Where is ventilation best in the lungs

T e : expiratory time: T i ; inspiratory time: V T ; tidal volume. During natural inspiration, the contraction of respiratory muscles mainly the diaphragm increases the chest volume, generating a temporary negative alveolar pressure P alv. Air is sucked into the lungs, and is mixed with the gases present there. This inhaled gas volume is called the inspiratory tidal volume. During inspiration the elastic recoil force shown as the stretched spring is loaded.

During expiration, the respiratory muscles relax, and the elastic recoil force pulls the chest and lungs back to their resting position, generating a temporary positive P alv.

A certain amount of gas is pushed out of the lungs. This expelled gas volume is called expiratory tidal volume. Inspiratory and expiratory tidal volumes are roughly equal.

The tidal volume of every breath contains two parts. The part that participates in alveolar gas exchange is alveolar tidal volume. The other part that does not participate in the gas exchange is anatomical dead space. Dead space volume is always moved in or out first. Dead space is inevitable. Do not forget it when setting and interpreting tidal volume or minute volume.

During mechanical ventilation, the dead space usually increases due to the presence of the artificial airway. The effective alveolar ventilation is determined by the difference between the tidal volume and the total dead space.

If the tidal volume is very close to, or equal to, the dead space volume, the alveolar ventilation is nearly zero, i. CO 2 removal is nearly zero. This unwanted situation is known as dead space ventilation. Note that after every breath, only a part of the alveolar gas is replaced. In addition to defining ventilation in terms of a single breath, we can also define it over a minute interval Fig. When we talk about minute ventilation or minute volume, we need to define a few common respiratory terms:.

The relationship can be expressed with a simple equation:. Think about how much energy you need during sleep compared to during physical exercise. Biochemically these activities differ greatly in metabolic rate, O 2 consumption, and CO 2 production.

There is no such thing as a normal value for energy demand. On the other hand, it is physiologically important to maintain the arterial partial pressure of oxygen and carbon dioxide PaO 2 , PaCO 2 , and pH within relatively narrow normal ranges even when energy demand changes.

This is achieved through a control mechanism that automatically and precisely adapts the breathing pattern i. To a limited extent, we can freely change our breathing pattern. This control mechanism uses a three-part sequence:. Central and peripheral chemoreceptors detect the current O 2 , CO 2 , and pH in the blood and cerebrospinal fluid.

The controller respiratory centre at the medulla and pons receives signals from the receptors, decides how to respond, and then sends the instruction to the effectors. The effectors respiratory muscles execute the commands received. Of these, PaCO 2 is the primary stimulant. As Fig. In most ventilated patients, this respiratory control mechanism remains intact. The mechanism plays a key role in respiratory distress syndrome, patient-ventilator asynchrony, and weaning. It may be abnormal in some neurological and neurosurgical patients.

In summary, respiration is a mechanism to maintain PaO 2 and PaCO 2 within their normal ranges even when energy demand fluctuates. Respiratory failure can occur due to severe functional impairment of the airway, lungs, chest wall, respiratory centre, respiratory nerves, and respiratory muscles for a variety of clinical reasons. Table 3. At this point, it is necessary to introduce two key terms.

Respiratory failure can be classified roughly into two types, type 1 and type 2. Type 1 respiratory failure is also known as hypoxic respiratory failure or lung failure.

Type 2 respiratory failure is also known as hypercapnic respiratory failure or pump failure. Type 2 respiratory failure is typically caused by inadequate lung ventilation due to: a excessive p. The clinical signs of respiratory failure include tachypnoea, tachycardia, cyanosis, sweating, intercostal retractions, grunting, and nose flaring. Pulse oximetry and blood gas analysis can help diagnose respiratory failure. Note that these clinical signs are non-specific.

For simplicity, we can think of the pathophysiologic process of respiratory failure as having several steps Fig. The underlying diseases lead to deterioration in the efficiency and effectiveness of respiratory function. Increased breathing efforts further increase the energy demand. If the patient can maintain normal PaO 2 and PaCO 2 with these intensified breathing efforts, the compensation is successful. If not, however, respiratory failure is inevitable.

