Pressure Ventilation: DISCUSSION

In the present study, HFJV applied with an identical Paw as during IPPV did not modify pulmonary gas exchange, MAP, and CI. Similarly, no significant differences in mean CBF values (Fb ISI) were noted.

Classic main determinants of CBF remained stable and were comparable during both modes of ventila­tion. The PaC02 and rectal temperature were main­tained in a narrow range fixed by our experimental protocol, and MAP did not change. Furthermore, Paw did not vary throughout the experiment, resulting in stable arterial oxygenation and probably in a similar lung volume, and with an identical effect on systemic hemodynamics. This is in agreement with previous studies showing a close relationship between Paw and systemic hemodynamics. In particular, our results confirm that HFJV has no different effect on cardiac output than during IPPV at similar Paw. Our patients had a relatively low dynamic total respiratory compli­ance (50 ml/cm H20), most probably due to a moderate interstitial pulmonary edema due to cardiopulmonary bypass. This relatively low compliance may have attenuated the influence of variations of intrathoracic pressure on cardiac filling pressures, ventricular vol­umes, and cardiac output.

The CBF measuring method used in the present study (Xe washout) provides an average flow over a period of 11 minutes. It does not detect changes of CBF that might occur during the course of a single respiratory cycle. Thus, although instant CBF could change in relation to ventilator-synchronous fluctua­tions of ICP, our results demonstrate that these varia­tions of ICP are too small in our patients to have an influence on average flow measured over several minutes. Moreover, as shown in animal studies,respiratory fluctuations of the ICP waveform which most likely reflect variations of cerebral blood volume, are not very important at normal ICP In presence of an abnormal cerebral compliance, the results of animal studies are conflicting and depend on the type of cerebral injury. Further studies investigating the effect of increase of intrathoracic pressure in patients with poor cerebral compliance are necessary.
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Table 4—Mean Arterial Pressure, PaCOt, Rectal Temperature, and Mean Airway Pressure of the Three Patients With Major Changes of CBF During Application of HFJV

Patient

IPPV,

HFJV

IPPV2

ISI, units

3

40

73

61

7

49

33

51

13

61

26

40

MAP, mm Hg

3

81

81

102

7

110

115

120

13

100

91

93

PaC02, kPa

3

5.54

5.52

5.47

7

5.19

5.52

5.67

13

5.51

5.35

4.86

Rectal temperature, °C

3

35.6

35.6

35.8

7

36.4

36.5

36.5

13

36.2

36.3

36.3

Paw, mm Hg

3

7.5

5.0

7.5

7

3.8

4.0

4.5

13

8.5

8.0

8.5

Although we did not find significant differences of mean CBF values between HFJV and IPPV for the group studied, HFJV induced in three patients im­portant variations of CBF (Fig 1) in spite of stable classic main determinants of CBF, ie, PaC02, cerebral perfusion pressure, Paw, and temperature (Table 4). Several of the following mechanisms could explain these findings: first, this could be related to technical problems of CBF measurement or the quality of our CBF determination was satisfactory, even for the three patients who demonstrated large changes in CBF during HFJV In addition, the variability of CBF measurement with this method is below 10 percent,and the mean change in these three patients was more than 40 percent. Second, the state of stimulation may have changed during HFJV because the intensive care environment could have been different between the IPPV and HFJV period, even with 5 mg morphine given before the beginning of the study. In addition, similar doses of midazolam and fentanyl were admin­istered during surgery, and the time between the end of surgery and the beginning of the study was similar for all patients. Third, CBF values could also be modified by other factors that we could not assess: eg, the state of neurogenic control of cerebral blood volume can be different for each patient. The marked response of CBF to the application of HFJV in three patients suggests that CBF also depends on uncon­trolled and possibly determinant factors. generic cialis 20mg

In conclusion, the results of this study indicate that HFJV has no specific effects on CBF when compared to a conventional mode of ventilation at similar Paw in patients with normal systemic hemodynamics. These observations are in agreement with animal studies.Whether HFJV can preserve CBF in patients with poor cerebral compliance or with circulatory shock, a situation during which HFJV has been shown to induce a lesser degree of hemodynamic impairment than during IPPV when identical levels of Paw are applied, remains to be investigated.