Introduction

Venous return to the chest supports cardiac output and, in turn, blood pressure (BP) as a manifestation of cardiac output. Invasive flow measurements in animals and a CO2 rebreathing technique in able-bodied human subjects have demonstrated this relationship.1, 2 Breathing supports venous return. Tidal volume over a wide range in the able-bodied is directly correlated with cardiac output.2 It has recently been shown that forced vital capacity (FVC), a measure of breathing capacity, is closely related to the level of a spinal cord lesion – the higher the lesion, the lower the FVC.3, 4, 5, 6 Similarly, BP has been correlated with the level of paralysis – the higher the level the lower the BP.7 If breathing supports venous return and cardiac output, then FVC, a parameter of breathing capacity, might be correlated with BP, a bedside measure of cardiac output. The FVC and blood pressure from these reports have been compared.

Methods and results

The subjects in the studies compared are similar (see Table 1). Taking the published data, the FVC has been compared with mean blood pressure (MBP). FVC fell from 89% of predicted (by height, age, ethnic group, sex) at the lumbar levels to 61% predicted at cervical levels. MBP fell from 113 mmHg at the lumbar levels to 96 mmHg at cervical levels. The progressive decline toward higher levels of paralysis for both measurements is shown in Figure 1. When FVC and MBP are normalized to fractions of the lumbar values, the correlation between FVC and MBP through the range of levels of paralysis is high. By linear regression analysis,8 n=13, r=0.85, P<0.001.

Table 1 Comparison of SCI cohorts surveyed for forced vital capacity (FVC) and BP
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Figure 1
figure 1

FVC and MBP versus level of paralysis in motor complete SCI patients. Levels of paralysis represented are cervical, thoracic 2–12 and lumbar

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Discussion

Since FVC and MBP have been related to levels of the spinal cord lesions, separately and in the same direction, the correlation between FVC and MBP was predictable. Nevertheless, the correlation coefficient for FVC to BP is quite high, despite the derivation of data from independent studies and the nonstandard position of the BP subjects. That being said, a causal relationship between FVC and BP can be claimed only if breathing at rest or tidal volume, which normally affects venous return through negative pleural pressure, is represented by FVC. A reduced FVC is, in fact, associated with a reduced tidal volume in severe spinal cord injury (SCI).9 There is reason to believe, therefore, that FVC represents tidal volume and its effect on venous return, cardiac output, and BP.

Two other factors that could affect venous return, cardiac output, and BP in SCI patients can be suggested. (A) Blood volume is reduced by SCI, factors identified being reduced red cell mass and increased vascular leakage of albumin.10, 11, 12, 13, 14 The role of the level of SCI in blood volume may parallel that of FVC and BP. (B) Increased venous capacitance could reduce venous return. In fact, however, venous capacitance is reduced.15 For ascending SCI lesions both breathing disability and intravascular volume might contribute to the observed BP reduction – and account for the reduced left ventricular mass seen in these patients.16, 17

Conclusion

Increasing severity of SCI is associated with decreasing breathing capacity and BP. These two deficits are indirectly related since breathing is a factor in venous return and cardiac output. Intravascular volume may be another factor in the reduced BP of more severe SCI. These factors together may affect response to illness such as sepsis, which carries a high mortality due to shock.18