Modification of a domiciliary ventilator to increase FiO2: an off-label modification which may be of value in COVID-19.

Although nasal continuous positive airway pressure or non-invasive ventilation is used to manage some patients with acute lung injury due to COVID-19, such patients also demonstrate increased minute ventilation which makes it hard, if the device is used in line with the manufacturer's instructions, to achieve adequate oxygen delivery. In addition, if a hospital contains many such patients, then it is possible that the oxygen requirements will exceed infrastructure capacity. Here we describe a simple modification of two exemplar ventilators normally used for domiciliary ventilation, which substantially increased the fraction of inspired oxygen (FiO2) delivered.

Resonance as the Mechanism of Daytime Periodic Breathing in Patients with Heart Failure.

RATIONALE: In patients with chronic heart failure, daytime oscillatory breathing at rest is associated with a high risk of mortality. Experimental evidence, including exaggerated ventilatory responses to CO2 and prolonged circulation time, implicates the ventilatory control system and suggests feedback instability (loop gain > 1) is responsible. However, daytime oscillatory patterns often appear remarkably irregular versus classic instability (Cheyne-Stokes respiration), suggesting our mechanistic understanding is limited. OBJECTIVES: We propose that daytime ventilatory oscillations generally result from a chemoreflex resonance, in which spontaneous biological variations in ventilatory drive repeatedly induce temporary and irregular ringing effects. Importantly, the ease with which spontaneous biological variations induce irregular oscillations (resonance "strength") rises profoundly as loop gain rises toward 1. We tested this hypothesis through a comparison of mathematical predictions against actual measurements in patients with heart failure and healthy control subjects. METHODS: In 25 patients with chronic heart failure and 25 control subjects, we examined spontaneous oscillations in ventilation and separately quantified loop gain using dynamic inspired CO2 stimulation. MEASUREMENTS AND MAIN RESULTS: Resonance was detected in 24 of 25 patients with heart failure and 18 of 25 control subjects. With increased loop gain-consequent to increased chemosensitivity and delay-the strength of spontaneous oscillations increased precipitously as predicted (r = 0.88), yielding larger (r = 0.78) and more regular (interpeak interval SD, r = -0.68) oscillations (P < 0.001 for all, both groups combined). CONCLUSIONS: Our study elucidates the mechanism underlying daytime ventilatory oscillations in heart failure and provides a means to measure and interpret these oscillations to reveal the underlying chemoreflex hypersensitivity and reduced stability that foretells mortality in this population.

Dynamic CO2 therapy in periodic breathing: a modeling study to determine optimal timing and dosage regimes.

We examine the potential to treat unstable ventilatory control (seen in periodic breathing, Cheyne-Stokes respiration, and central sleep apnea) with carefully controlled dynamic administration of supplementary CO(2), aiming to reduce ventilatory oscillations with minimum increment in mean CO(2). We used a standard mathematical model to explore the consequences of phasic CO(2) administration, with different timing and dosing algorithms. We found an optimal time window within the ventilation cycle (covering approximately 1/6 of the cycle) during which CO(2) delivery reduces ventilatory fluctuations by >95%. Outside that time, therapy is dramatically less effective: indeed, for more than two-thirds of the cycle, therapy increases ventilatory fluctuations >30%. Efficiency of stabilizing ventilation improved when the algorithm gave a graded increase in CO(2) dose (by controlling its duration or concentration) for more severe periodic breathing. Combining gradations of duration and concentration further increased efficiency of therapy by 22%. The (undesirable) increment in mean end-tidal CO(2) caused was 300 times smaller with dynamic therapy than with static therapy, to achieve the same degree of ventilatory stabilization (0.0005 vs. 0.1710 kPa). The increase in average ventilation was also much smaller with dynamic than static therapy (0.005 vs. 2.015 l/min). We conclude that, if administered dynamically, dramatically smaller quantities of CO(2) could be used to reduce periodic breathing, with minimal adverse effects. Algorithms adjusting both duration and concentration in real time would achieve this most efficiently. If developed clinically as a therapy for periodic breathing, this would minimize excess acidosis, hyperventilation, and sympathetic overactivation, compared with static treatment.

Novel cardiac pacemaker-based human model of periodic breathing to develop real-time, pre-emptive technology for carbon dioxide stabilisation.

