Ventilation and blood gases

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Chapter 6. Ventilation and blood gases
Problems with respiration provide a substantial proportion of the acute workload of a neonatologist. Early respiratory problems lead to physiological instability, which in turn contributes to other pathological processes that lead to mortality and morbidity. The management of early respiratory difficulties itself contributes to subsequent problems, with severe chronic lung disease rarely occurring in infants that have not received mechanical ventilation. The knowledge that positive pressure ventilation may in itself be harmful has led to a quest for methods to reduce the need for such support. Different modalities of ventilation have been developed and are widely used – not necessarily with evidence to support their use or inform on the best methods of using that particular technique. Surfactant is widely used and continuous positive airway pressure is gaining wider acceptance as a primary means of providing respiratory support as opposed to a modality principally for weaning babies from positive pressure ventilation. Whatever the method used, the mode in which it is used and the adjuncts used in association with it (e.g. surfactant) there will be changes in respiratory status which may be rapid and potentially very serious. Central to effective monitoring of respiratory status is the measurement of blood gases and any individual responsible for the care of acutely unwell infants must be comfortable with the interpretation of blood gas results and with the impact of changes in ventilation on the different parameters being measured.
In this chapter a series of blood gas results are given. Where appropriate, ventilator settings and relevant history are also given. The results should be interpreted and a decision made as to what the result implies – acidosis or alkalosis, metabolic or respiratory. An explanation for the abnormality should be given where possible and a plan formulated for the action that needs to be taken to rectify the problem. On occasion you may conclude that the result is normal and therefore no action need be taken. Think carefully. Making no changes is not the same as doing nothing and a normal result does not necessarily mean that some action should not be taken to see whether a normal result can still be obtained when the amount of support is reduced. Neonatal care must always be proactive. Doing nothing is rarely the best option.

QUESTIONS

1. You are presented with the following blood gas result on a 31 week infant who is 2 hours old.
pH 7.35 HCO 3 20.4
PcO 2 3.10 BE −4.2
PcCO 2 4.9 FiO 2 0.21
a. What does this show?
b. What further piece of information do you need to be able to interpret it correctly?
2. A 34 week infant is delivered by emergency caesarean section for fetal distress. At 4 hours she is settled and breathing spontaneously in room air. The following blood gas result is obtained from a capillary sample.
pH 7.37 HCO 3 24.5
PcO 2 4.2 BE −0.4
PcCO 2 5.6 FiO 2 0.21
a. What does this show?
b. What further information would help you to decide what action to take?
3. A term infant has been born following a difficult delivery during which the CTG had shown frequent decelerations. She is quietly breathing room air. Respiratory rate is 48/minute and there is no recession and grunting. The baby responds appropriately to handling. A capillary blood sample is taken 30 minutes after birth and the following blood gas result obtained.
pH 7.26 HCO 3 21.8
PcO 2 2.8 BE −5.5
PcCO 2 6.5 FiO 2 0.21
a. What does this show?
b. What factors in the history should be considered before taking action in response to this result?
4. An infant is born at 28 weeks gestation, has received surfactant and is on CPAP. An umbilical arterial line has been inserted and a blood sample is taken through it while an x-ray is awaited and sent for blood gas analysis. The following result is obtained.
pH 7.35 PIP
PaO 2 3.5 PEEP 4
PaCO 2 5.7 TI
HCO 3 23.7 TE
BE −1.6 Flow 8 L/min
FiO 2 0.34
The infant is being monitored. Heart rate is 134 bpm, respiratory rate 50/minute, SaO 2 is 100%.

5. A 37 week infant is born by emergency caesarean section after a haemorrhage due to a torn umbilical cord. The baby is moderately bradycardic and approximately one minute of cardiac compression is given. As the heart rate remains relatively slow a bolus of 10 mL/kg of 0.9% saline is given with some improvement. The infant is transferred to the neonatal unit. Monitoring is started and oxygen saturation is 97%. A blood sample is obtained by arterial stab 20 minutes after birth and the result of blood gas analysis is shown below.
pH 7.22
PaO 2 8.5
PaCO 2 5.0
HCO 3 15.2
BE −11.7
FiO 2 0.25
Hb 13.2 g/dl
a. What does the blood gas show?
b. What must be done when such a result is obtained?
c. What is the most likely explanation in this case and what action should be taken?
