Respiratory Physiology Flashcards
Why is the functional residual capacity particular important in anaesthesia?
- Oxygen reserve during apnoea
- Major influence on the distribution of ventilation within the lung by determining where the starting position of each area on the lung is on the compliance curve
How many times does the trachea branch until reaching the alveoli.
23 times
Up until which airway generation does gas flow occur and how does gas move in the divisions subsequent to this point
Flow of gas occurs up until division 16. From airway generations 17 to 23 –> gases move passively by diffusion along partial pressure gradients.
What unwanted effect does IPPV have on the Cardiovascular system. Contrast IPPV with spontaneous ventilation
During spontaneous ventilation: negative intrathoracic pressure created by diaphragm and external intercostal muscles generates pressure gradient between the atmosphere and the alveoli and hence flow. This negative intrathoracic pressure also contributes to increasing right atrial filling through a ‘sucking effect’ into the RA.
During IPPV, the pressure gradient between the Atmosphere and alveoli is created by a high inspiratory pressure which effectively increases the intrathoracic pressure. This negates the ‘sucking effect’ present in spontaneous ventilation and potentially hinders venous return to the right atrium. This decreases RVEDV and from the F-S law possibly reduces cardiac output and perfusion of the lungs. This may have forward effects with a decrease in filling volumes on the left side of the heart.
What do opioids and general anaesthetic agents do to the PaCO2 response curve
Make the respiratory centre less sensitive to PaCO2 shifting the curve down, to the right and less steep.
What happens to the energy used to overcome lung elasticity during lung expansion?
What happens to the energy used to overcome resistive forces (airway resistance and friction between lung tissues)
Energy to overcome lung elasticity –> Stored as potential energy in elastic tissues and is used during the passive process of expiration.
Energy to overcome resistive forces –> dissipated as heat and therefore is expended by the body to achieve ventilation = work of breathing
Why are the bases of the lungs worse aerated but better ventilated
Apices - high aeration due to gravitational forces on the lung parenchyma here to expand these alveoli at baseline.
Bases - Relatively ‘squashed’ alveoli as the weight of the lung parenchyma and other structures are exerted on the bases. Less aerated.
However, the relative difference in the baseline level of alveolar aeration/distention affects where the different parts of the lung are on the lung compliance curve. The middle and basal aspects of the lungs exist on the steep part of the compliance curve meaning there is a larger change in volume for a lower change in pressure. Therefore ventilation in the middle and basal zones of the lungs is more than in the apical zones.
How does the effect of gravity influence the lung perfusion?
The effect of gravity reduces the perfusion pressure by 1cm H20 for every 1 cm above the level of the heart, with a similar increase below. This means that the perfusion in the apices reduces to virtually zero, and increases progressively down to the bases.
Why does ventilation increase towards the base of the lungs?
Because they are on the most favourable part of the compliance curve
Why does perfusion increase towards the base of the lungs?
Because the effect of gravity decreases the perfusion pressure at the apices and increases it at the bases
Will increasing FiO2 improve hypoxaemia in pathological shunt? Explain.
No. The blood passing normally ventilated alveoli is fully oxygenated. The dysfunctional alveoli does not permit the entry of the FIO2 and therefore the oxygenation of the portion of blood perfusing this dysfunctional alveoli will not be affected by the increased FiO2 and this deoxygenated blood will continue on to mix with blood oxygenated by other functional alveoli.
Which monitored value will change if there is a sudden increase in alveolar dead space?
ETCO2 will fall because gas not involved in respiratory exchange will dilute the CO2 coming from the areas of the lungs that have been.
Does increasing the FiO2 improve hypoxaemia in the case of V/Q mismatch?
Yes. The area of V/Q mismatch still has some ventilation and a higher FiO2 will result in improved oxygenation of blood perfusing this region.
How does general anaesthesia produce V/Q mismatching
Reduced FRC (supine and muscle relaxation) leads to the bases now being on a less favourable position on the compliance curve –> lower volumes –> lower down on the sigmoid curve.
Distribution of perfusion is largely unchanged.
V/Q mismatch