Depending on the underlying diseases, both types of respiratory failure can be acute, with symptoms occurring rapidly; as in near drowning, asthma attack, respiratory arrest, drug overdose, upper airway obstruction, or chest and lung injury. Respiratory failure can also be progressive chronic , as in emphysema, chronic bronchitis, or neuromuscular disease. For purposes of clinical treatment, it is important to differentiate between types 1 and 2, as shown in Table 3.

The treatment of respiratory failure typically involves: a oxygen therapy, b ventilatory support with a ventilator system or a continuous positive airway pressure CPAP system, c treatment of the underlying cause, and d other supporting measures, such as administration of fluids and nutrition.

Acute respiratory failure is usually treated in an intensive care unit, while chronic respiratory failure is usually treated at home or in a long-term care facility. Today, mechanical ventilation is the principal therapy used to treat severe respiratory failure caused by a serious disease or injury of any of the six key parts of respiratory system i.

If applied appropriately, this therapy effectively assists, supports, or replaces compromised natural lung ventilation, artificially satisfying the vital demands of respiration.

This gives the clinician valuable time to treat the underlying diseases and improve the general condition of the patient. The therapy should be terminated as soon as the patient can breathe adequately on their own. The only exception is the patient with permanently damaged pulmonary function, who may be ventilator dependent for their entire life.

Is there a limit to what mechanical ventilation can accomplish? This is a critical but seldom asked question. The answer is yes. In this case, ECMO extracorporeal membrane oxygenation should be used. As we noted in Chapter 1 , mechanical ventilation can be realized with one of three principles: intermittent positive pressure ventilation IPPV , intermittent negative pressure ventilation INPV , and high-frequency ventilation HFV.

Intermittent positive pressure ventilation IPPV. When the airway pressure is temporarily higher than the alveolar pressure, fresh gas is pushed into the lungs, the process of inspiration. When the airway pressure is lower than alveolar pressure, the gas is expelled out of the lungs, the process of expiration. IPPV is the common principle of most modern ventilators, whether invasive or non-invasive. During inspiration, a negative pressure is applied to the surface of the chest wall, temporarily reducing the alveolar pressure.

Fresh air is now sucked into the lungs. During expiration, the applied negative pressure is removed. The elastic recoil force temporarily generates a positive alveolar pressure, squeezing the stale gas out of the lungs.

The driving pressure changes opposite to the way it does in IPPV. A negative pressure is generated intermittently inside the container, resulting in inspiration and expiration. Different variations of the iron lung were developed during the s and s. They were used widely during the polio outbreaks in the s and s. The second type is the cuirass ventilator, in which a rigid shell or cuirass fits over the thoracic area only.

An intermittent negative pressure is applied locally to change the thoracic volume. Today, INPV ventilators are used primarily for non-invasive ventilatory assistance. The tidal volume is much smaller than the physiological range, often smaller than dead space. Clinically HFV is often applied in neonates.

Sometimes it is also used to treat patients with ARDS, especially those who require very high positive airway pressures. Of all the three principles, IPPV currently dominates. To recap, the objective of mechanical ventilation is to restore or maintain adequate lung ventilation by using artificial means to intermittently change lung volume.

Mechanical ventilation requires a ventilator system, which will be described in depth in Chapters 4 and 5. The pressure on the device side is called the airway opening pressure P ao , and the pressure on the lung side is called the alveolar pressure P alv. Both P ao and P alv fluctuate regularly during mechanical ventilation.

If the ventilated patient is passive, P ao is actively changed, while P alv follows. If the patient is active, both P ao and P alv can change actively. During inspiration, alveolar pressure rises as more and more gas enters the lungs.

In a pressure-based breath, inspiratory flow drops to zero if P ao and P alv are equal at any level. Under normal conditions of mechanical ventilation, at any time point P ao , P alv , and lung volume are determined by these forces Fig. This is the force to pull the lung and chest wall back to the resting position. This situation is comparable to a stretched rubber band.

Whenever the stretching force is removed, the recoil force causes the band to retract. The lungs are at their resting position typically at the end of an adequate expiration. This is a static state with zero airway flow, because the inherent forces to open and retract the lungs are equal and there is no externally applied force.