6. A 29 week gestation infant is 2 hours old. He has a respiratory rate of 70 and there is mild recession. A soft end-expiratory grunt is audible. An arterial blood sample is taken and blood gas analysis is performed.
pH 7.32
PaO 2 6.5
PaCO 2 6.2
HCO 3 24.0
BE −2.1
FiO 2 0.32
a. What does the blood gas show?
b. What two actions should be taken?
c. What three investigations might you want to be sure had been performed?
7. A 38 week gestation infant was well at birth. At 6 hours of age she is noted to be tachycardic and tachypnoeic. An end-expiratory grunt is heard. An arterial stab is performed and blood gas analysis performed.
pH 7.36
PaO 2 9.6
PaCO 2 4.8
HCO 3 20.5
BE −3.8
FiO 2 0.21
a. What does blood the gas show?
b. What three investigations would you make sure had been done?
c. What therapy would you start?
pH 7.30
PaO 2 5.4
PaCO 2 6.2
HCO 3 22.9
BE −3.5
FiO 2 0.60
a. What does blood the gas show?
b. A septic screen and chest x-ray have already been performed. What further management would you consider?
9. A 27 week gestation infant is 12 hours old. She is grunting and there is obvious recession. Her respiratory rate is 90/minute and there are frequent bradycardias. The following result is obtained from an umbilical arterial line blood sample.
pH 7.35
PaO 2 4.4
PaCO 2 6.8
HCO 3 28.3
BE 2.05
FiO 2 0.55
a. What does the blood gas show?
b. What are the main problems that must be treated?
c. What measures could be taken to address them?
10. A 32 week gestation infant is born to a mother whose membranes ruptured 60 hours before delivery. Group B streptococcus had been grown from an introital swab taken when mother was admitted at 20 weeks with some abdominal pains that subsequently resolved. Mother received no medication between rupture of membranes and delivery other than analgesia. At 4 hours of age the baby is grunting and appears poorly perfused. A capillary sample is attempted but perfusion is very poor and the sample clots before it is completed. After some discussion it is decided that a radial artery stab should be performed. The following result is obtained.
pH 7.20
PaO 2 9.9
PaCO 2 5.1
HCO 3 14.8
BE −12.6
FiO 2 0.35
a. What is the most important treatment to be commenced?
b. Why?
c. What investigations are urgently needed?
d. What serious error in management has been made?
pH 7.18 PIP
PaO 2 4.6 PEEP 6
PaCO 2 7.3 TI
HCO 3 20.2 TE
BE −8.9 Flow 8 L/min
FiO 2 0.55
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
12. Following on from question 11, the infant was intubated, given surfactant and positive pressure ventilation was commenced. Oxygen saturation has been above 95% and a further arterial blood gas sample is taken at three hours.
pH 7.5 PIP 18
PaO 2 12.5 PEEP 4
PaCO 2 2.6 TI 0.35
HCO 3 15 .5 TE 0.65
BE −4.0 Flow 8.5 L/min
FiO 2 0.45
MAP 8. 9 mBar, VT 10.4 mL, MV 0.62 L/min
a. What do these results show?
b. What is the explanation?
c. What actions will you take?
d. What errors in management have been made?
13. A 28 week gestation, 1.15 kg baby has been ventilated for respiratory distress syndrome. He has received surfactant and oxygenation has started to improve. He is now 6 hours old. An umbilical artery catheter has been inserted and an arterial blood gas taken. Arterial blood pressure is 32/18 with a mean of 24.
pH 7.2 PIP 22
PaO 2 7.7 PEEP 5
PaCO 2 4.8 TI 0.4
HCO 3 13.9 TE 0.6
BE −13.2 Flow 8 L/min
FiO 2 0.29
MAP 14. 6 mBar, VT 5.75 mL, MV 0.37 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
pH 7.16 PIP 22
PaO 2 5.8 PEEP 5
PaCO 2 8.0 TI 0.35
HCO 3 21.1 TE 0.65
BE −8.7 Flow 7.5 L/min
FiO 2 0.65
MAP 10. 5 mBar, VT 2.4 mL, MV 0.15 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
15. A 29 week gestation infant, birth weight 1.5 kg, has been ventilated for two days and has been stable and ventilation is weaning. He suddenly becomes bradycardic and remains with a heart rate below 60. At the same time his oxygen requirement rises sharply. The following arterial blood gas is obtained within two minutes of the bradycardia.
