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