Positive end-expiratory pressure may alter breathing cardiovascular variability and baroreflex gain in mechanically ventilated patients

Abstract

Background: Baroreflex allows to reduce sudden rises or falls of arterial pressure through parallel RR interval fluctuations induced by autonomic nervous system. During spontaneous breathing, the application of positive end-expiratory pressure (PEEP) may affect the autonomic nervous system, as suggested by changes in baroreflex efficiency and RR variability. During mechanical ventilation, some patients have stable cardiorespiratory phase difference and high-frequency amplitude of RR variability (HF-RR amplitude) over time and others do not. Our first hypothesis was that a steady pattern could be associated with reduced baroreflex sensitivity and HF-RR amplitude, reflecting a blunted autonomic nervous function. Our second hypothesis was that PEEP, widely used in critical care patients, could affect their autonomic function, promoting both steady pattern and reduced baroreflex sensitivity.

Methods: We tested the effect of increasing PEEP from 5 to 10 cm H2O on the breathing variability of arterial pressure and RR intervals, and on the baroreflex. Invasive arterial pressure, ECG and ventilatory flow were recorded in 23 mechanically ventilated patients during 15 minutes for both PEEP levels. HF amplitude of RR and systolic blood pressure (SBP) time series and HF phase differences between RR, SBP and ventilatory signals were continuously computed by complex demodulation. Cross-spectral analysis was used to assess the coherence and gain functions between RR and SBP, yielding baroreflex-sensitivity indices.

Results: At PEEP 10, the 12 patients with a stable pattern had lower baroreflex gain and HF-RR amplitude of variability than the 11 other patients. Increasing PEEP was generally associated with a decreased baroreflex gain and a greater stability of HF-RR amplitude and cardiorespiratory phase difference. Four patients who exhibited a variable pattern at PEEP 5 became stable at PEEP 10. At PEEP 10, a stable pattern was associated with higher organ failure score and catecholamine dosage.

Conclusions: During mechanical ventilation, stable HF-RR amplitude and cardiorespiratory phase difference over time reflect a blunted autonomic nervous function which might worsen as PEEP increases.

Figure 1

Comparison between an unstable and a stable subject at PEEP 10, through significant spectral (summarized in Table 2) and CDM (summarized in Table 3) parameters. At the top, solid lines represented the CDM instantaneous parameters (HF-RR phase and amplitude). At the bottom, each dot represented an averaged value over 16 seconds, from the spectral gain method. The CDM method revealed a greater variability in HF-RR phase and amplitude in the unstable group, with great phase drifts and time spent below the amplitude threshold (dashed line). The spectral gain method revealed a higher gain in the unstable group, due to a greater HF-RR spectral density (close to the HF-RR CDM amplitude). The grey bands represented Fast Fourier Transform epochs with no efficient gain, corresponding to losses of CDM HF-RR amplitude, explaining the great correlation between the two following parameters: the percentage of time (in seconds) spent under an amplitude threshold provided by the CDM and the percentage of time (in epochs of 16 seconds) spent with a coherence value below 0.5.

Figure 2

Comparison of the spectral gain parameters between PEEP 5 and PEEP 10, in one subject, representative of the mean behavior of all patients. Each dot represented an averaged value over 16 seconds. Raw RR and SBP remained at the same level, while the gain was lower at PEEP 10, due to the increase in SBP spectral density. The mean coherence value and the percentage of time (in number of 16-second epochs) with coherence under 0.5 did not significantly differ between PEEP 5 and PEEP 10. Dashed lines showed 3 epochs with low coherence, discarding them from the analysis.

Figure 3

Comparison of the CDM parameters between PEEP 5 and PEEP 10 in a subject representative of the mean behavior of all patients. This subject exhibited great differences as he was classified in the unstable group at PEEP 5 and reclassified in the stable group at PEEP 10. So, the figure mainly focused on the greater variability of the HF phase and amplitude of RR and SBP, at PEEP 5. This greater variability was estimated by the standard-deviation of the time series, the maximal drift of the phase, the percentage of time spent below the amplitude threshold (dashed line).