pH 7.28 PIP 18
PaO 2 2.5 PEEP 4
PaCO 2 10.9 TI 0.4
HCO 3 35.5 TE 0.9
BE 5.3 Flow 8.5 L/min
FiO 2 0.75
MAP 8. 3 mBar, VT 3.2 mL, MV 0.14 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
16. In the same infant the peak inspiratory pressure has been increased. The infant remains bradycardic and the oxygen saturation has fallen and is no longer measurable. A repeat arterial blood gas is obtained.
pH 7.06 PIP 26
PaO 2 1.2 PEEP 4
PaCO 2 11.8 TI 0.4
HCO 3 24.5 TE 0.9
BE −8.7 Flow 8.5 L/min
FiO 2 1.0
MAP 10. 8 mBar, VT 1.1 mL, MV 0.11 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
17. In the same infant as question 16 a chest drain has now been inserted and good chest movement is seen. Oxygen saturation improves and inspired oxygen concentration can be reduced. The following arterial blood gas is obtained an hour later.
pH 7.48 PIP 22
PaO 2 10.1 PEEP 5
PaCO 2 3.5 TI 0.35
HCO 3 19.9 TE 0.65
BE −1.1 Flow 7.5 L/min
FiO 2 0.55
MAP 10. 7 mBar, VT 12.1 mL, MV 0.56 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
d. What basic error has been made?
18. A 26 week infant is 8 days old. He weighed 1.0 kg at birth. He was initially maintained on CPAP but changed to positive pressure ventilation on day two. He reached maximum pressures of 28/4 and inspired oxygen of 0.8. The following arterial gas is obtained.
pH 7.50 PIP 24
PaO 2 4.7 PEEP 4
PaCO 2 3.1 TI 0.4
HCO 3 18.5 TE 0.6
BE −1.7 Flow 9.1 L/min
FiO 2 0.60
MAP 11 . 5 mBar, VT 7.1 mL, MV 0.4 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
19. A 37 week gestation female infant weighing 2.8 kg is admitted following an elective caesarean section as the mother’s two previous deliveries had also been by caesarean section. She develops quite severe respiratory signs and has a respiratory acidosis. She will not tolerate CPAP and gases deteriorate further. She is intubated and positive pressure ventilation is commenced. Oxygenation remains poor and an arterial blood gases obtained.
pH 7.28 PIP 24
PaO 2 5.1 PEEP 4
PaCO 2 6.7 TI 0.5
HCO 3 23.6 TE 0.5
BE −3.5 Flow 3.0 L/min
FiO 2 0.71
MAP 9. 2 mBar, VT 8.1 mL, MV 0.49 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
pH 7.16 PIP 22
PaO 2 5.8 PEEP 4
PaCO 2 6.7 TI 0.6
HCO 3 17.7 TE 0.8
BE −11.4 Flow 8.0 L/min
FiO 2 0.65
MAP 11. 6 mBar, VT 4.4 mL, MV0.18 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
21. A 28 week infant has required ventilation following a difficult delivery. Initial ventilation was minimal but his condition has deteriorated over the first 36 hours despite three doses of surfactant. The lungs appear poorly aerated on x-ray and there are widespread inflammatory changes. The following arterial blood gas is obtained. He weighs 1.3 kg.
pH 7.17 PIP 24
PaO 2 4.9 PEEP 5
PaCO 2 9.9 TI 0.35
HCO 3 26.7 TE 0.65
BE −4.2 Flow 9.0 L/min
FiO 2 0.85
MAP 11. 2 mBar, VT 4.9 mL, MV 0.28 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
22. A 34 week infant has developed severe abdominal distension and an ileal atresia has been diagnosed. Laparotomy is performed for which he requires ventilation. Postoperatively ventilatory requirements steadily increase and 48 hours after surgery the following arterial blood gas is obtained. Chest x-ray shows a ground-glass appearance and low lung volumes. His weight at birth was 2.1 kg.
pH 7.20 PIP 24
PaO 2 6.1 PEEP 4
PaCO 2 10.1 TI 0.4
HCO 3 29.3 TE 0.9
BE −1.2 Flow 10.2 L/min
FiO 2 0.85
MAP 10. 1 mBar, VT 6.3 mL, MV 0.29 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
23. In the same infant surfactant has been given and some changes made to ventilation. The following gas is obtained 90 minutes later.