BACKGROUND: Constant flow and concentration CO2 has previously been efficacious in attenuating ventilatory oscillations in periodic breathing (PB) where oscillations in CO2 drive ventilatory oscillations. However, it has the undesirable effect of increasing end-tidal CO2, and ventilation. We tested, in a model of PB, a dynamic CO2 therapy that aims to attenuate pacemaker-induced ventilatory oscillations while minimising CO2 dose. METHODS: First, pacemakers were manipulated in 12 pacemaker recipients, 6 with heart failure (ejection fraction (EF)=23.7±7.3%) and 6 without heart failure, to experimentally induce PB. Second, we applied a real-time algorithm of pre-emptive dynamic exogenous CO2 administration, and tested different timings. RESULTS: We found that cardiac output alternation using pacemakers successfully induced PB. Dynamic CO2 therapy, when delivered coincident with hyperventilation, attenuated 57% of the experimentally induced oscillations in end-tidal CO2: SD/mean 0.06±0.01 untreated versus 0.04±0.01 with treatment (p<0.0001) and 0.02±0.01 in baseline non-modified breathing. This translated to a 56% reduction in induced ventilatory oscillations: SD/mean 0.19±0.09 untreated versus 0.14±0.06 with treatment (p=0.001) and 0.10±0.03 at baseline. Of note, end-tidal CO2 did not significantly rise when dynamic CO2 was applied to the model (4.84±0.47 vs 4.91± 0.45 kPa, p=0.08). Furthermore, mean ventilation was also not significantly increased by dynamic CO2 compared with untreated (7.8±1.2 vs 8.4±1.2 L/min, p=0.17). CONCLUSIONS: Cardiac pacemaker manipulation can be used to induce PB experimentally. In this induced PB, delivering CO2 coincident with hyperventilation, ventilatory oscillations can be substantially attenuated without a significant increase in end-tidal CO2 or ventilation. Dynamic CO2 administration might be developed into a clinical treatment for PB. TRIAL REGISTRATION NUMBER: ISRCTN29344450.

Underestimation of duration of ventricular activation by 12-lead ECG compared with direct measurement of activation duration derived from implanted pacemaker leads.

AIM: To determine extent to which 12-lead ECG QRS duration (QRSd) reflects ventricular activation duration compared with time relations from unpaced ventricular myograms in cardiac resynchronisation therapy (CRT) patients. METHODS: Left (LV) and right ventricular (RV) myograms were recorded during spontaneous rhythm from in-situ pacemaker leads in 77 patients receiving CRT; 14 'normal activation' (unpaced QRSd <12 ms), 10 'simple left bundle branch block' (LBBB, QRSd 120-149 ms), 40 'advanced LBBB' (QRS ≥ 150 ms) and 13 right bundle branch block. Delay in onset (Q-LV, Q-RV) and duration (dur-LV, dur-RV) of activation were measured. Interventricular delay (ΔT: Q-LV minus Q-RV) and 'LV-overrun' (time between end 12-lead QRS and Q-end LV myogram) were calculated. RESULTS: 'Normal activation': Neither Q-LV, Q-RV (38 ± 6 ms, 39 ± 11 ms), nor dur-LV, dur-RV (66 ± 9 ms, 81 ± 25 ms) differed. ΔT (-1 ± 11 ms) was not different from zero, nor was Q-end LV (104 ± 10 ms) different from QRSd (p=0.09). 'Simple LBBB': Q-LV (102 ± 28 ms) was longer than 'normal activation' (p<0.001), but Q-RV, dur-LV, and dur-RV were no different. ΔT (54 ± 23 ms) was increased (p<0.001) and Q-end LV (187 ± 48 ms) was longer than QRSd (p=0.005). 'Advanced LBBB': Q-LV (115 ± 52 ms) was longer than 'normal activation' (p<0.001) but Q-RV was no different, so ΔT (72 ± 47 ms) was increased (p<0.001 compared to normal, p=0.04 compared to simple LBBB). Dur-LV (102 ± 27 ms) was also prolonged, so Q-end LV (218 ± 48 ms) was longer than QRSd (p<0.001). Longer LV-overrun was associated with longer ΔT (p<0.001). CONCLUSIONS: Prolonged LV myopotential duration, associated with interventricular delay, is electrically silent on 12-lead QRSd. Unpaced surface QRSd underestimates true duration of native LV activation in CRT patients.

Real-time dynamic carbon dioxide administration: a novel treatment strategy for stabilization of periodic breathing with potential application to central sleep apnea.

