Module 2: Invasive Respiratory Support Flashcards
Ventilation
movement of gases in and out of the pulmonary system
Ventilation is comprised of both pulmonary and alveolar ventilation.
- Pulmonary ventilation is the volume of air exchanged between the environment and the lungs.
- Alveolar ventilation is the volume of air entering the alveoli that takes part in gas exchange per minute.
Respiration
involves the exchange of oxygen and carbon dioxide at the alveolar-capillary level and at the capillary-cellular level.
Ventilation occurs unconsciously and is maintained by various bodily functions. These include:
The Central Nervous System:
The respiratory centre is located in the medulla oblongata, located in the brain stem. Here, breathing patterns are adjusted in response to various levels of pCO2 and pO2 in the blood.
Stretch Reflexes:
These are located in the chest wall and airways and serve to alter the breathing pattern to maintain adequate minute ventilation (Minute Ventilation is the product of Rate X Volume).
Chemoreceptors:
Located in the aorta and the common carotid artery, chemoreceptors respond to increases and decreases in Pa02, PCO2 and pH, particularly in the presence of hypoxemia and acidocis. Examples:
↑pCO2 will lead to increased minute ventilation.
↓pCO2 will lead to decreased minute ventilation.
↓pO2 leads to increased minute ventilation.
Normal alveolar ventilation occurs
when an infant has a sufficient respiratory drive and sufficient energy to inflate and deflate the lungs, maintaining some volume at the end of every breath known as the functional residual capacity (FRC).
functional residual capacity (FRC)
FRC is maintained when sufficient surfactant minimizes surface tension (tendency for alveoli to collapse) and the alveoli remain slightly open at the end of each exhalation. Some gas remains, making the next inhalation significantly easier, decreasing the work of breathing (WOB).
Recall that the younger the gestational age, the more likely they are to have the following:
↓ surfactant ↓ alveoli ↓ capillaries ↑ distance between alveoli and capillaries Small airways Underdeveloped, weak muscles Underdeveloped pulmonary vasculature Cartilaginous rib cage
Respiration
is the act of inhalation and exhalation for the purpose of gas exchange within the lungs and at a cellular level. It allows the exchange of gas, (primarily oxygen and carbon dioxide) between an individual and his or her environment. Respiration is more than ventilation. It is dependent on:
Sufficient alveolar ventilation Alveolar/capillary diffusion Pulmonary perfusion Hemoglobin Peripheral perfusion
Gas is exchanged in two places:
in the lungs and at the cellular level. Alveolar respiration occurs in the lungs; cellular respiration occurs in the tissues.
The cardiovascular system acts as a conduit between these two sites of gas exchange. The brain acts as a central controller. The whole process looks something like this:
see picture on desktop
The entire process of respiration occurs in all body cells. The three main systems responsible for gas exchange are:
pulmonary system
circulatory system
nervous system.
The pulmonary system plays a key role in gas exchange or respiration. Alveolar respiration (gas exchange that occurs between the alveoli and the pulmonary capillaries in the lungs) is a result of three interdependent processes:
alveolar ventilation
pulmonary perfusion
diffusion across the alveolar-capillary membrane to allow for pulmonary perfusion.
Pulmonary perfusion
is also an important requirement for gas exchange. Pulmonary perfusion refers to the flow of blood through the portion of the circulatory system that supplies the lungs. De-oxygenated blood travels from the right side of the heart through the pulmonary arteries to the lungs where oxygen will be picked up and returned to the left heart through the pulmonary veins. The oxygenated blood can then travel out the aorta and to the rest of the body. At the same time, the blood transported to the lungs by the pulmonary artery will deliver CO2 picked up from the tissues and carry it to the lungs for elimination.
Pulmonary perfusion is dependent on oxygen and pH. In response to hypoxia and acidosis, pulmonary vasoconstriction leads to diminished pulmonary perfusion and pulmonary hypertension.
Diffusion
across the alveolar-capillary membrane occurs because of the pressure gradients for oxygen and carbon dioxide. These gases move from areas of high pressure toward areas of low pressure. Oxygen moves from the alveoli to the capillaries and carbon dioxide moves from the capillaries to the alveoli.
Oxygen and CO2 diffuse across the alveolar-capillary membrane differently.
CO2 is much more diffusible than O2. Even with diminished perfusion, CO2 diffuses readily from pulmonary venous and capillary blood to the alveoli. Once in the alveoli, CO2 is dependent on adequate alveolar or minute ventilation for elimination.
