CB[028] Respiratory Function Workshop without answers PDF

Preview

Summary

Download CB[028] Respiratory Function Workshop without answers PDF

Description

Respiratory Function Workshop The Alveolar Ventilation Equation •

This equation is a more practical way of assessing alveolar ventilation.

VA = (VCO2 / P ACO 2 ) x 115.1 or PACO2 α (VCO2/VA) •

VA = alveolar ventilation per unit time

•

VCO2= volume of expired CO2per unit time

•

PACO2 = alveolar PCO 2

•

115.1 = constant

[Note on origin of constant 115.1 If you start with Boyles Law, P 1V1/T 1 = P2V2/T2 and then give the relevant numbers for BTPS and STPD, then you will generate the conversion factor between STPD and BTPS. P1/V1 = P2/V2(i), therefore 8 kPa ÷ 200mls STPD converted to BTPS = 95.0 kPa ÷ minute alveolar ventilation BTPS. Rearranging this equation: . VA BTPS = [200mls STPD converted to BTPS ÷ 8 kPa] x 95.0 kPa

200mls STPD is converted to BTPS using equation P 1 V1/T 1 = P 2V 2/T2 (ii). The constant 115.1 is generated by combining equations i and ii]

Questions 6) i) What is the relationship between alveolar ventilation and PACO2?

7)

8)

ii) What happens to PACO2 if alveolar ventilation doubles? How does alveolar PA CO2 relate to Pa CO2 and can you think of a way of measuring PACO2? (Pa = Parterial) What factors can influence the volume of CO2 expired per unit time?

Clinical Examples

A – Ventilation under anaesthesia During a general anaesthetic, patients are paralysed and unable to breathe for themselves. The anaesthetist inserts and endotracheal ube, which is attached to a ventilator, which the anaesthetist controls. During the anaesthetic, arterial blood samples are taken regularly to assess the adequacy of the patient’s ventilation. 9) You are anaesthetising a 70kg man. You take an arterial blood sample and find that the PaCO2 is 10.7kPa ( normal = 5.3kPa) Thinking about V A = (VCO2 / PACO2 ) x 115.1 (i.e., the alveolar ventilation equation) what is happening to this man? What should the anaesthetist do? [P ACO2 α (VCO2/VA )] )

B - Comatose State •

A 38 year old woman is brought into A&E in a comatose state. The relative with her thinks she has taken an overdose of sleeping pills. She is breathing spontaneously about 10 times every minute.

•

STPD = standard temperature (273K) and pressure (101.3kPa) dry (SVP = 0kPa)

•

BTPS = body temperature (310K) and pressure (101.3kPa) saturated (= 6.3 kPa)

•

While she is breathing room air (FiO 2 = 0.21) at normal barometric pressure (101.3kPa) you take a blood sample. [ PiO2 = FiO2 x ( barometric pressure - saturated vapour pressure) ]

•

Analysis of the blood shows a PaO 2 of 11.5kPa ( normal 13.5kPa) P A CO2 of 8 kPa ( normal 5.3kPa) and a pH of 7.2 ( normal 7.4)

Questions a) •

What is her alveolar ventilation? ( Assume a CO 2 production of 200ml/min STPD).

•

Is her alveolar ventilation above normal, normal or below normal? [ PACO2 = (VCO2 STPD ÷ VABTPS) x 115.1]

b) • •

c)

What is her PAO2 ? Given than: PAO2 = PiO2 - PACO2 To simplify the equation assume her steady state R ( respiratory exchange ratio which equals volume of CO2 produced/volume of O2 consumed) = 1.0. (in reality it would probably be in the range 0.8 - 0.85) Saturated Vapour Pressure (SVP) at 370C = 6.3 kPa You realize from the analysis of blood that she is hypoxic, hypercapnic and has a pH disturbance. Which of the treatment options is best? (Think about effect on blood gas and pH). Treatment Options: allow her to continue breathing spontaneously and give her a 50% O2 to breathe.

*artificially ventilate her on room air but with an increased frequency of breathing of 15 breaths per minute?(assume CO2 production is unchanged).

C – ASTHMA A 16 year old boy with known asthma and hay fever becomes acutely short of breath during sports day at school. He is taken to see the school nurse by which time he is progressively worsening. Questions a) Can you give 3 stimuli for an asthma attack? During an asthma attack there is an acute response leading to narrowing of the airways and increased mucous secretions with oedema (fluid leak into tissues making them turgid) •

What effect will this have on? i) VT? ii)VA?

•

iii)PACO2/ PAO2? Analysis of his blood shows: PaO2 of 8.5kPa ( normal 9.3 - 13.3kPa) PaCO2 of 6.7 kPa ( normal 4.7 - 6.0) pHof 7.3 ( normal 7.35 - 7.45) a) What is the relationship between arterial carbon dioxide, serum bicarbonate and blood pH? 3

The school nurse notices that he has increased his respiratory rate (frequency) as well as the depth of his breathing. On examination he has an increased heart rate. ii) How does the body detect changes in carbon dioxide and pH? b) What compensatory mechanisms are initiated in an attempt to bring blood gases back to within the normal range/and or maintain oxygen delivery to the tissues? ?

What will these compensatory mechanisms achieve with regard to: •

i) VT ?

•

ii)VA?

•

iii)PACO 2/ PA O2 ?

After initially settling down the boy became drowsy. Repeat blood analysis shows: P A O2 of 6.7kPa ( normal range 9.3 - 13.3kPa) P A CO2of 9.3 kPa ( normal range 4.7 - 6.0 kPa) pH of 7.1 ( normal 7.35 - 7.45) Can you explain the cause of this change in blood gas? [Is Asthma Clear in Your Mind? If not ask........]

AIMS •

By the end of this workshop you will:

•

Understand the principles of alveolar ventilation

•

Use the alveolar gas equation to assess respiratory function

•

Apply this to clinical scenarios

DO YOU?...