OBJECTIVES: This study targeted carbon dioxide (CO(2)) oscillations seen in oscillatory ventilation with dynamic pre-emptive CO(2) administration. BACKGROUND: Oscillations in end-tidal CO(2) (et-CO(2)) drive the ventilatory oscillations of periodic breathing (PB) and central sleep apnea in heart failure (HF). METHODS: Seven healthy volunteers simulated PB, while undergoing dynamic CO(2) administration delivered by an automated algorithm at different concentrations and phases within the PB cycle. The algorithm was then tested in 7 patients with HF and PB. RESULTS: In voluntary PB, the greatest reduction (74%, p < 0.0001) in et-CO(2) oscillations was achieved when dynamic CO(2) was delivered at hyperventilation; when delivered at the opposite phase, the amplitude of et-CO(2) oscillations increased (35%, p = 0.001). In HF patients, oscillations in et-CO(2) were reduced by 43% and ventilatory oscillations by 68% (both p < 0.05). During dynamic CO(2) administration, mean et-CO(2) and ventilation levels remained unchanged. Static CO(2) (2%, constant flow) administration also attenuated spontaneous PB in HF patients (p = 0.02) but increased mean et-CO(2) (p = 0.03) and ventilation (by 45%, p = 0.03). CONCLUSIONS: Dynamic CO(2) administration, delivered at an appropriate time during PB, can almost eliminate oscillations in et-CO(2) and ventilation. This dynamic approach might be developed to treat central sleep apnea, as well as minimizing undesirable increases in et-CO(2) and ventilation.

Novel use of cardiac pacemakers in heart failure to dynamically manipulate the respiratory system through algorithmic changes in cardiac output.

BACKGROUND: Alternation of heart rate between 2 values using a pacemaker generates oscillations in end-tidal CO(2) (et-CO(2)). This study examined (a) whether modulating atrioventricular delay can also do this, and (b) whether more gradual variation of cardiac output can achieve comparable changes in et-CO(2) with less-sudden changes in blood pressure. METHODS AND RESULTS: We applied pacemaker fluctuations by adjusting heart rate (by 30 bpm) or atrioventricular delay (between optimal and nonoptimal values) or both, with period of 60 s in 19 heart failure patients (age 73+/-11, EF 29+/-12%). The changes in cardiac output, by either heart rate or atrioventricular delay or both, were made either as a step ("square wave") or more gradually ("sine wave"). We obtained changes in cardiac output sufficient to engender comparable oscillations in et-CO(2) (P=NS) in all 19 patients either by manipulation of heart rate (14), or by atrioventricular delay (2) or both (3). The square wave produced 191% larger and 250% more sudden changes in blood pressure than the sine wave alternations (22.4+/-11.7 versus 13.6+/-4.5 mm Hg, P<0.01 and 19.8+/-10.0 versus 7.9+/-3.2 mm Hg over 5 s, P<0.01), but peak-to-trough et-CO(2) elicited was only 45% higher (0.45+/-0.18 versus 0.31+/-0.13 kPa, P=0.01). CONCLUSIONS: This study shows that cardiac output is the key to dynamically manipulating the respiratory system with pacing sequences. When manipulating respiration by this route, a sine wave pattern may be preferable to a square wave, because it minimizes sudden blood pressure fluctuations.

Induction of oscillatory ventilation pattern using dynamic modulation of heart rate through a pacemaker.

For disease states characterized by oscillatory ventilation, an ideal dynamic therapy would apply a counteracting oscillation in ventilation. Modulating respiratory gas transport through the circulation might allow this. We explore the ability of repetitive alternations in heart rate, using a cardiac pacemaker, to elicit oscillations in respiratory variables and discuss the potential for therapeutic exploitation. By incorporating acute cardiac output manipulations into an integrated mathematical model, we observed that a rise in cardiac output should yield a gradual rise in end-tidal CO2 and, subsequently, ventilation. An alternating pattern of cardiac output might, therefore, create oscillations in CO2 and ventilation. We studied the effect of repeated alternations in heart rate of 30 beats/min with periodicity of 60 s, on cardiac output, respiratory gases, and ventilation in 22 subjects with implanted cardiac pacemakers and stable breathing patterns. End-tidal CO2 and ventilation developed consistent oscillations with a period of 60 s during the heart rate alternations, with mean peak-to-trough relative excursions of 8.4 +/- 5.0% (P < 0.0001) and 24.4 +/- 18.8% (P < 0.0001), respectively. Furthermore, we verified the mathematical prediction that the amplitude of these oscillations would depend on those in cardiac output (r = 0.59, P = 0.001). Repetitive alternations in heart rate can elicit reproducible oscillations in end-tidal CO2 and ventilation. The size of this effect depends on the magnitude of the cardiac output response. Harnessed and timed appropriately, this cardiorespiratory mechanism might be exploited to create an active dynamic responsive pacing algorithm to counteract spontaneous respiratory oscillations, such as those causing apneic breathing disorders.