CO2 is a primary indicator of adequate alveolar ventilation. Changes in carbon dioxide levels are primarily due to changes in minute ventilation. Specifically, elevated CO2 indicates hypoventilation; CO2 depletion indicates hyperventilation.
Hypoventilation is the term used to describe a breathing pattern that results in decreased minute ventilation. The pattern may be one of:
diminished tidal volume
decreased rate
both
The circulatory system
The circulatory system acts as a courier, transporting gases between tissues and the lungs. It is responsible for delivering oxygen from the lungs to the cells and carbon dioxide from the cells to the lungs. The heart is the pump and the vasculature is the conduit.
Cardiac output is a reflection of the heart’s ability to perform this important function. Cardiac output is the volume of blood pumped by the heart in one minute, (mls/min). Cardiac output is a product of heart rate × stroke volume. Stroke volume is the volume of blood pumped with each beat (Litres/beat).
Stroke volume is a product of three factors:
preload
afterload
contractility.
Together these three factors determine stroke volume. Combined with heart rate, these factors determine cardiac output.
Preload
refers to the volume of blood in the ventricles prior to contraction (systole).
Afterload
refers to the pressure or resistance against which the ventricles are pumping (diastolic blood pressure).
Contractility
refers to the strength of contraction of the heart muscle.
CO2 is transported in three ways:
dissolved in plasma
attached to hemoglobin
combined with H20 to form H2CO3 (carbonic acid)
Most CO2 is dissolved in plasma or as carbonic acid; very little is carried by hemoglobin.
Oxygen is transported in two ways:
dissolved in plasma
attached to hemoglobin
Most oxygen is carried attached to hemoglobin (98%) and a very small amound is dissolved in plasma (2%).
Once arterial blood (containing oxygen and CO2) reaches capillary beds in the tissues, the process of cellular respiration occurs.
Central Nervous System
Chemoreceptors in the aorta and carotid arteries as well as in the cerebrospinal fluid (CSF) detect changes in blood gas composition. Nerves relay the information to the brain, and necessary adjustments in ventilation are made. Respiration is controlled by two types of chemoreceoptors:
Central Chemoreceptors
Peripheral Chemoreceptors
Central Chemoreceptors
located in the brain stem and respond to the acidity of the CSF
involved in control of respiratory rate and depth of breath
Example: acidic CSF results in hyperventilation, alkalotic CSF results in hypoventilation.
Peripheral Chemoreceptors
located in the aortic arch and carotid arteries
respond to changes in oxygen concentrations, carbon dioxide and pH
regulate breathing breath to breath
What happens if CNS is damaged?
Central nervous system depression or damage can lead to respiratory problems such as bradypnea, bradycardia, apnea, hypoventilation, hyperventilation, and hypotension. When using mechanical ventilation, these central nervous system responses are bypassed, thereby implicating an external control that is subject to error and ongoing adjusting.
key point
Mechanical ventilation is largely aimed at improving alveolar ventilation, but alveolar ventilation is only one part of what happens in the lungs and only one part of the whole process of respiration. When working with mechanical ventilation, look at the case in the context of factors that can influence respiration such as pulmonary hypoperfusion, anemia, poor peripheral perfusion, and hypovolemia (too name a few).
Which parameters of the blood gas are used to assess ventilation? Acid-base balance? Oxygenation?
Ventilation is assessed by looking at the pCO2. Acid-base balance is assessed by looking at the pH, pCO2, and
HCO3. Oxygenation is assessed by looking at the pO2 and the O2 saturation.
Which parameter is used to determine the respiratory component of acid-base abnormalities? Which one determines the metabolic component?
The pCO2 determines the ventilatory component. An increase in pCO2 creates a decrease in pH which is called “respiratory acidosis” (or ventilatory acidosis). It is due to hypoventilation.
The HCO3 determines the metabolic component. A net decrease in HCO3 causes a decrease in pH and is called metabolic acidosis. It is due to many etiologies, such as anaerobic metabolism producing lactic acid, a loss of HCO3 from the kidney or failure of the kidney to eliminate excess H+.
Why is the amount of carbon dioxide in the blood a direct measure of ventilation?
Because the amount of carbon dioxide in the blood is a reflection of the amount produced by metabolism and the amount exhaled through ventilation, or breathing. As metabolism does not change greatly, pCO2 is an indication of ventilation.
Explain “compensation” using compensated respiratory acidosis as an example.
