Pulmonary blood flow generates cardiogenic oscillations

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Abstract

Cardiogenic oscillations are small waves produced by heartbeats, which are superimposed on the pressure and flow signals at the airway opening. The aim of this study was to investigate the role of the two main factors believed to generate these oscillations: (1) contact between heart and lungs and (2) pulmonary blood flow. We studied 15 heart surgery patients on cardiopulmonary bypass so both factors could be manipulated independently.

At minimal heart–lung contact pressure and flow oscillations were larger than during maximal contact (1.20 ± 0.17 cmH2O and 2.36 ± 0.08 L min−1 vs 0.92 ± 0.15 cmH2O and 1.78 ± 0.26 L min−1, mean ± SD, p < 0.05). Cardiogenic oscillations for pressure and flow were smaller at 50% compared to 100% pulmonary blood flow (0.80 ± 0.12 cmH2O and 1.56 ± 0.34 L min−1 vs 1.19 ± 0.14 cmH2O and 2.38 ± 0.19 L min−1). We conclude that the amount of pulmonary blood flow and not the contact between heart and lungs is the main factor determining the amplitude of cardiogenic oscillations.

Introduction

Cardiogenic oscillations (COS) are heart-synchronous variations in the gas, flow and pressure signals recorded at the airway opening (Dahlstrom et al., 1954, Michels and West, 1978, Arieli, 1983, Bijaoui et al., 2001, Lichtwark-Aschoff et al., 2004, Montmerle and Linnarsson, 2005). COS of a particular gas (COSgas) are caused by two factors: (1) pulsatile waves that push gas molecules towards the airway opening, and (2) a difference in the gas concentration within the small airways. The amplitude of COSgas is directly related to both of these effects and would increase in the presence of a heterogeneous distribution of ventilation (Michels and West, 1978, Guy et al., 1994).

Many studies have focused on COS in airway flow (COSflow) and pressure (COSpaw) signals. These studies investigated COS as the cause of auto-triggering during assisted ventilation (Imanaka et al., 2000), tried to differentiate between central and obstructive apnea (Ayappa et al., 1999), to estimate respiratory compliance (Lichtwark-Aschoff et al., 2004) and impedance (Bijaoui et al., 2001) by means of COS or to study the effects of COS on gases measured at the airway (COSgas) (Montmerle and Linnarsson, 2005).

The origin of COSflow and COSpaw is still under debate and not well understood. A widely accepted explanation is that COSflow and COSpaw result from a direct physical transfer of heartbeats onto the surrounding lungs (West and Hugh-Jones, 1961, Bosman and Lee, 1965, Fukuchi et al., 1977, Arieli et al., 1981, Heckman et al., 1982). However, other factors like the time-dependent variations in thoracic blood volume during the cardiac cycle or the transmission of the pulse wave through the main pulmonary vessels have been proposed as origins of COS (Rohdin et al., 2003, Montmerle and Linnarsson, 2005, Dahlstrom et al., 1954).

The knowledge of the exact mechanism involved in the genesis of COSflow and COSpaw has potentially important clinical implications. First, it could improve our understanding of studies using COSgas for analyzing the distribution of lung ventilation and/or perfusion. Second, due to the information they contain, COS may have a future role in cardio-respiratory monitoring (Heckman et al., 1982).

The aim of this work was to study the role of each one of the two main factors known to cause COSflow and COSpaw: pulmonary blood flow (PBF) and the physical contact between heart and lungs. We studied patients during cardiac surgery assisted by cardiopulmonary bypass (CPB). This artificial circuit temporarily replaces in part or in full the lung's gas exchange and thus, we used it as an in vivo model, in which PBF could be adjusted as needed and the heart isolated from the lung parenchyma.

Section snippets

Material and methods

After approval by the local ethic committee and after written informed consent by the patient, we studied 11 men and 4 women aged 62 ± 5 years, with weights of 81 ± 13 kg and heights of 1.69 ± 0.16 m (mean ± SD) undergoing elective aorto-coronary bypass surgery. Patients had normal pre-operative lung function according to their clinical history, chest X-ray and arterial blood gases. Exclusion criteria were emergency surgery, left ventricular ejection fraction <30%, age >70 years, chronic pulmonary or

Results

The data in Table 1 represent group mean values of all patients for each one of the protocol phases. The amplitudes of COSflow and COSpaw were similar along the different phases of the protocol. The SBW with its slow expiration resulted in COS of good resolution and quality in the pressure and flow signals. During the breath-holds phases, COSpaw had a higher amplitude and better resolution during the inspiratory holds whereas COSflow had a higher amplitude and better resolution during the

Discussion

This is the first invasive study analyzing COSflow and COSpaw in patients undergoing mechanical ventilation and cardiopulmonary bypass. The two main findings of this study can be summarized as follows: (1) At similar stroke volumes, the amplitude of COS increased when heart and lungs were physically isolated from each other during cardiopulmonary bypass as compared to a condition of maximum contact. (2) Amplitudes of COSflow and COSpaw were proportional to the variations in PBF.

Acknowledgment

We thank Rodolfo Plit for his technical assistance during the cardiopulmonary bypass.

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