Reducing the within-patient variability of breathing for radiotherapy delivery in conscious, unsedated cancer patients using a mechanical ventilator

Abstract

Objective: Variability in the breathing pattern of patients with cancer during radiotherapy requires mitigation, including enlargement of the planned treatment field, treatment gating and breathing guidance interventions. Here, we provide the first demonstration of how easy it is to mechanically ventilate patients with breast cancer while fully conscious and without sedation, and we quantify the resulting reduction in the variability of breathing.

Methods: 15 patients were trained for mechanical ventilation. Breathing was measured and the left breast anteroposterior displacement was measured using an Osiris surface-image mapping system (Qados Ltd, Sandhurst, UK).

Results: Mechanical ventilation significantly reduced the within-breath variability of breathing frequency by 85% (p < 0.0001) and that of inflation volume by 29% (p < 0.006) when compared with their spontaneous breathing pattern. During mechanical ventilation, the mean amplitude of the left breast marker displacement was 5 ± 1 mm, the mean variability in its peak inflation position was 0.5 ± 0.1 mm and that in its trough inflation position was 0.4 ± 0.0 mm. Their mean drifts were not significantly different from 0 mm min(-1) (peak drift was -0.1 ± 0.2 mm min(-1) and trough drift was -0.3 ± 0.2 mm min(-1)). Patients had a normal resting mean systolic blood pressure (131 ± 5 mmHg) and mean heart rate [75 ± 2 beats per minute (bpm)] before mechanical ventilation. During mechanical ventilation, the mean blood pressure did not change significantly, mean heart rate fell by 2 bpm (p < 0.05) with pre-oxygenation and rose by only 4 bpm (p < 0.05) during pre-oxygenation with hypocapnia. No patients reported discomfort and all 15 patients were always willing to return to the laboratory on multiple occasions to continue the study.

Conclusion: This simple technique for regularizing breathing may have important applications in radiotherapy.

Advances in knowledge: Variations in the breathing pattern introduce major problems in imaging and radiotherapy planning and delivery and are currently addressed to only a limited extent by asking patients to breathe to auditory or visual guidelines. We provide the first demonstration that a completely different technique, of using a mechanical ventilator to take over the patients' breathing for them, is easy for patients who are conscious and unsedated and reduces the within-patient variability of breathing. This technique has potential advantages in radiotherapy over currently used breathing guidance interventions because it does not require any active participation from or feedback to the patient and is therefore worthy of further clinical evaluation.

Figure 1.

Equipment on the patient during mechanical ventilation in the simulator room. The figure shows a patient lying supine on a breast board, with blood pressure (BP) and oxygen saturation (SpO₂) measured with non-invasive monitors on the fingers, a three-lead electrocardiogram (ECG) to measure heart rate and airway pressure and exhaled partial pressure of carbon dioxide (PCO₂) measured in the face mask. The mechanical ventilator drives breathing via the face mask. The Osiris measures the movement of the chest with markers positioned at the centre of the chest, on the left breast and with right and left lateral markers oriented at 90° to the horizontal. The hypothetical X-ray target is indicated by the dotted circle. The Osiris surface-image mapping system was obtained from Qados Ltd, Sandhurst, UK.

Figure 2.

Mechanical ventilation regularizes breathing and breast movement in Patient 1. (a) Airway pressure during 2 min of spontaneous (irregular) breathing. Pressure measurements are uncalibrated [in arbitrary (arb.) units]. Spontaneous breaths in the mask cause negative pressure waves (downwards in the figure); but, for comfort, the ventilator then added approximately 10 cm water (approximately 10 mbar) of inspiratory assist, so each spontaneous breath continues as a positive pressure wave (upwards in the figure). For spontaneous breathing, the mean breathing frequency with its variability [±standard deviation (SD)%] within this patient in breaths per minute, inflation volume with its variability (±SD%) in arb. units seconds, the recording period duration (minutes) and number (#) of breaths during this period are indicated. (b) Airway pressure during 2 min of mechanical ventilation. The regular frequency and pressure amplitude indicate that the patient is passive (i.e. has allowed the ventilator to take over their breathing). For mechanical ventilation, the mean breathing frequency (f) with its variability (±SD%) within this patient in breaths per minute, inflation volume with its variability (±SD%) in arb. units seconds, the recording period duration (minutes) and number (#) of breaths during this period are indicated. (c) Left breast anteroposterior (ant.-post.) movement during 50 s of mechanical ventilation. The regular frequency and movement amplitude of the left breast confirms that the patient is passive and demonstrates how predictable is the breast movement during mechanical ventilation. The mean displacement (mm), variability (var) in peak and trough position (mm) and peak and trough drift (mm min⁻¹) of the left breast marker are indicated. (d) Airway pressure during the same 50 s of mechanical ventilation as in (c). This demonstrates the correspondence between airway pressure and breast movement during mechanical ventilation and therefore that inflation volume is the same for each mechanically induced breath. For mechanical ventilation, the mean breathing frequency with its variability (±SD%) within this patient in breaths per minute, inflation volume with its variability (±SD%) in arb. units seconds, the recording period duration (minutes) and number (#) of breaths during this period are indicated.

Figure 3.

Mechanical ventilation regularizes breathing and breast movement in Patient 2. For descriptions on (a–d), refer to the legend of Figure 2.

Figure 4.

Mechanical ventilation regularizes breathing and breast movement in Patient 3. For descriptions on (a–d), refer to the legend of Figure 2.

Figure 5.

Mechanical ventilation regularizes breathing and breast movement in Patient 4. For descriptions on (a)–(d), refer to the legend of Figure 2.