pH 7.38 PIP 24
PaO 2 14.6 PEEP 4
PaCO 2 4.1 TI 0.35
HCO 3 18.3 TE 0.35
BE −5.0 Flow 10.2 L/min
FiO 2 0.55
MAP 14. 0 mBar, VT 12.1 mL, MV 1.04 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
24. In the same infant some changes in ventilation are made. The following arterial blood gas is obtained two hours later.
pH 7.52 PIP 22
PaO 2 9.8 PEEP 4
PaCO 2 3.3 TI 0.35
HCO 3 21 TE 0.35
BE 1.11 Flow 10.2 L/min
FiO 2 0.35
MAP 12. 5 mBar, VT 13.2 mL, MV 1.13 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
25. A 34 week gestation infant required air only at birth. Her condition has steadily deteriorated and she required ventilation 36 hours after birth. The following arterial blood gas is obtained at 72 hours of age. She weighs 2.1 kg.
pH 7.26 PIP 22
PaO 2 9.1 PEEP 5
PaCO 2 8.3 TI 0.4
HCO 3 27.8 TE 0.9
BE −0.7 Flow 8.7 L/min
FiO 2 0.80
MAP 10. 8 mBar, VT 12.2 mL, MV 0.56 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
26. In the same infant as question 25 the expiratory time has been reduced and the ventilator rate increased. The following arterial blood gas is obtained one hour after the change.
pH 7.38 PIP 22
PaO 2 11.4 PEEP 5
PaCO 2 6.1 TI 0.4
HCO 3 27.3 TE 0.6
BE 2.1 Flow 8.7 L/min
FiO 2 0.65
MAP 11. 8 mBar, VT 12.2 mL, MV 0.3 L/min
27. A 27 week gestation infant weighing 0.98 kg has moderately severe RDS and has required ventilation from birth. Carbon dioxide is good but oxygenation is difficult. The following arterial blood gas is obtained.
pH 7.36 PIP 22
PaO 2 4.7 PEEP 4
PaCO 2 5.1 TI 0.3
HCO 3 21.7 TE 0.5
BE −2.8 Flow 3.0 L/min
FiO 2 0.85
MAP 7. 7 mBar, VT 4.4 mL, MV 0.33 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
28. Continuing with the case above. You have elected to increase the flow and have found that after setting 9 L/min you are able to get good chest movement and the ventilator display shows that you now have a good airway pressure curve. Oxygenation improves and you are able to reduce the inspired oxygen concentration. The unit is busy and you are unable to see the baby for a further six hours. However, a colleague performed a blood gas analysis after three hours and reassured you that the pH was acceptable and oxygenation was better. When you are able to return you check the arterial gas, which is shown below.
pH 7.35 PIP 22
PaO 2 8.9 PEEP 5
PaCO 2 2.1 TI 0.35
HCO 3 8.8 TE 0.65
BE −13.4 Flow 7.5 L/min
FiO 2 0.35
MAP 10. 8 mBar, VT 6.1 mL, MV 0.37 L/min
a. What do these results show?
b. What is the most likely explanation?
c. What actions will you take?
29. A 29 week gestation infant weighing 1.15 kg has been placed on synchronised intermittent positive pressure ventilation. He is fighting the ventilator and appears distressed. You are considering either paralysing him or returning him to conventional ventilation. The following arterial blood gas is obtained.
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pH 7.20 PIP 20
PaO 2 9.1 PEEP 5
PaCO 2 7.1 TI 0.6
HCO 3 20.6 TE 0.6
BE −8.0 Flow 9.2 L/min
FiO 2 0.55

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