When hypoventilation leads to respiratory acidosis (↓ pH, ↑ pCO2, and HCO3), the body will compensate by creating a compensated metabolic alkalosis. The kidneys will retain HCO3 and when the levels elevate, it looks like this: Normal pH, ↑pCO2, and ↑ HCO3.
Whereas elimination of CO2 is only dependent on adequate ventilation, tissue oxygenation is dependent on several processes. What are they?
Tissue oxygenation is dependent on adequate ventilation. In addition, tissue oxygenation is dependent on: oxygen delivery to the tissues (cold, hypovolemia, cardiac failure can lead to tissue hypoxia) and presence of anemia (reduced oxygen carrying capacity resulting in reduced oxygen content of the blood). When hypoventilation leads to respiratory acidosis (↓ pH, ↑ pCO2, and HCO3), the body will attempt to bring the pH back to nomal by retaining bicarbonate.
it is important to note the primary goal of mechanical respiration: to improve minute ventilation. The desired outcomes of mechanical ventilation include:
to achieve and maintain adequate pulmonary gas exchange
to minimize the risk of lung injury
to reduce patient work of breathing (WOB)
to optimize patient comfort
Positive Pressure Ventilation
Positive pressure ventilation occurs when we try to move air into the patient’s lungs by way of an endotracheal tube or non-invasive mask to create a positive pressure there by creating a forceful delivery of gas to inflate the lungs. Mechanical Ventilation in the neonatal population will be dependent not only on the airway anatomy and pulmonary pathophysiology of this particular population, but also on how various diseases will impact them. Thus, there is no one way to ventilate the neonatal patient but a variety of ventilators, modes of ventilation, and methods of ventilation that will be chosen based on the airway anatomy, pathophysiology, and disease process specific to each individual.
Achieving and Maintaining Adequate Pulmonary Gas Exchange
Mechanical ventilation is the application of positive inspiratory pressure to inflate the lungs. Remember that mechanical ventilation is aimed primarily at improving gas exchange by increasing minute ventilation. Minute ventilation is improved by increasing either the tidal volume or the rate. When minute ventilation increases, oxygen supply and carbon dioxide elimination increase.
Key point
The lungs have an inherent ability to match ventilation and perfusion (by increasing perfusion to well-ventilated areas). Therefore, re-inflation of atelectatic alveoli, through mechanical ventilation, will help to increase pulmonary perfusion. As well, the increased oxygen levels associated with improved alveolar ventilation and supplemental oxygen will further improve pulmonary perfusion. Finally, alkalinization through hyperventilation (by increasing respiratory rates and tidal volumes to decrease CO2 and pH) is thought to increase pulmonary perfusion by reducing pulmonary vasoconstriction caused by acidosis and hypoxia.
Diffusion is improved by increased mean airway pressure. An increased mean airway pressure allow for recruitment of collapsed alveoli and/or reistribution of lung fluid to decrease deadspace ventilation (an area of ventilation in absense of perfusion), and shunting (an area of normal perfusion but absent of ventilation). In particular, oxygen diffusion is enhanced when mean airway pressure is increased. This will allow for better exchange of gases across the alveolar-capillary membrane, assisting with alveolar ventilation.
Minimizing the Risk of Lung Injury
As gas is delivered under pressure to the newborn lung, the airway expands in proportion to its compliance; therefore, the more compliant the airway, the greater the change in airway diameter with mechanical ventilation (Greenspan & Shaffer, 2006). These authors go on to explain that the newborn airway is much like a balloon:
Once a balloon has been inflated, it is generally easier to inflate it again because of the structural changes to the balloon wall. Similarly, once the structure of the airway wall is compromised by mechanical ventilation, it will behave differently when subsequent pressure is applied. Although the airway may recoil to its original shape after the exhalation of each breath, some elasticity will be lost over time and the airway will become larger than normal.
Reducing WOB
Some infants will be born with immature or compromised respiratory, circulatory, or central nervous systems. This may lead to increased work of breathing which may present as tachpnea, tachycardia, indrawing, nasal flaring, grunting or decreased or absent respiratory drive. Mechanical ventilation is aimed at decreasing the work of breathing. This is accomplished by setting the appropriate tidal volume, respiratory rate with the option of pressure support to achieve required alveolar ventilation and also by using positive end-expiratory pressure to maintain FRC and thereby improve compliance.
