAP

We describe five patients with severe nocturnal cough and daytime somnolence in whom the coughing attacks are triggered by assuming the supine body position. Quantity and quality of the nocturnal cough were evaluated in the sleep laboratory with and without nasal continuous positive airway pressure (N-CPAP). Air flow characteristics were assessed using flow volume and airway resistance loops. Airway anatomy was evaluated bronchoscopically. In all five patients, the cough had a barking quality. Flow-volume loops showed an expiratory collapse phenomenon in two of the patients. Endoscopically, all five patients had signs of airway collapse. All patients had difficulty falling asleep because of coughing and were awakened by it frequently. Sleep times ranged from 2.5 to 4.5 h per night. With N-CPAP pressures ranging from 5 to 13 cm H2O, all five patients had clinically significant improvement in their symptoms. Their sleep times increased to a range of 5 to 7.5 h per night and the daytime somnolence markedly improved or resolved. All five patients requested a N-CPAP unit for home use. We conclude that a cough that is predominantly associated with or exacerbated by the supine body position may be treated effectively with N-CPAP.