1.1 Respiratory Mechanics Flashcards
What is the FRC
Elastic recoil?
Sum of what volumes?
How much is it
How can it be measured
Functional residual capacity (FRC) is the volume of air present in the lungs, specifically the parenchyma tissues, at the end of passive expiration.
At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.
FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV) and measures approximately 2400 ml in a 70 kg, average-sized male.
It cannot be estimated through spirometry, since it includes the residual volume.
In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.
The helium dilution technique is a common way of measuring the functional residual capacity of the lungs.
Functions of lungs
x 6
- The main function of the lungs is gas exchange, but they have many other functions including:
- The synthesis of surfactant, prostaglandins and histamine
- Activation and deactivation of angiotensin, bradykinin, 5-hydroxytryptamine and the handling of amide local anaesthetics..
- They act as a blood reservoir, in that the pulmonary circulation contains up to 900 ml of blood and this volume increases by up to 400 ml when in the supine position.
- Substances are filtered from the pulmonary circulation, for example, thrombus and air;
- the mucous lining the air passages also traps particles.
Humidification is a function of the nose and the upper airway rather than of the lungs themselves.
Compliance is defined as
Decreased with -
Explain complaince at different position of volumes
Normal lung compliance
Chest wall compliance -
Compliance is the volume change per unit change in pressure (mL per cmH2O).
Compliance is decreased with
- pulmonary oedema,
- Increased venous pressure
- pulmonary fibrosis.
Compliance describes the ability of the lung to stretch. It is the volume change per unit change in pressure. At resting lung volumes the compliance curve is steep, meaning that for a small change in pressure there is a large volume change.
At higher lung volumes the curve is flatter meaning there is a smaller change in volume for the same pressure change. Compliance is hence smaller at higher lung volumes.
Normal lung compliance is 200 ml/cmH2O and combined lung and chest wall compliance is normally about 70-80 ml/cmH2O
Blood gas changes in asthma attack varying severtiy
Arterial Pco2
FRC
Serum Bic
FEV1
Po2
In mild to moderate asthma attacks the arterial pCO2 may decrease. With increasing severity the pCO2 returns to normal and in severe asthma attacks it may be increased.
With airways obstruction the functional residual capacity increases (not decreases).
The serum bicarbonate concentration would not be raised in moderately severe asthma but it could be in a life-threatening attack for the same reasons as the arterial PCO2 increases.
The forced expiratory volume in 1 sec (FEV1) is a good indication of airway obstruction.
Do not be confused with chronic obstructive pulmonary disease (COPD) where reduced FEV1 is normally mentioned. It measures airways obstruction and is reduced in acute asthma.
The arterial pO2 is usually normal in a moderate attack but may decrease during a severe asthma attack.
If a pneumothorax occurs then the fall in arterial PO2 may be greater.
ODC
What is the signinficance of the shape
Steep part =
Right shift
Decreased Hb =
The curved shape of the oxygen dissociation curve (ODC) means that the loading of oxygen to the tissues is little affected by significant drops in alveolar pO2.
The steep lower part of the dissociation curve means that peripheral tissues can take off large amounts of oxygen for only a small drop in capillary pO2, assisting the diffusion of oxygen into the tissues.
The oxygen dissociation curve is shifted to the right by
Acidosis
Hypercapnia, raising the temperature and
Increasing the amount of 2,3-DPG (2,3- diphosphoglycerate)
which is an end product of red cell metabolism, the concentration of which increases in chronic hypoxia, at altitude, or in chronic lung disease.
A decreased haemoglobin reduces the total oxygen carrying capacity of the blood, but does not change the shape of the curve.
In chronic anaemia there is a compensatory increase in 2,3-DPG that does cause a rightward shift in the ODC, but this effect does not occur immediately.
Hct
Is what
Normally what
Is it higher in venous or arterial blood - why
What does a fall in Hct do to consitution of blood
The haematocrit or packed cell volume (PVC) is the total red blood cell volume as a proportion of blood volume. It is the volume percentage (%) of the red blood cells in blood. It is sometimes expressed as a fraction.
Normal values are 40-54% (0.4-0.54) in males and 37-47% (0.37-0.47) in females.
Venous blood has a higher haematocrit than arterial blood because of the entry of chloride ions into red cells (chloride shift) which is followed by water entry by osmosis.
A fall in haematocrit decreases the viscosity and thus increases the flow. Therefore, a haematocrit of about 30% (0.3) after acute blood loss is thought to be optimal.