Optimizing Patient Comfort
Imagine struggling to breathe – what an upsetting and stressful sensation. Supporting the infant to breathe is of utmost importance; not only for their survival, but as a comfort measure as well. These vulnerable infants need to be utilizing their reserves for growth – not using their energy to breath. Strategies to optimize infant comfort will ideally reduce their WOB and minimize “fighting the ventilator.”Interventions may include:
Ensuring the best mode of ventilation is being utilized Non-pharmacologic measures Facilitated tucking Skin-to-skin Nesting Noise reduction Pharmacologic sedation/pain control
Using Positive Pressure Ventilation
Positive pressure ventilation is when air is moved into the patient’s lungs via and endotracheal or tracheostomy tube to create a positive pressure to inflate the lungs. Mechanical ventilation in the neonatal population will be dependent on the airway anatomy, pulmonary pathophysiology and disease process. There is no one way to ventilate a baby, but options in terms of ventilators, modes and methods by which to ventilate that meet their individual needs. Most ventilators use positive pressure inflation of the lungs.
Rememeber that the goals of mechanical ventilation are to:
achieve and maintain adequate pulmonary gas excahnge
minimize the risk of lung injury
to reduce work of breathing
to optimize patient comfort
While we initiate mechanical ventilation to mitigate problems of hypercapnia, hypoxia, apnea and WOB, the possible complications associated with this treatment can have life-long consequences.
mechanical ventilation is NOT like normal breathing. Specifically, it differs from normal breathing in two important ways:
- The pressure needed to inflate the lungs with mechanical ventilation is much greater than the pressure needed with normal breathing.
- Mechanical ventilation bypasses the normal internal biochemical feedback mechanism that controls breathing.
Normal Ventilation
Normally, when we inhale, our lungs expand as a result of negative intrathoracic and intrapleural pressure. This negative pressure actually pulls the lungs open from the outside — much the way a vacuum bag is expanded inside the vacuum chamber of a vacuum cleaner. As a result, the pressure within the lungs does not increase significantly as the lungs inflate.
Positive Pressure Ventilation
Mechanical ventilation does not work in this way. Rather, mechanical ventilation uses high positive pressure exerted on the inside of the lungs to push them open from the inside — much like blowing up a balloon. As a result, the pressure within the lungs increases significantly as the lungs inflate. In fact, without this significant increase in pressure, the lungs would not inflate.
What is the problem using mechanical ventilation?
The problem with using positive pressure to inflate an infant’s lungs is that this pressure causes damage to fragile and developing lung tissue. Edema, inflammation, scarring, over-inflation, and loss of elasticity are a few of the more common and more serious problems that result from the barotrauma, (damage due to pressure) caused by positive pressure ventilation, and volutrauma, (damage due to over-distension of the lung).
Pressure
refers to force. For example, in mechanical ventilation, pressure refers to the force being applied to the inside of the alveoli as they are being inflated. Think about blowing up a balloon. Pressure is the force you use to inflate the balloon.
Volume
refers to the space something occupies. For example, in ventilation, volume refers to the amount of gas inside the alveoli. Think about the balloon you have just used force to inflate. Volume refers to the amount of gas inside the balloon after it is inflated.
Generally pressure and volume are directly related. The greater pressure you apply to inflate the lung, the greater the volume you will achieve. However, the relationship between volume and pressure is mediated by how easy the lung is to inflate. Normally two types of forces impose the inflation of the lungs. These forces are compliance and resistance:
These forces are compliance and resistance:
Compliance: (Litres/cmH20)
How compliant a baby’s lungs are affects how much volume moves into a lung at a given pressure. It affects how much pressure is required to achieve a given end-inflation volume within a lung. Think about two balloons. One is slightly inflated and the other is collapsed. It takes less pressure to inflate the first balloon because it is more compliant. It would take more pressure to achieve the same volume in the collapsed balloon because it requires more pressure to inflate, and is therefore less compliant.
Resistance (cmH20/Litre/second)
Resistance is the measurement of the frictional forces that must be overcome during breathing. An increase in force can be a result of anatomical structures of the airways, decrease diameter of the endotracheal tube size, increase in secretions, bronchospasm, mucosal edema (to name a few). An increase in resistance will require more pressure to achieve the same end-inflation volume within a lung.
Types of Mechanical Ventilation
There are three basic methods that have been developed to replace the mechanisms of normal breathing in mechanical ventilation. They are negative-pressure ventilation, positive-pressure ventilation, and high frequency ventilation.