Mechanics of Breathing

Introduction

The human infant has at least two important advantages over the adult in the evaluation of respiratory mechanical function. First, the infant, like newborn mammals of many species, is often asleep and does not immediately react to gently performed maneuvers. Hence, a maneuver consisting of a very brief occlusion of the airways during spontaneous breathing is often used in the evaluation of the mechanical properties of the respiratory system in infants. Second, infants have a chest wall compliance ( C _w ) some five times higher than lung compliance ( C _L ). Therefore because the reciprocal of respiratory system compliance 1/ C _rs equals 1/ C _L + 1/ C _w , C _rs is about 83% of C _L . This means that C _rs essentially reflects C _L in infants, which is an important practical advantage because C _rs can be measured much more easily than C _L . This chapter reviews some basic concepts and focuses on simple techniques that permit measurements of respiratory mechanics in newborn infants.

General Concepts and Terminology

In the chain of events that contribute to the translation of the output of the respiratory rhythm generator into pulmonary ventilation, the mechanical properties of the respiratory system are a critical aspect ( Fig. 63.1 ). After activation of the inspiratory muscles, the magnitude of the force generated depends on the force-length and force-velocity characteristics of the muscles. The physical translation of force into inspiratory pressure depends on the configuration of the muscle and the mechanical properties of the structure to which the force is applied. Finally, of the total muscle pressure generated, a part overcomes the elastic properties of the respiratory system to change lung volume (V) and a part is dissipated to overcome the resistive characteristics of the respiratory system to generate flow ( $\dot{V}$ ). An additional pressure component for the acceleration of the gas depends on the inertia of the respiratory system; it is usually very small and can be disregarded. Hence, the total pressure P produced equals the sum of elastic ( P _el ) and resistive ( P _res ) components; the former is proportional to V and 1/ C, while the latter is proportional to $\dot{V}$ and R :

where C (compliance) and R (resistance) are proportionality factors determined by, respectively, the elastic and resistive characteristics of the system.

During spontaneous breathing, the total P for inflation is generated by the respiratory muscles. Hence, muscle pressure ( P _mus ) equals P , and inflow occurs whenever P _mus − P _el is greater than 0. During resting breathing, because the tidal volume is entirely above the resting volume of the respiratory system ( V _r ), the inspiratory muscles generate P _mus and expiration is passive. However, in some conditions, such as certain cases of hyperventilation or during breathing against a positive airway pressure, the expiratory muscles may become active and breathing occurs both above and below V _r . In these cases, inspiration originates from the recoil of the respiratory system after expiratory muscle relaxation plus the active contraction of the inspiratory muscles.

During external (mechanical) ventilation, P _mus = 0, and the driving pressure is generated by the ventilator, which opposes P _el and P _res . Whenever P _mus = 0, the system is in passive mode; this is the case during lung expansion by a ventilator or in the last portion of expiration during resting breathing. During this phase, the elastic pressure stored during inspiration ( P _el ) is the sole driving pressure generating expiratory flow—that is, $P_{\text{el}}=P_{\text{res}}=\dot{V}\cdot R_{\text{rs}}\cdot$ .

Whenever P _mus differs from 0, the system is in an active mode; of course, this is the case during spontaneous inspiration or when $\dot{V}=0$ during voluntary breath holding (in which case P _mus = P _el ).

Whenever $\dot{V}=0$ , Equation 63.1 simplifies to

This condition is defined as static, irrespective of whether P _el is offset by muscle activity (e.g., during breath holding) or by external means with P _mus = 0 (e.g., relaxation against an artificial occlusion of the airways). In contrast, whenever $\dot{V}$ differs from 0, the system is in a dynamic condition again, irrespective of whether the respiratory muscles are active (e.g., inflation during spontaneous inspiration) or not (e.g., inflation by a ventilator). In conclusion, the operating modes of the respiratory system can be summarized into four combinations, static or dynamic, depending on the absence or presence of $\dot{V}$ , each either active or passive, depending on the presence or absence of muscle activity ( Table 63.1 ).

A dynamic condition with changes in V so small that the component V ⋅1/ C in Equation 63.1 is negligible is approached whenever $\dot{V}$ is very high and V is very small, such as with thermal panting or high-frequency ventilation. During an inspiratory effort against closed airways, because the change in lung volume equals 0 (and therefore $\dot{V}=0$ ), no external work is performed and the energy of isometric contraction is dissipated entirely as heat.

Passive Mechanical Properties of the Respiratory System

As defined already, the respiratory system is in a passive mode when P _mus = 0. This is the case during paralysis, with the lungs ventilated by external means (e.g., during positive pressure ventilation) or without paralysis after a period of hyperventilation. Hyperventilation lowers the arterial partial pressure of carbon dioxide below the threshold for muscle activation, which is just a few millimeters of mercury below the resting arterial partial pressure of carbon dioxide. During spontaneous breathing, the passive mode occurs when the subject relaxes at lung volumes above the resting position of the respiratory system; in infants, this is the case after an occlusion of the airways at the end of inspiration. The sustained lung inflation activates the airway slowly adapting stretch receptors, which trigger the vagally mediated Hering-Breuer inflation reflex and cause relaxation of the inspiratory muscles. This brief period of artificially provoked muscle relaxation is important in the context of pulmonary function testing in human neonates and is sufficient to evaluate the passive mechanical properties of the respiratory system ( C _rs , R _rs ) during spontaneous breathing.