In addition to reducing the viscosity and improving tissue blood flow the hazards of blood transfusion and deep vein thrombosis are reduced. However, a value below 30% is undesirable because of reduced oxygen carrying capacity.
Smoking & FEV1
What happens to Fev1 through childhood
when does it decline
What is the deceline compared to smokers
What age do smoker need to stop
The forced expiratory volume in 1-second (FEV1) increases through childhood and peaks between the ages of 18 and 25 years. There is then a slow decline. This decline of 50-70 mls per year in 15-20% of smokers compares with a decline of 30 mls per year in non-smokers. Cessation of smoking returns the decline in FEV1 to that of non-smokers thus preserving lung function.
Smokers who abstain before the age of 40 years eventually have an FEV1 similar to those who have never smoked. The FEV1 is lower by 7 % in smokers who gave up between the ages of 40 and 60 years of age. However, If a lifelong smoker gives up after the age of 60 the FEV1 will at best be 14% less than that of a non-smoker of the same age. Lung function is related inversely to pack-years of cigarette use.
Therefore the FEV1 will not return to normal in this gentleman in five years, let alone six months.
PHTN recog compication of
x5
ulmonary hypertension is a recognised complication of:
Thromboembolic disease Life at high altitude Chronic alveolar hypoventilation, and Patent ductus arteriosus. Polycythaemia rubra vera is usually associated with systemic hypertension but pulmonary hypertension1 can also be a feature
Change if PFTs and obesity
FRC
Airway closure
how much is frc decrease
Resp complaince reduced du to
In morbid obesity, even in the upright position, the fall in FRC can be so marked that it approaches residual volume (RV).
The patient is at risk of premature airway closure and significant ventilation perfusion mismatch resulting in hypoxia. The FRC decreases by 40% when supine as the diaphragm is pushed upward into the thorax by the abdominal contents. Further falls in FRC occur following induction of anaesthesia.
Total respiratory compliance is reduced by up to two-thirds of normal:
Chest wall; due to fat deposition around diaphragm, ribs and abdomen, and
Lung; due to increased pulmonary blood.
This can further deteriorate in the recumbent position volume as the chest wall become less compliant and increase in respiratory resistance.
Whilst morbid obesity results in an increase in inspiratory reserve volume and reduced FEV1, PEFR and ERV, the reduction of FRC (an oxygen reservoir) is very important.
Chemoreceptors
Central located where
Sensitive to what
CSF sensitive because
What receieves high blood flow per 100g tissue
Dopaine contained in what type cell
The central chemoreceptors are on the ventral surface of the medulla (not dorsal) and are sensitive to hydrogen ion concentration [H+].
The cerebrospinal fluid (CSF) is very sensitive to changes in [H+] because it lacks proteins and other buffers (it does not have a good buffering system). The pH of CSF is 7.32 and for a given change in PCO2 the change in CSF pH is greater than in the blood.
Each carotid body receives 0.04 ml of blood per min, which is the equivalent of 2 litres per100g of tissue per min, and is the highest blood flow to any body tissue.
Dopamine is contained within glomus type 1 cells (not type 2 cells).
What is staic lung compliance
when is measured
what is the formaula
How man it be measured
Normal compliance is what
Static lung compliance is the change in volume for any given change in pressure. Static compliance is measured at a period when there is no gas flow, for example during an inspiratory or expiratory pause.
Compliance = ΔV/ΔP
The units are ml/cmH2O or L/cmH2O.
Intrapleural pressure can be measured indirectly using an oesophageal manometer. Respiratory volumes can be measured at the mouth using a pneumotachograph or spirometer.
The normal compliance (Cl) of a normal lung is 200 ml/cmH2O.
For example, if a patient inhales 600 mL of air from a spirometer with an intrapleural pressure before inspiration of -6 cm H2O and -12 cm H2O at the end of inspiration.
Cl = 600mL/-6 -(-12)cmH2O = 600/6 = 100ml/cmH2O.
In this example with a ventilated patient the static compliance represents pulmonary compliance during periods without gas flow, such as during an inspiratory pause.
It can be calculated with the formula:
Cstat = Vt/Pplateau-PEEP
where:
Vt = tidal volume
Pplateau = plateau pressure
PEEP = peak end-expiratory pressure.
So:
Cstat = 800/50-10 = 20 ml/cmH2O.
