Lung resistance and Neonatal Respiratory Disease Flashcards
What are volume and pressure changes?
- Respiratory muscles initiate changes in the intraplueral pressure which sets up a partial pressure gradient enables air to flow
- 1cm diaphragm movement creates a change in 3 cmH20 which is enough
- The more muscular effort the more work
- Ideally the lungs would be stretchy to allow inflation
Normal respiratory pressure around 1 L
What is Boyle’s Law?
The pressure of a given mass of a n ideal gas is inversely proportional to its volume at a constant temperature.
What are the two resistances that breathing effort work needs to overcome?
- Elastic resistance ( Lungs and Chest wall = surface tension and surfactant) - contributes to 65% resistance (Static state)
- Tissue - Lungs, Pleura
- skeletal, muscular skin
Dynamic state - Tissue Viscous Resistance and Airway Resistance:
- 35% of resistance - tissue viscuous resistance and airway resistance
- 20% = Lungs, Ribs,, Diaphragm and organs
- Airways, Gas flow
What is compliance and specific compliance?
Compliance: Distensibility (stretch) of the lunge i.e. the change in lung volume that occurs with each unit change in transpulmonary pressure e.e. V/P i.e. for both lung by 200 mL.
- Specific compliance - the corrected for volume at that time e.e. compliance / V
- This standardises compliances for different volumes
What is Elastance?
-Property of resisting deformation (resistance) or desite to return to original shape
E = 1/C
What is the V/P (compliance) relationship (TP CHNAGES)?
- Shows the capacity of the lungs to adapt to transpulmonary pressure changes
- You need muscular effort so the work of breathing is the area in this loop plus the hatched area
- Resistance are overcome
What is static resistance of the chest wall?
- Stiffness of chest wall
- Force of movement of chest wall is opposite that of lung
- Chest wall has tendency to expand but held by negatove pleural pressure
- Unopposed reaches 70% of TLC (resting position)
- When thorax is intact, in equilibrium with the lungs, its restinf level is Functional Residual Capacity (FRC)
What are some causes of increased chest wall resistance?
- Structural abnotmality - decreased flexibility/expansion
- Deformities of thorax
- Ossification of the costal cartilage
- Paralysis of intercostal muscles
- Chest trauma - paralysis, strain, pain
- Loss of elasticity
- Burns
- Blockage of the smaller respiratory passages with mucus or fluid
- Raised abdominal pressure
- Obesity
- Pregnancy
What is the static resistance of the lungs?
-Elastic recoil 0 the tendency of an elastic structure to oppose stretching
- The lungs naturally have a tendency to collapse because of elastic recoil
- Held open by the negative intraplueral pressure (established by lymphatic pumping of fluid)
- Collapse caused by :
- Elastin and collagen in lung tissues (1/3 of this)
- Surface tension (2/3 of this resistance)
Describe elastin and collagen in lung tissues
- Interwoven in the lung parenchyma
- Tendency to oppose stretching
What is surface tension in lungs?
- Caused by an air-liquid interface
- Small alveoli are unstable and have a greater tendency to collapse and empty its air into connected larger alveoli (i.e. atelectasis)
- Reduction of ST forces allows interdependent/stability
What are the benefits of surfactant?
- 3 X less transmural pressure needed to expamd lungs
- Keeps alveoli dry - lowers the inwardly directed pressure which draws water into the alveoli
- Maximises area for ventilation and perfusion
- ST of small alveoli are reduced more than larger alveoli as syrfactant molecules crowd into the smaller space
- Maintains ‘alveolar interdependence’
- Aided by fibrous tissue seprtal and septal walls between alveoli of different sizes which act as additional splints
What are some disorders that influence surface tension?
- Neonatal Respiratory Distress Syndrome (NRDS)- Alters or destroys surfactant
- Adult respiratory distress syndrome
- Oxygen toxicity
What is NRDS?
- Increasing respiratory distress commencing at or shortly after birth. Increases in severity until progressive resolution occurs among survivors, usually around 2nd to 7th day . Caused by deficiency of surfactant
- Can be primary or secondary
- Mortality is 50% in infants <1 kg
- Incidence and severity is inversely proportional to gestational
What are the symptoms of NRDS?
Cyanosis
Preterm birth
Tachypnea
Nasal flaring
Chest retractions and grunting
What are the causes and contributor factors of NRDS?
- Fetal head injury during birth
- Aspiration of blood or amniotic fluid
- Excessive sedation of mother during birth
- Maternal diabetes (excess insulin suppresses surfactant production)
- Cold stress
- Males
- Genetic disorders (e.g. abnormalities in proteins B and C or transport proteins ABC transporter 3 (ABCA3)
What is the pathogenesis of NRDS?
-Starts with immature and damaged Type II pneumocytes due to risk factor or specific cause
Low surfactant = increased Surface Tension = Lung collapse = Hypoxia = Pulmonary vasoconstriction and Alveolar epithelial damage TO fibrin hyaline membrane
Compare normal vs hyaline membranes?
Hyaline:
-collapsed alveoli
- Wax layers of hyaline
- Bleeding and vascular congestion
- Fibrin, cell debris, erythrocytles, Neutrophils and macrophages
What are consequences of NRDS?
- Inflammation - neutrophils in lungs
- Atelectasis
- Leads to ventilation-perfusion mismatch (VQ)
- Right to left shunting
- Arterial hypoxaemia
- Hyaline membrane
- Epithelial necrosis
- Decreases fluid absorption and lung oedema
- May haemorrhage
- Can lead to complications e.g. hypoxic related intracerebrral bleeding, GIT necrosis , failure to close the hole in the heart (i.e. ductus arteriosis, normally caused by oxygen) and also some fibrosis due to fibrin exudation
How is NRDS treated?
In babies:
- Synthetic surfactant therapy (endotracheal)
- Application of surfactant to a preterm infant
- Surfactant therapy - effect on the flow loops
- Assisted ventilation
- Supportive care e.g., thermoregulation, fluid management and nutrition
- Pre-term mothers :
- I.v. betamethasone 26-28 weeks gestation
- Administered - single dose or repeated doses
