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Correspondence |
Sapporo, Japan
To the Editor:
We non-invasively investigated the relationship between waveform variation determined by pulse oximetry and the diameter of the inferior vena cava (IVC) determined by ultrasound imaging, the diameter of the IVC directly reflecting intravascular volume.1
Twenty ASA physical status I or II adult patients who required general anesthesia were enrolled in this study. After tracheal intubation, anesthesia was maintained with 1% sevoflurane and nitrous oxide (1 L•min–1)/oxygen (1 L•min–1). Ventilation was controlled with a ventilatory rate of 10 min–1 (durations of inspiratory and expiratory periods were two and four seconds, respectively) with an inspiratory pressure of +15 cm H2O. Under stable anesthesia but before surgery, "pulse amplitude (PA)" and diameter of the IVC were measured by a pulse oximeter (NELLCOR N-595TM; Tyco Healthcare, Pleasanton, CA, USA) attached on the left second finger tip and by a simpleminded ultrasound imaging system M2430A (OptiGoTM; Philips, Eindhoven, the Netherlands), respectively. PA was measured automatically as a relative PA (alternating current component) to a background light absorption (direct current component); thus, PA is defined as (max – min)•1/2 (max + min) –1 x 100 (max and min = maximal and minimal light absorption intensities). This variable was recorded automatically at two-second intervals in a personal computer with SatCollector version 2.2 software (NELLCOR). The diameter of the IVC was measured longitudinally with an OptiGoTM probe (2.5 MHz sector transducer) from a window below the xiphoid process by an independent expert. Maximal and minimal values of the IVC diameter were recorded over the mechanical positive-pressure respiratory cycle.2
The relationship between the respiratory-dependent variations of the PA and IVC diameter is shown in the Figure
. Mean (± SD) percent variations of PA and IVC diameter were 10.7% ± 4.8% and 7.4% ± 3.3%, respectively. There was a significant linear correlation between these variables (r = 0.82, n = 20, P < 0.01). Rescue ephedrine was administered in patients with a higher variation (indicated by an asterisk; 15.8% ± 4.3% for PA and 9.4% ± 5.5% for the IVC diameter, n = 4) compared to patients with a lesser variation (11.5% ± 3.0% for PA and 6.3% ± 3.3% for the IVC diameter, n = 16). The results of our preliminary study suggest that the respiratory-dependent variation of PA measured by a pulse oximeter may be a reliable and early predictor of hypovolemia.3
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References
1 Kelman GR. Interpretation of CVP measurements. Anaesthesia 1971; 26: 209–15.[Medline]
2 Perel A, Pizov R. Cardiovascular effects of mechanical ventilation. In: Perel A, Stock MC (Eds). Handbook of Mechanical Ventilatory Support. Baltimore: Williams & Wilkins; 1992: 51–65.
3 Shamir M, Eidelman LA, Floman Y, Kaplan L, Pizov R. Pulse oximetry plethysmographic waveform during changes in blood volume. Br J Anaesth 1999; 82: 178–81.
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