Ficks lawWhat is the formula
The rate of gas transfer of gas across a semi-permeable membrane is directly proportional to its area, the gas partial pressure differential and the diffusion permeability coefficient.
Fick’s law relates to the rate of transfer of a gas across a sheet of tissue.
The rate of gas transfer is directly proportional to the area of tissue (A), the difference in gas partial pressure between the two sides (P1 − P2) and the diffusion permeability coefficient (D). The rate transfer of gas across a sheet of tissue is inversely proportional to the thickness of the membrane (T).
dV/dt = A/T × D × (P1 − P2)
V = mL/minute A = area T = tissue thickness D = diffusion constant P1 − P2 = partial pressure gradient
PVR - lung volumes
increased by
Lowered by
Anaemia
Pulmonary vascular resistance varies with lung volume in a U shaped curve, such that PVR is at its lowest when lung volumes approximate to the functional residual capacity.
PVR is increased by:
Vasoconstrictor drugs
5-HT, and
Histamine.
Hypoxia, hypercapnia and acidosis
It is lowered by:
Vasodilator drugs
Prostacycline, and
Acetylcholine.
Hypoxia, hypercapnia and acidosis increase PVR whilst their opposites decrease PVR.
Anaemia reduces blood viscosity which decreases PVR and SVR.
Compliance is defined
Which is related to airways resitance
what is specific compliance
Which is greater - static or dynamic
why x3
What is normal
Compliance is defined as the volume change per unit pressure change.
Dynamic (not static) compliance is related to airways resistance.
Specific compliance allows comparison to be made between patients with varying body sizes.
Dynamic compliance is less than static compliance (not greater) for the following reasons.
Airway resistance affects airflow.
Incomplete filling of alveoli in the available time
True pressure equilibrium between applied and alveolar pressure is not obtained, and lung appears stiffer than it really is
Static compliance in the normal lung during spontaneous ventilation is 200 ml per cm of water.
What has the leftest ODC
What is the p50 for that odc
The myoglobin ODC is a rectangular hyperbola with a very low P50 0.37 kPa (2.75 mmHg). It needs a lower P50 to enable offloading of oxygen from haemoglobin and it is low enough to be able to offload oxygen onto myoglobin where it is stored. The myoglobin, however, does release its oxygen at the very low PO2 values found inside the mitochondria.
what is the p50 decribing
What is it for adult hb
The term P50 is used to describe the affinity of haemoglobin for oxygen. The P50 is the PO2 at which the haemoglobin becomes 50% saturated with oxygen. The P50 of adult haemoglobin under normal conditions is 3.47 kPa(26 mmHg).
Whats foetal hb
Whats it ODC
Foetal haemoglobin has 2 α and 2 γchains. The ODC is left shifted (P50 lies between 2.34-2.67 kPa [18-20 mmHg]) compared with the adult curve it has a higher affinity for oxygen. Because foetal haemoglobin has no β chains there is less binding of 2.3 diphosphoglycerate (2.3 DPG).
What does CO do to Hb
Whats the p50 in SCD
Carbon monoxide binds to haemoglobin with an affinity more than 200-fold higher than that of oxygen, and thus decreases the amount of haemoglobin available for oxygen transport. Carbon monoxide binding also increases the affinity of haemoglobin for oxygen, thus shifting the oxygen-haemoglobin dissociation curve to the left and impeding oxygen unloading in the tissues.
In sickle cell disease (HbSS) has a P50 of 4.53 kPa(34 mmHg).
HPV
Response to
what
where
Dependent on innervation to vessel walls?
What does it result in
what level of nitric will inhibit HPV
is it affected by volatiles?
Hypoxic pulmonary vasoconstriction (HPV) is the reflex constriction of pulmonary arterioles in response to low a PO2 (below 80 - 100 mmHg; 11 - 13 kPa) in adjacent alveoli.
It is independent of the innervation to the vessel walls and can occur when blood with a high PO2 is perfused through lung which has a low alveolar PO2.
Thus a low PO2 in the alveoli has been shown to influence hypoxic pulmonary vasoconstriction (HPV) more than a low PO2 in the blood.
HPV results in the blood flow being directed away from poorly ventilated areas of the lung and helps to reduce the ventilation/perfusion mismatch (not increase).
Volatile anaesthetic agents reduce HPV in animals, but in adults the evidence is less convincing, although it certainly does not increase the effect.
20 parts per million (ppm) of nitric oxide will inhibit the HPV response.