Note that in this position, the lungs are not totally empty, but instead contain a volume of gas known as the FRC. The FRC is physiologically important.

Some lung diseases can cause FRC to abnormally increase e. COPD or decrease e. The inflated position of the lungs represents another static state with zero airway flow. Here the forces to inflate the lungs and the forces to deflate the lungs are equal. The lungs are at their inflated position in these two cases: a at the end of an adequate inspiration in a pressure-based breath, and b at the end of inspiration in a volume breath with inspiratory pause.

Inspiration is the process to increase lung volume. Typically it begins at the resting position and may or may not end at the inflated position. The process differs in pressure breaths and volume breaths. In a pressure breath, P ao rises quickly to and stays at a preset level Fig. The pressure gradient is the greatest at the beginning, resulting in the maximum inspiratory flow.

Over time, P alv increases as more and more gas enters the lungs. The pressure gradient diminishes, causing p. If the inspiration is sufficiently long, the lungs reach the inflated position.

In a volume breath with constant flow, the most common inspiratory flow pattern, the applied positive P ao pushes gas into the lungs at a constant, defined inspiratory flow. The applied P ao must increase steadily to maintain the required pressure gradient. At the end of inspiration, the lungs do not reach their inflated position unless an inspiratory pause is imposed. Expiration is the process to decrease lung volume. The applied positive P ao drops suddenly to the baseline. The additional recoil force causes the lungs to retract.

Over time, P alv and the resultant expiratory flow decrease. The lungs return to their resting position if sufficient expiratory time is allowed. The expiration process is the same in both pressure and volume breaths. So far, we have discussed the four driving forces, the lung resting and inflated positions, and the inspiration and expiration processes.

All of them share the same condition: the patient is passive. In reality, however, many ventilated patients are actively breathing. But the situation is more complicated than that. We know there are two external forces to inflate p. A similar situation may also be present at expiration, as the contraction of expiratory muscles generates additional positive P alv.

This gives us six forces altogether Table 3. Applied positive P ao to expand the lungs. Additional recoil force to bring the lungs back to the resting position. Contraction of inspiratory muscles to enlarge the chest cavity and lower P alv. Contraction of expiratory muscles to reduce the chest cavity and raise P alv. The ultimate goal of lung ventilation, for both natural and mechanical ventilation, is to alternately increase and decrease the lung volume.

Both natural and mechanical ventilation are realized through the natural pulmonary system, although the functioning of this system is often deteriorated in mechanically ventilated patients. The direction of gas movement during inspiration and expiration is the same for both forms of lung ventilation. Natural and mechanical expiration are similar. The patient breathes exclusively through the connected ventilator, isolated from atmospheric air. With natural inspiration, the contraction of inspiratory muscles generates a negative P alv.

It can be regarded as an addition to the inherent force to inflate the lungs, leading to an increased FRC. With natural ventilation, the respiratory centre automatically and precisely regulates respiratory rate and breath intensity to satisfy current physiological demand.

With mechanical ventilation, however, an operator must set the mechanical breath rate and intensity. Actively breathing patients may refuse to accept the imposed ventilation, including the rate, tidal volume, and inspiratory pressure, causing patient-ventilator asynchrony. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use for details see Privacy Policy and Legal Notice.

Oxford Medicine Online. Publications Pages Publications Pages. Recently viewed 0 Save Search. Medical Ventilator System Basics: A clinical guide. Read More. Your current browser may not support copying via this button.

Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username. Password Please enter your Password. Forgot password? Don't have an account? Sign in via your Institution. Pulmonary ventilation is commonly referred to as breathing.

It is the process of air flowing into the lungs during inspiration inhalation and out of the lungs during expiration exhalation. Air flows because of pressure differences between the atmosphere and the gases inside the lungs. Air, like other gases, flows from a region with higher pressure to a region with lower pressure. Muscular breathing movements and recoil of elastic tissues create the changes in pressure that result in ventilation.

Pulmonary ventilation involves three different pressures:. Atmospheric pressure is the pressure of the air outside the body. Intraalveolar pressure is the pressure inside the alveoli of the lungs. Intrapleural pressure is the pressure within the pleural cavity. These three pressures are responsible for pulmonary ventilation.